kernel - Add callout debugging

[dragonfly.git] / sys / kern / usched_dfly.c

/*
 * Copyright (c) 2012-2017 The DragonFly Project.  All rights reserved.
 * Copyright (c) 1999 Peter Wemm <peter@FreeBSD.org>.  All rights reserved.
 *
 * This code is derived from software contributed to The DragonFly Project
 * by Matthew Dillon <dillon@backplane.com>,
 * by Mihai Carabas <mihai.carabas@gmail.com>
 * and many others.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in
 *    the documentation and/or other materials provided with the
 *    distribution.
 * 3. Neither the name of The DragonFly Project nor the names of its
 *    contributors may be used to endorse or promote products derived
 *    from this software without specific, prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
 * FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE
 * COPYRIGHT HOLDERS OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
 * INCIDENTAL, SPECIAL, EXEMPLARY OR CONSEQUENTIAL DAMAGES (INCLUDING,
 * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
 * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
 * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
 * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
 * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 */
#include <sys/param.h>
#include <sys/systm.h>
#include <sys/kernel.h>
#include <sys/lock.h>
#include <sys/queue.h>
#include <sys/proc.h>
#include <sys/rtprio.h>
#include <sys/uio.h>
#include <sys/sysctl.h>
#include <sys/resourcevar.h>
#include <sys/spinlock.h>
#include <sys/cpu_topology.h>
#include <sys/thread2.h>
#include <sys/spinlock2.h>

#include <sys/ktr.h>

#include <machine/cpu.h>
#include <machine/smp.h>

/*
 * Priorities.  Note that with 32 run queues per scheduler each queue
 * represents four priority levels.
 */

int dfly_rebalanced;

#define MAXPRI                  128
#define PRIMASK                 (MAXPRI - 1)
#define PRIBASE_REALTIME        0
#define PRIBASE_NORMAL          MAXPRI
#define PRIBASE_IDLE            (MAXPRI * 2)
#define PRIBASE_THREAD          (MAXPRI * 3)
#define PRIBASE_NULL            (MAXPRI * 4)

#define NQS     32                      /* 32 run queues. */
#define PPQ     (MAXPRI / NQS)          /* priorities per queue */
#define PPQMASK (PPQ - 1)

/*
 * NICE_QS      - maximum queues nice can shift the process
 * EST_QS       - maximum queues estcpu can shift the process
 *
 * ESTCPUPPQ    - number of estcpu units per priority queue
 * ESTCPUMAX    - number of estcpu units
 *
 * Remember that NICE runs over the whole -20 to +20 range.
 */
#define NICE_QS         24      /* -20 to +20 shift in whole queues */
#define EST_QS          20      /* 0-MAX shift in whole queues */
#define ESTCPUPPQ       512
#define ESTCPUMAX       (ESTCPUPPQ * EST_QS)
#define PRIO_RANGE      (PRIO_MAX - PRIO_MIN + 1)

#define ESTCPULIM(v)    min((v), ESTCPUMAX)

TAILQ_HEAD(rq, lwp);

#define lwp_priority    lwp_usdata.dfly.priority
#define lwp_forked      lwp_usdata.dfly.forked
#define lwp_rqindex     lwp_usdata.dfly.rqindex
#define lwp_estcpu      lwp_usdata.dfly.estcpu
#define lwp_estfast     lwp_usdata.dfly.estfast
#define lwp_uload       lwp_usdata.dfly.uload
#define lwp_rqtype      lwp_usdata.dfly.rqtype
#define lwp_qcpu        lwp_usdata.dfly.qcpu
#define lwp_rrcount     lwp_usdata.dfly.rrcount

static __inline int
lptouload(struct lwp *lp)
{
        int uload;

        uload = lp->lwp_estcpu / NQS;
        uload -= uload * lp->lwp_proc->p_nice / (PRIO_MAX + 1);

        return uload;
}

/*
 * DFly scheduler pcpu structure.  Note that the pcpu uload field must
 * be 64-bits to avoid overflowing in the situation where more than 32768
 * processes are on a single cpu's queue.  Since high-end systems can
 * easily run 900,000+ processes, we have to deal with it.
 */
struct usched_dfly_pcpu {
        struct spinlock spin;
        struct thread   *helper_thread;
        struct globaldata *gd;
        u_short         scancpu;
        short           upri;
        long            uload;          /* 64-bits to avoid overflow (1) */
        int             ucount;
        int             flags;
        struct lwp      *uschedcp;
        struct rq       queues[NQS];
        struct rq       rtqueues[NQS];
        struct rq       idqueues[NQS];
        u_int32_t       queuebits;
        u_int32_t       rtqueuebits;
        u_int32_t       idqueuebits;
        int             runqcount;
        int             cpuid;
        cpumask_t       cpumask;
        cpu_node_t      *cpunode;
} __cachealign;

/*
 * Reflecting bits in the global atomic masks allows us to avoid
 * a certain degree of global ping-ponging.
 */
#define DFLY_PCPU_RDYMASK       0x0001  /* reflect rdyprocmask */
#define DFLY_PCPU_CURMASK       0x0002  /* reflect curprocmask */

typedef struct usched_dfly_pcpu *dfly_pcpu_t;

static void dfly_acquire_curproc(struct lwp *lp);
static void dfly_release_curproc(struct lwp *lp);
static void dfly_select_curproc(globaldata_t gd);
static void dfly_setrunqueue(struct lwp *lp);
static void dfly_setrunqueue_dd(dfly_pcpu_t rdd, struct lwp *lp);
static void dfly_schedulerclock(struct lwp *lp, sysclock_t period,
                                sysclock_t cpstamp);
static void dfly_recalculate_estcpu(struct lwp *lp);
static void dfly_resetpriority(struct lwp *lp);
static void dfly_forking(struct lwp *plp, struct lwp *lp);
static void dfly_exiting(struct lwp *lp, struct proc *);
static void dfly_uload_update(struct lwp *lp);
static void dfly_yield(struct lwp *lp);
static void dfly_changeqcpu_locked(struct lwp *lp,
                                dfly_pcpu_t dd, dfly_pcpu_t rdd);
static dfly_pcpu_t dfly_choose_best_queue(struct lwp *lp);
static dfly_pcpu_t dfly_choose_worst_queue(dfly_pcpu_t dd, int forceit);
static dfly_pcpu_t dfly_choose_queue_simple(dfly_pcpu_t dd, struct lwp *lp);
static void dfly_need_user_resched_remote(void *dummy);
static struct lwp *dfly_chooseproc_locked(dfly_pcpu_t rdd, dfly_pcpu_t dd,
                                          struct lwp *chklp, int worst);
static void dfly_remrunqueue_locked(dfly_pcpu_t dd, struct lwp *lp);
static void dfly_setrunqueue_locked(dfly_pcpu_t dd, struct lwp *lp);
static void dfly_changedcpu(struct lwp *lp);

struct usched usched_dfly = {
        { NULL },
        "dfly", "Original DragonFly Scheduler",
        NULL,                   /* default registration */
        NULL,                   /* default deregistration */
        dfly_acquire_curproc,
        dfly_release_curproc,
        dfly_setrunqueue,
        dfly_schedulerclock,
        dfly_recalculate_estcpu,
        dfly_resetpriority,
        dfly_forking,
        dfly_exiting,
        dfly_uload_update,
        NULL,                   /* setcpumask not supported */
        dfly_yield,
        dfly_changedcpu
};

/*
 * We have NQS (32) run queues per scheduling class.  For the normal
 * class, there are 128 priorities scaled onto these 32 queues.  New
 * processes are added to the last entry in each queue, and processes
 * are selected for running by taking them from the head and maintaining
 * a simple FIFO arrangement.  Realtime and Idle priority processes have
 * and explicit 0-31 priority which maps directly onto their class queue
 * index.  When a queue has something in it, the corresponding bit is
 * set in the queuebits variable, allowing a single read to determine
 * the state of all 32 queues and then a ffs() to find the first busy
 * queue.
 *
 * curprocmask is used to publish cpus with assigned curprocs to the rest
 * of the cpus.  In certain situations curprocmask may leave a bit set
 * (e.g. a yield or a token-based yield) even though dd->uschedcp is
 * NULL'd out temporarily).
 */
                                        /* currently running a user process */
static cpumask_t dfly_curprocmask = CPUMASK_INITIALIZER_ALLONES;
static cpumask_t dfly_rdyprocmask;      /* ready to accept a user process */
static struct usched_dfly_pcpu dfly_pcpu[MAXCPU];
static struct sysctl_ctx_list usched_dfly_sysctl_ctx;
static struct sysctl_oid *usched_dfly_sysctl_tree;
static struct lock usched_dfly_config_lk = LOCK_INITIALIZER("usdfs", 0, 0);

/* Debug info exposed through debug.* sysctl */

static int usched_dfly_debug = -1;
SYSCTL_INT(_debug, OID_AUTO, dfly_scdebug, CTLFLAG_RW,
           &usched_dfly_debug, 0,
           "Print debug information for this pid");

static int usched_dfly_pid_debug = -1;
SYSCTL_INT(_debug, OID_AUTO, dfly_pid_debug, CTLFLAG_RW,
           &usched_dfly_pid_debug, 0,
           "Print KTR debug information for this pid");

static int usched_dfly_chooser = 0;
SYSCTL_INT(_debug, OID_AUTO, dfly_chooser, CTLFLAG_RW,
           &usched_dfly_chooser, 0,
           "Print KTR debug information for this pid");

/*
 * WARNING!
 *
 * The fork bias can have a large effect on the system in the face of a
 * make -j N or other high-forking applications.
 *
 * Larger values are much less invasive vs other things that
 * might be running in the system, but can cause exec chains
 * such as those typically generated by make to have higher
 * latencies in the face of modest load.
 *
 * Lower values are more invasive but have reduced latencies
 * for such exec chains.
 *
 *      make -j 10 buildkernel example, build times:
 *
 *           +0 3:04
 *           +1 3:14    -5.2%   <-- default
 *           +2 3:22    -8.9%
 *
 * This issue occurs due to the way the scheduler affinity heuristics work.
 * There is no way to really 'fix' the affinity heuristics because when it
 * comes right down to it trying to instantly schedule a process on an
 * available cpu (even if it will become unavailable a microsecond later)
 * tends to cause processes to shift around between cpus and sockets too much
 * and breaks the affinity.
 *
 * NOTE: Heavily concurrent builds typically have enough things on the pan
 *       that they remain time-efficient even with a higher bias.
 */
static int usched_dfly_forkbias = 1;
SYSCTL_INT(_debug, OID_AUTO, dfly_forkbias, CTLFLAG_RW,
           &usched_dfly_forkbias, 0,
           "Fork bias for estcpu in whole queues");

/*
 * Tunning usched_dfly - configurable through kern.usched_dfly.
 *
 * weight1 - Tries to keep threads on their current cpu.  If you
 *           make this value too large the scheduler will not be
 *           able to load-balance large loads.
 *
 *           Generally set to a fairly low value, but high enough
 *           such that estcpu jitter doesn't move threads around.
 *
 * weight2 - If non-zero, detects thread pairs undergoing synchronous
 *           communications and tries to move them closer together.
 *           Behavior is adjusted by bit 4 of features (0x10).
 *
 *           WARNING!  Weight2 is a ridiculously sensitive parameter,
 *           change the default at your peril.
 *
 * weight3 - Weighting based on the number of recently runnable threads
 *           on the userland scheduling queue (ignoring their loads).
 *
 *           A nominal value here prevents high-priority (low-load)
 *           threads from accumulating on one cpu core when other
 *           cores are available.
 *
 *           This value should be left fairly small because low-load
 *           high priority threads can still be mostly idle and too
 *           high a value will kick cpu-bound processes off the cpu
 *           unnecessarily.
 *
 * weight4 - Weighting based on other cpu queues being available
 *           or running processes with higher lwp_priority's.
 *
 *           This allows a thread to migrate to another nearby cpu if it
 *           is unable to run on the current cpu based on the other cpu
 *           being idle or running a lower priority (higher lwp_priority)
 *           thread.  This value should be large enough to override weight1
 *
 * weight5 - Weighting based on the relative amount of ram connected
 *           to the node a cpu resides on.
 *
 *           This value should remain fairly low to allow assymetric
 *           NUMA nodes to get threads scheduled to them.  Setting a very
 *           high level will prevent scheduling on assymetric NUMA nodes
 *           with low amounts of directly-attached memory.
 *
 *           Note that when testing e.g. N threads on a machine with N
 *           cpu cores with assymtric NUMA nodes, a non-zero value will
 *           cause some cpu threads on the low-priority NUMA nodes to remain
 *           idle even when a few process threads are doubled-up on other
 *           cpus.  But this is typically more ideal because it deschedules
 *           low-priority NUMA nodes at lighter nodes.
 *
 *           Values between 50 and 200 are recommended.  Default is 50.
 *
 * weight6 - rdd transfer weight hysteresis.  Defaults to 0, can be increased
 *           to improve stabillity at the cost of more mis-schedules.
 *
 * features - These flags can be set or cleared to enable or disable various
 *            features.
 *
 *            0x01      Enable idle-cpu pulling                 (default)
 *            0x02      Enable proactive pushing                (default)
 *            0x04      Enable rebalancing rover                (default)
 *            0x08      Enable more proactive pushing           (default)
 *            0x10      (flip weight2 limit on same cpu)        (default)
 *            0x20      choose best cpu for forked process
 *            0x40      choose current cpu for forked process
 *            0x80      choose random cpu for forked process    (default)
 */
static int usched_dfly_smt = 0;
static int usched_dfly_cache_coherent = 0;
static int usched_dfly_weight1 = 10;    /* keep thread on current cpu */
static int usched_dfly_weight2 = 180;   /* synchronous peer's current cpu */
static int usched_dfly_weight3 = 10;    /* number of threads on queue */
static int usched_dfly_weight4 = 160;   /* availability of idle cores */
static int usched_dfly_weight5 = 50;    /* node attached memory */
static int usched_dfly_weight6 = 0;     /* rdd trasnfer weight */
static int usched_dfly_features = 0x8F; /* allow pulls */
static int usched_dfly_fast_resched = PPQ / 2; /* delta priority / resched */
static int usched_dfly_swmask = ~PPQMASK; /* allow pulls */
static int usched_dfly_rrinterval = (ESTCPUFREQ + 9) / 10;
static int usched_dfly_decay = 8;
static long usched_dfly_node_mem;

/* KTR debug printings */

KTR_INFO_MASTER(usched);

#if !defined(KTR_USCHED_DFLY)
#define KTR_USCHED_DFLY KTR_ALL
#endif

KTR_INFO(KTR_USCHED_DFLY, usched, chooseproc, 0,
    "USCHED_DFLY(chooseproc: pid %d, old_cpuid %d, curr_cpuid %d)",
    pid_t pid, int old_cpuid, int curr);

/*
 * This function is called when the kernel intends to return to userland.
 * It is responsible for making the thread the current designated userland
 * thread for this cpu, blocking if necessary.
 *
 * The kernel will not depress our LWKT priority until after we return,
 * in case we have to shove over to another cpu.
 *
 * We must determine our thread's disposition before we switch away.  This
 * is very sensitive code.
 *
 * WARNING! THIS FUNCTION IS ALLOWED TO CAUSE THE CURRENT THREAD TO MIGRATE
 * TO ANOTHER CPU!  Because most of the kernel assumes that no migration will
 * occur, this function is called only under very controlled circumstances.
 */
static void
dfly_acquire_curproc(struct lwp *lp)
{
        globaldata_t gd;
        dfly_pcpu_t dd;
        dfly_pcpu_t rdd;
        thread_t td;
        int force_resched;

        /*
         * Make sure we aren't sitting on a tsleep queue.
         */
        td = lp->lwp_thread;
        crit_enter_quick(td);
        if (td->td_flags & TDF_TSLEEPQ)
                tsleep_remove(td);
        dfly_recalculate_estcpu(lp);

        gd = mycpu;
        dd = &dfly_pcpu[gd->gd_cpuid];

        /*
         * Process any pending interrupts/ipi's, then handle reschedule
         * requests.  dfly_release_curproc() will try to assign a new
         * uschedcp that isn't us and otherwise NULL it out.
         */
        force_resched = 0;
        if ((td->td_mpflags & TDF_MP_BATCH_DEMARC) &&
            lp->lwp_rrcount >= usched_dfly_rrinterval / 2) {
                force_resched = 1;
        }

        if (user_resched_wanted()) {
                if (dd->uschedcp == lp)
                        force_resched = 1;
                clear_user_resched();
                dfly_release_curproc(lp);
        }

        /*
         * Loop until we are the current user thread.
         *
         * NOTE: dd spinlock not held at top of loop.
         */
        if (dd->uschedcp == lp)
                lwkt_yield_quick();

        while (dd->uschedcp != lp) {
                /*
                 * Do not do a lwkt_yield_quick() here as it will prevent
                 * the lwp from being placed on the dfly_bsd runqueue for
                 * one cycle (possibly an entire round-robin), preventing
                 * it from being scheduled to another cpu.
                 */
                /* lwkt_yield_quick(); */

                spin_lock(&dd->spin);

                /* This lwp is an outcast; force reschedule. */
                if (__predict_false(
                    CPUMASK_TESTBIT(lp->lwp_cpumask, gd->gd_cpuid) == 0) &&
                    (rdd = dfly_choose_best_queue(lp)) != dd) {
                        dfly_changeqcpu_locked(lp, dd, rdd);
                        spin_unlock(&dd->spin);
                        lwkt_deschedule(lp->lwp_thread);
                        dfly_setrunqueue_dd(rdd, lp);
                        lwkt_switch();
                        gd = mycpu;
                        dd = &dfly_pcpu[gd->gd_cpuid];
                        continue;
                }

                /*
                 * We are not or are no longer the current lwp and a forced
                 * reschedule was requested.  Figure out the best cpu to
                 * run on (our current cpu will be given significant weight).
                 *
                 * Doing this on many cpus simultaneously leads to
                 * instability so pace the operation.
                 *
                 * (if a reschedule was not requested we want to move this
                 * step after the uschedcp tests).
                 */
                if (force_resched &&
                   (usched_dfly_features & 0x08) &&
                   (u_int)sched_ticks / 8 % ncpus == gd->gd_cpuid &&
                   (rdd = dfly_choose_best_queue(lp)) != dd) {
                        dfly_changeqcpu_locked(lp, dd, rdd);
                        spin_unlock(&dd->spin);
                        lwkt_deschedule(lp->lwp_thread);
                        dfly_setrunqueue_dd(rdd, lp);
                        lwkt_switch();
                        gd = mycpu;
                        dd = &dfly_pcpu[gd->gd_cpuid];
                        continue;
                }

                /*
                 * Either no reschedule was requested or the best queue was
                 * dd, and no current process has been selected.  We can
                 * trivially become the current lwp on the current cpu.
                 */
                if (dd->uschedcp == NULL) {
                        atomic_clear_int(&lp->lwp_thread->td_mpflags,
                                         TDF_MP_DIDYIELD);
                        if ((dd->flags & DFLY_PCPU_CURMASK) == 0) {
                                ATOMIC_CPUMASK_ORBIT(dfly_curprocmask,
                                                     gd->gd_cpuid);
                                dd->flags |= DFLY_PCPU_CURMASK;
                        }
                        dd->uschedcp = lp;
                        dd->upri = lp->lwp_priority;
                        KKASSERT(lp->lwp_qcpu == dd->cpuid);
                        spin_unlock(&dd->spin);
                        break;
                }

                /*
                 * Can we steal the current designated user thread?
                 *
                 * If we do the other thread will stall when it tries to
                 * return to userland, possibly rescheduling elsewhere.
                 * Set need_user_resched() to get the thread to cycle soonest.
                 *
                 * It is important to do a masked test to avoid the edge
                 * case where two near-equal-priority threads are constantly
                 * interrupting each other.
                 *
                 * In the exact match case another thread has already gained
                 * uschedcp and lowered its priority, if we steal it the
                 * other thread will stay stuck on the LWKT runq and not
                 * push to another cpu.  So don't steal on equal-priority even
                 * though it might appear to be more beneficial due to not
                 * having to switch back to the other thread's context.
                 *
                 * usched_dfly_fast_resched requires that two threads be
                 * significantly far apart in priority in order to interrupt.
                 *
                 * If better but not sufficiently far apart, the current
                 * uschedcp will be interrupted at the next scheduler clock.
                 */
                if (dd->uschedcp &&
                   (dd->upri & ~PPQMASK) >
                   (lp->lwp_priority & ~PPQMASK) + usched_dfly_fast_resched) {
                        dd->uschedcp = lp;
                        dd->upri = lp->lwp_priority;
                        KKASSERT(lp->lwp_qcpu == dd->cpuid);
                        need_user_resched();
                        spin_unlock(&dd->spin);
                        break;
                }

                /*
                 * Requeue us at lwp_priority, which recalculate_estcpu()
                 * set for us.  Reset the rrcount to force placement
                 * at the end of the queue.
                 *
                 * We used to move ourselves to the worst queue, but
                 * this creates a fairly serious priority inversion
                 * problem.
                 */
                if (lp->lwp_thread->td_mpflags & TDF_MP_DIDYIELD) {
                        spin_unlock(&dd->spin);
                        lp->lwp_rrcount = usched_dfly_rrinterval;
                        lp->lwp_rqindex = (lp->lwp_priority & PRIMASK) / PPQ;

                        lwkt_deschedule(lp->lwp_thread);
                        dfly_setrunqueue_dd(dd, lp);
                        atomic_clear_int(&lp->lwp_thread->td_mpflags,
                                         TDF_MP_DIDYIELD);
                        lwkt_switch();
                        gd = mycpu;
                        dd = &dfly_pcpu[gd->gd_cpuid];
                        continue;
                }

                /*
                 * We are not the current lwp, figure out the best cpu
                 * to run on (our current cpu will be given significant
                 * weight).  Loop on cpu change.
                 */
                if ((usched_dfly_features & 0x02) &&
                    force_resched == 0 &&
                    (rdd = dfly_choose_best_queue(lp)) != dd) {
                        dfly_changeqcpu_locked(lp, dd, rdd);
                        spin_unlock(&dd->spin);
                        lwkt_deschedule(lp->lwp_thread);
                        dfly_setrunqueue_dd(rdd, lp);
                        lwkt_switch();
                        gd = mycpu;
                        dd = &dfly_pcpu[gd->gd_cpuid];
                        continue;
                }

                /*
                 * We cannot become the current lwp, place the lp on the
                 * run-queue of this or another cpu and deschedule ourselves.
                 *
                 * When we are reactivated we will have another chance.
                 *
                 * Reload after a switch or setrunqueue/switch possibly
                 * moved us to another cpu.
                 */
                spin_unlock(&dd->spin);
                lwkt_deschedule(lp->lwp_thread);
                dfly_setrunqueue_dd(dd, lp);
                lwkt_switch();
                gd = mycpu;
                dd = &dfly_pcpu[gd->gd_cpuid];
        }

        /*
         * Make sure upri is synchronized, then yield to LWKT threads as
         * needed before returning.  This could result in another reschedule.
         * XXX
         */
        crit_exit_quick(td);

        KKASSERT((lp->lwp_mpflags & LWP_MP_ONRUNQ) == 0);
}

/*
 * DFLY_RELEASE_CURPROC
 *
 * This routine detaches the current thread from the userland scheduler,
 * usually because the thread needs to run or block in the kernel (at
 * kernel priority) for a while.
 *
 * This routine is also responsible for selecting a new thread to
 * make the current thread.
 *
 * NOTE: This implementation differs from the dummy example in that
 * dfly_select_curproc() is able to select the current process, whereas
 * dummy_select_curproc() is not able to select the current process.
 * This means we have to NULL out uschedcp.
 *
 * Additionally, note that we may already be on a run queue if releasing
 * via the lwkt_switch() in dfly_setrunqueue().
 */
static void
dfly_release_curproc(struct lwp *lp)
{
        globaldata_t gd = mycpu;
        dfly_pcpu_t dd = &dfly_pcpu[gd->gd_cpuid];

        /*
         * Make sure td_wakefromcpu is defaulted.  This will be overwritten
         * by wakeup().
         */
        if (dd->uschedcp == lp) {
                KKASSERT((lp->lwp_mpflags & LWP_MP_ONRUNQ) == 0);
                spin_lock(&dd->spin);
                if (dd->uschedcp == lp) {
                        dd->uschedcp = NULL;    /* don't let lp be selected */
                        dd->upri = PRIBASE_NULL;

                        /*
                         * We're just going to set it again, avoid the global
                         * cache line ping-pong.
                         */
                        if ((lp->lwp_thread->td_mpflags & TDF_MP_DIDYIELD) == 0) {
                                if (dd->flags & DFLY_PCPU_CURMASK) {
                                        ATOMIC_CPUMASK_NANDBIT(dfly_curprocmask,
                                                               gd->gd_cpuid);
                                        dd->flags &= ~DFLY_PCPU_CURMASK;
                                }
                        }
                        spin_unlock(&dd->spin);
                        dfly_select_curproc(gd);
                } else {
                        spin_unlock(&dd->spin);
                }
        }
}

/*
 * DFLY_SELECT_CURPROC
 *
 * Select a new current process for this cpu and clear any pending user
 * reschedule request.  The cpu currently has no current process.
 *
 * This routine is also responsible for equal-priority round-robining,
 * typically triggered from dfly_schedulerclock().  In our dummy example
 * all the 'user' threads are LWKT scheduled all at once and we just
 * call lwkt_switch().
 *
 * The calling process is not on the queue and cannot be selected.
 */
static
void
dfly_select_curproc(globaldata_t gd)
{
        dfly_pcpu_t dd = &dfly_pcpu[gd->gd_cpuid];
        struct lwp *nlp;
        int cpuid = gd->gd_cpuid;

        crit_enter_gd(gd);

        spin_lock(&dd->spin);
        nlp = dfly_chooseproc_locked(dd, dd, dd->uschedcp, 0);

        if (nlp) {
                if ((dd->flags & DFLY_PCPU_CURMASK) == 0) {
                        ATOMIC_CPUMASK_ORBIT(dfly_curprocmask, cpuid);
                        dd->flags |= DFLY_PCPU_CURMASK;
                }
                dd->upri = nlp->lwp_priority;
                dd->uschedcp = nlp;
#if 0
                dd->rrcount = 0;                /* reset round robin */
#endif
                spin_unlock(&dd->spin);
                lwkt_acquire(nlp->lwp_thread);
                lwkt_schedule(nlp->lwp_thread);
        } else {
                spin_unlock(&dd->spin);
        }
        crit_exit_gd(gd);
}

/*
 * Place the specified lwp on the user scheduler's run queue.  This routine
 * must be called with the thread descheduled.  The lwp must be runnable.
 * It must not be possible for anyone else to explicitly schedule this thread.
 *
 * The thread may be the current thread as a special case.
 */
static void
dfly_setrunqueue(struct lwp *lp)
{
        dfly_pcpu_t dd;
        dfly_pcpu_t rdd;

        /*
         * First validate the process LWKT state.
         */
        KASSERT(lp->lwp_stat == LSRUN, ("setrunqueue: lwp not LSRUN"));
        KASSERT((lp->lwp_mpflags & LWP_MP_ONRUNQ) == 0,
            ("lwp %d/%d already on runq! flag %08x/%08x", lp->lwp_proc->p_pid,
             lp->lwp_tid, lp->lwp_proc->p_flags, lp->lwp_flags));
        KKASSERT((lp->lwp_thread->td_flags & TDF_RUNQ) == 0);

        /*
         * NOTE: dd/rdd do not necessarily represent the current cpu.
         *       Instead they may represent the cpu the thread was last
         *       scheduled on or inherited by its parent.
         */
        dd = &dfly_pcpu[lp->lwp_qcpu];
        rdd = dd;

        /*
         * This process is not supposed to be scheduled anywhere or assigned
         * as the current process anywhere.  Assert the condition.
         */
        KKASSERT(rdd->uschedcp != lp);

        /*
         * Ok, we have to setrunqueue some target cpu and request a reschedule
         * if necessary.
         *
         * We have to choose the best target cpu.  It might not be the current
         * target even if the current cpu has no running user thread (for
         * example, because the current cpu might be a hyperthread and its
         * sibling has a thread assigned).
         *
         * If we just forked it is most optimal to run the child on the same
         * cpu just in case the parent decides to wait for it (thus getting
         * off that cpu).  As long as there is nothing else runnable on the
         * cpu, that is.  If we did this unconditionally a parent forking
         * multiple children before waiting (e.g. make -j N) leaves other
         * cpus idle that could be working.
         */
        if (lp->lwp_forked) {
                lp->lwp_forked = 0;
                if (usched_dfly_features & 0x20)
                        rdd = dfly_choose_best_queue(lp);
                else if (usched_dfly_features & 0x40)
                        rdd = &dfly_pcpu[lp->lwp_qcpu];
                else if (usched_dfly_features & 0x80)
                        rdd = dfly_choose_queue_simple(rdd, lp);
                else if (dfly_pcpu[lp->lwp_qcpu].runqcount)
                        rdd = dfly_choose_best_queue(lp);
                else
                        rdd = &dfly_pcpu[lp->lwp_qcpu];
        } else {
                rdd = dfly_choose_best_queue(lp);
                /* rdd = &dfly_pcpu[lp->lwp_qcpu]; */
        }
        if (lp->lwp_qcpu != rdd->cpuid) {
                spin_lock(&dd->spin);
                dfly_changeqcpu_locked(lp, dd, rdd);
                spin_unlock(&dd->spin);
        }
        dfly_setrunqueue_dd(rdd, lp);
}

/*
 * Change qcpu to rdd->cpuid.  The dd the lp is CURRENTLY on must be
 * spin-locked on-call.  rdd does not have to be.
 */
static void
dfly_changeqcpu_locked(struct lwp *lp, dfly_pcpu_t dd, dfly_pcpu_t rdd)
{
        if (lp->lwp_qcpu != rdd->cpuid) {
                if (lp->lwp_mpflags & LWP_MP_ULOAD) {
                        atomic_clear_int(&lp->lwp_mpflags, LWP_MP_ULOAD);
                        atomic_add_long(&dd->uload, -lp->lwp_uload);
                        atomic_add_int(&dd->ucount, -1);
                }
                lp->lwp_qcpu = rdd->cpuid;
        }
}

/*
 * Place lp on rdd's runqueue.  Nothing is locked on call.  This function
 * also performs all necessary ancillary notification actions.
 */
static void
dfly_setrunqueue_dd(dfly_pcpu_t rdd, struct lwp *lp)
{
        globaldata_t rgd;

        /*
         * We might be moving the lp to another cpu's run queue, and once
         * on the runqueue (even if it is our cpu's), another cpu can rip
         * it away from us.
         *
         * TDF_MIGRATING might already be set if this is part of a
         * remrunqueue+setrunqueue sequence.
         */
        if ((lp->lwp_thread->td_flags & TDF_MIGRATING) == 0)
                lwkt_giveaway(lp->lwp_thread);

        rgd = rdd->gd;

        /*
         * We lose control of the lp the moment we release the spinlock
         * after having placed it on the queue.  i.e. another cpu could pick
         * it up, or it could exit, or its priority could be further
         * adjusted, or something like that.
         *
         * WARNING! rdd can point to a foreign cpu!
         */
        spin_lock(&rdd->spin);
        dfly_setrunqueue_locked(rdd, lp);

        /*
         * Potentially interrupt the currently-running thread
         */
        if ((rdd->upri & ~PPQMASK) <= (lp->lwp_priority & ~PPQMASK)) {
                /*
                 * Currently running thread is better or same, do not
                 * interrupt.
                 */
                spin_unlock(&rdd->spin);
        } else if ((rdd->upri & ~PPQMASK) <= (lp->lwp_priority & ~PPQMASK) +
                   usched_dfly_fast_resched) {
                /*
                 * Currently running thread is not better, but not so bad
                 * that we need to interrupt it.  Let it run for one more
                 * scheduler tick.
                 */
                if (rdd->uschedcp &&
                    rdd->uschedcp->lwp_rrcount < usched_dfly_rrinterval) {
                        rdd->uschedcp->lwp_rrcount = usched_dfly_rrinterval - 1;
                }
                spin_unlock(&rdd->spin);
        } else if (rgd == mycpu) {
                /*
                 * We should interrupt the currently running thread, which
                 * is on the current cpu.  However, if DIDYIELD is set we
                 * round-robin unconditionally and do not interrupt it.
                 */
                spin_unlock(&rdd->spin);
                if (rdd->uschedcp == NULL)
                        wakeup_mycpu(rdd->helper_thread); /* XXX */
                if ((lp->lwp_thread->td_mpflags & TDF_MP_DIDYIELD) == 0)
                        need_user_resched();
        } else {
                /*
                 * We should interrupt the currently running thread, which
                 * is on a different cpu.
                 */
                spin_unlock(&rdd->spin);
                lwkt_send_ipiq(rgd, dfly_need_user_resched_remote, NULL);
        }
}

/*
 * This routine is called from a systimer IPI.  It MUST be MP-safe and
 * the BGL IS NOT HELD ON ENTRY.  This routine is called at ESTCPUFREQ on
 * each cpu.
 */
static
void
dfly_schedulerclock(struct lwp *lp, sysclock_t period, sysclock_t cpstamp)
{
        globaldata_t gd = mycpu;
        dfly_pcpu_t dd = &dfly_pcpu[gd->gd_cpuid];

        /*
         * Spinlocks also hold a critical section so there should not be
         * any active.
         */
        KKASSERT(gd->gd_spinlocks == 0 || dumping);

        /*
         * If lp is NULL we might be contended and lwkt_switch() may have
         * cycled into the idle thread.  Apply the tick to the current
         * process on this cpu if it is contended.
         */
        if (gd->gd_curthread == &gd->gd_idlethread) {
                lp = dd->uschedcp;
                if (lp && (lp->lwp_thread == NULL ||
                           lp->lwp_thread->td_contended == 0)) {
                        lp = NULL;
                }
        }

        /*
         * Dock thread for tick
         */
        if (lp) {
                /*
                 * Do we need to round-robin?  We round-robin 10 times a
                 * second.  This should only occur for cpu-bound batch
                 * processes.
                 */
                if (++lp->lwp_rrcount >= usched_dfly_rrinterval) {
                        lp->lwp_thread->td_wakefromcpu = -1;
                        need_user_resched();
                }

                /*
                 * Adjust estcpu upward using a real time equivalent
                 * calculation, and recalculate lp's priority.  Estcpu
                 * is increased such that it will cap-out over a period
                 * of one second.
                 */
                lp->lwp_estcpu = ESTCPULIM(lp->lwp_estcpu +
                                           ESTCPUMAX / ESTCPUFREQ + 1);
                dfly_resetpriority(lp);
        }

        /*
         * Rebalance two cpus every 8 ticks, pulling the worst thread
         * from the worst cpu's queue into a rotating cpu number.
         * Also require that the moving of the highest-load thread
         * from rdd to dd does not cause the uload to cross over.
         *
         * This mechanic is needed because the push algorithms can
         * steady-state in an non-optimal configuration.  We need to mix it
         * up a little, even if it means breaking up a paired thread, so
         * the push algorithms can rebalance the degenerate conditions.
         * This portion of the algorithm exists to ensure stability at the
         * selected weightings.
         *
         * Because we might be breaking up optimal conditions we do not want
         * to execute this too quickly, hence we only rebalance approximately
         * ~7-8 times per second.  The push's, on the otherhand, are capable
         * moving threads to other cpus at a much higher rate.
         *
         * We choose the most heavily loaded thread from the worst queue
         * in order to ensure that multiple heavy-weight threads on the same
         * queue get broken up, and also because these threads are the most
         * likely to be able to remain in place.  Hopefully then any pairings,
         * if applicable, migrate to where these threads are.
         */
        if ((usched_dfly_features & 0x04) &&
            ((u_int)sched_ticks & 7) == 0 &&
            (u_int)sched_ticks / 8 % ncpus == gd->gd_cpuid) {
                /*
                 * Our cpu is up.
                 */
                struct lwp *nlp;
                dfly_pcpu_t rdd;

                rdd = dfly_choose_worst_queue(dd, 1);
                if (rdd && dd->uload + usched_dfly_weight6 / 2 < rdd->uload) {
                        spin_lock(&dd->spin);
                        if (spin_trylock(&rdd->spin)) {
                                nlp = dfly_chooseproc_locked(rdd, dd, NULL, 1);
                                spin_unlock(&rdd->spin);
                                if (nlp == NULL)
                                        spin_unlock(&dd->spin);
                        } else {
                                spin_unlock(&dd->spin);
                                nlp = NULL;
                        }
                } else {
                        nlp = NULL;
                }
                /* dd->spin held if nlp != NULL */

                /*
                 * Either schedule it or add it to our queue.
                 */
                if (nlp &&
                    (nlp->lwp_priority & ~PPQMASK) < (dd->upri & ~PPQMASK)) {
                        if ((dd->flags & DFLY_PCPU_CURMASK) == 0) {
                                ATOMIC_CPUMASK_ORMASK(dfly_curprocmask,
                                                      dd->cpumask);
                                dd->flags |= DFLY_PCPU_CURMASK;
                        }
                        dd->upri = nlp->lwp_priority;
                        dd->uschedcp = nlp;
#if 0
                        dd->rrcount = 0;        /* reset round robin */
#endif
                        spin_unlock(&dd->spin);
                        lwkt_acquire(nlp->lwp_thread);
                        lwkt_schedule(nlp->lwp_thread);
                } else if (nlp) {
                        dfly_setrunqueue_locked(dd, nlp);
                        spin_unlock(&dd->spin);
                }
        }
}

/*
 * Called from acquire and from kern_synch's one-second timer (one of the
 * callout helper threads) with a critical section held.
 *
 * Adjust p_estcpu based on our single-cpu load, p_nice, and compensate for
 * overall system load.
 *
 * Note that no recalculation occurs for a process which sleeps and wakes
 * up in the same tick.  That is, a system doing thousands of context
 * switches per second will still only do serious estcpu calculations
 * ESTCPUFREQ times per second.
 */
static
void
dfly_recalculate_estcpu(struct lwp *lp)
{
        globaldata_t gd = mycpu;
        sysclock_t cpbase;
        sysclock_t ttlticks;
        int estcpu;
        int decay_factor;
        int ucount;

        /*
         * We have to subtract periodic to get the last schedclock
         * timeout time, otherwise we would get the upcoming timeout.
         * Keep in mind that a process can migrate between cpus and
         * while the scheduler clock should be very close, boundary
         * conditions could lead to a small negative delta.
         */
        cpbase = gd->gd_schedclock.time - gd->gd_schedclock.periodic;

        if (lp->lwp_slptime > 1) {
                /*
                 * Too much time has passed, do a coarse correction.
                 */
                lp->lwp_estcpu = lp->lwp_estcpu >> 1;
                dfly_resetpriority(lp);
                lp->lwp_cpbase = cpbase;
                lp->lwp_cpticks = 0;
                lp->lwp_estfast = 0;
        } else if (lp->lwp_cpbase != cpbase) {
                /*
                 * Adjust estcpu if we are in a different tick.  Don't waste
                 * time if we are in the same tick.
                 *
                 * First calculate the number of ticks in the measurement
                 * interval.  The ttlticks calculation can wind up 0 due to
                 * a bug in the handling of lwp_slptime  (as yet not found),
                 * so make sure we do not get a divide by 0 panic.
                 */
                ttlticks = (cpbase - lp->lwp_cpbase) /
                           gd->gd_schedclock.periodic;
                if ((ssysclock_t)ttlticks < 0) {
                        ttlticks = 0;
                        lp->lwp_cpbase = cpbase;
                }
                if (ttlticks < 4)
                        return;
                updatepcpu(lp, lp->lwp_cpticks, ttlticks);

                /*
                 * Calculate instant estcpu based percentage of (one) cpu
                 * used and exponentially average it into the current
                 * lwp_estcpu.
                 */
                ucount = dfly_pcpu[lp->lwp_qcpu].ucount;
                estcpu = lp->lwp_cpticks * ESTCPUMAX / ttlticks;

                /*
                 * The higher ttlticks gets, the more meaning the calculation
                 * has and the smaller our decay_factor in the exponential
                 * average.
                 *
                 * The uload calculation has been removed because it actually
                 * makes things worse, causing processes which use less cpu
                 * (such as a browser) to be pumped up and treated the same
                 * as a cpu-bound process (such as a make).  The same effect
                 * can occur with sufficient load without the uload
                 * calculation, but occurs less quickly and takes more load.
                 * In addition, the less cpu a process uses the smaller the
                 * effect of the overload.
                 */
                if (ttlticks >= hz)
                        decay_factor = 1;
                else
                        decay_factor = hz - ttlticks;

                lp->lwp_estcpu = ESTCPULIM(
                                (lp->lwp_estcpu * ttlticks + estcpu) /
                                (ttlticks + 1));
                if (usched_dfly_debug == lp->lwp_proc->p_pid)
                        kprintf(" finalestcpu %d %d\n", estcpu, lp->lwp_estcpu);

                dfly_resetpriority(lp);
                lp->lwp_cpbase += ttlticks * gd->gd_schedclock.periodic;
                lp->lwp_cpticks = 0;
        }
}

/*
 * Compute the priority of a process when running in user mode.
 * Arrange to reschedule if the resulting priority is better
 * than that of the current process.
 *
 * This routine may be called with any process.
 *
 * This routine is called by fork1() for initial setup with the process of
 * the run queue, and also may be called normally with the process on or
 * off the run queue.
 */
static void
dfly_resetpriority(struct lwp *lp)
{
        dfly_pcpu_t rdd;
        int newpriority;
        u_short newrqtype;
        int rcpu;
        int checkpri;
        int estcpu;
        int delta_uload;

        crit_enter();

        /*
         * Lock the scheduler (lp) belongs to.  This can be on a different
         * cpu.  Handle races.  This loop breaks out with the appropriate
         * rdd locked.
         */
        for (;;) {
                rcpu = lp->lwp_qcpu;
                cpu_ccfence();
                rdd = &dfly_pcpu[rcpu];
                spin_lock(&rdd->spin);
                if (rcpu == lp->lwp_qcpu)
                        break;
                spin_unlock(&rdd->spin);
        }

        /*
         * Calculate the new priority and queue type
         */
        newrqtype = lp->lwp_rtprio.type;

        switch(newrqtype) {
        case RTP_PRIO_REALTIME:
        case RTP_PRIO_FIFO:
                newpriority = PRIBASE_REALTIME +
                             (lp->lwp_rtprio.prio & PRIMASK);
                break;
        case RTP_PRIO_NORMAL:
                /*
                 * Calculate the new priority.
                 *
                 * nice contributes up to NICE_QS queues (typ 32 - full range)
                 * estcpu contributes up to EST_QS queues (typ 24)
                 *
                 * A nice +20 process receives 1/10 cpu vs nice+0.  Niced
                 * process more than 20 apart may receive no cpu, so cpu
                 * bound nice -20 can prevent a nice +5 from getting any
                 * cpu.  A nice+0, being in the middle, always gets some cpu
                 * no matter what.
                 */
                estcpu = lp->lwp_estcpu;
                newpriority = (lp->lwp_proc->p_nice - PRIO_MIN) *
                              (NICE_QS * PPQ) / PRIO_RANGE;
                newpriority += estcpu * PPQ / ESTCPUPPQ;
                if (newpriority < 0)
                        newpriority = 0;
                if (newpriority >= MAXPRI)
                        newpriority = MAXPRI - 1;
                newpriority += PRIBASE_NORMAL;
                break;
        case RTP_PRIO_IDLE:
                newpriority = PRIBASE_IDLE + (lp->lwp_rtprio.prio & PRIMASK);
                break;
        case RTP_PRIO_THREAD:
                newpriority = PRIBASE_THREAD + (lp->lwp_rtprio.prio & PRIMASK);
                break;
        default:
                panic("Bad RTP_PRIO %d", newrqtype);
                /* NOT REACHED */
        }

        /*
         * The LWKT scheduler doesn't dive usched structures, give it a hint
         * on the relative priority of user threads running in the kernel.
         * The LWKT scheduler will always ensure that a user thread running
         * in the kernel will get cpu some time, regardless of its upri,
         * but can decide not to instantly switch from one kernel or user
         * mode user thread to a kernel-mode user thread when it has a less
         * desireable user priority.
         *
         * td_upri has normal sense (higher values are more desireable), so
         * negate it (this is a different field lp->lwp_priority)
         */
        lp->lwp_thread->td_upri = -(newpriority & usched_dfly_swmask);

        /*
         * The newpriority incorporates the queue type so do a simple masked
         * check to determine if the process has moved to another queue.  If
         * it has, and it is currently on a run queue, then move it.
         *
         * Since uload is ~PPQMASK masked, no modifications are necessary if
         * we end up in the same run queue.
         *
         * Reset rrcount if moving to a higher-priority queue, otherwise
         * retain rrcount.
         */
        if ((lp->lwp_priority ^ newpriority) & ~PPQMASK) {
                if (lp->lwp_priority < newpriority)
                        lp->lwp_rrcount = 0;
                if (lp->lwp_mpflags & LWP_MP_ONRUNQ) {
                        dfly_remrunqueue_locked(rdd, lp);
                        lp->lwp_priority = newpriority;
                        lp->lwp_rqtype = newrqtype;
                        lp->lwp_rqindex = (newpriority & PRIMASK) / PPQ;
                        dfly_setrunqueue_locked(rdd, lp);
                        checkpri = 1;
                } else {
                        lp->lwp_priority = newpriority;
                        lp->lwp_rqtype = newrqtype;
                        lp->lwp_rqindex = (newpriority & PRIMASK) / PPQ;
                        checkpri = 0;
                }
        } else {
                /*
                 * In the same PPQ, uload cannot change.
                 */
                lp->lwp_priority = newpriority;
                checkpri = 1;
                rcpu = -1;
        }

        /*
         * Adjust effective load.
         *
         * Calculate load then scale up or down geometrically based on p_nice.
         * Processes niced up (positive) are less important, and processes
         * niced downard (negative) are more important.  The higher the uload,
         * the more important the thread.
         */
        /* 0-511, 0-100% cpu */
        delta_uload = lptouload(lp);
        delta_uload -= lp->lwp_uload;
        if (lp->lwp_uload + delta_uload < -32767) {
                delta_uload = -32768 - lp->lwp_uload;
        } else if (lp->lwp_uload + delta_uload > 32767) {
                delta_uload = 32767 - lp->lwp_uload;
        }
        lp->lwp_uload += delta_uload;
        if (lp->lwp_mpflags & LWP_MP_ULOAD)
                atomic_add_long(&dfly_pcpu[lp->lwp_qcpu].uload, delta_uload);

        /*
         * Determine if we need to reschedule the target cpu.  This only
         * occurs if the LWP is already on a scheduler queue, which means
         * that idle cpu notification has already occured.  At most we
         * need only issue a need_user_resched() on the appropriate cpu.
         *
         * The LWP may be owned by a CPU different from the current one,
         * in which case dd->uschedcp may be modified without an MP lock
         * or a spinlock held.  The worst that happens is that the code
         * below causes a spurious need_user_resched() on the target CPU
         * and dd->pri to be wrong for a short period of time, both of
         * which are harmless.
         *
         * If checkpri is 0 we are adjusting the priority of the current
         * process, possibly higher (less desireable), so ignore the upri
         * check which will fail in that case.
         */
        if (rcpu >= 0) {
                if (CPUMASK_TESTBIT(dfly_rdyprocmask, rcpu) &&
                    (checkpri == 0 ||
                     (rdd->upri & ~PRIMASK) >
                     (lp->lwp_priority & ~PRIMASK))) {
                        if (rcpu == mycpu->gd_cpuid) {
                                spin_unlock(&rdd->spin);
                                need_user_resched();
                        } else {
                                spin_unlock(&rdd->spin);
                                lwkt_send_ipiq(globaldata_find(rcpu),
                                               dfly_need_user_resched_remote,
                                               NULL);
                        }
                } else {
                        spin_unlock(&rdd->spin);
                }
        } else {
                spin_unlock(&rdd->spin);
        }
        crit_exit();
}

static
void
dfly_yield(struct lwp *lp)
{
        if (lp->lwp_qcpu != mycpu->gd_cpuid)
                return;
        KKASSERT(lp == curthread->td_lwp);

        /*
         * Don't set need_user_resched() or mess with rrcount or anything.
         * the TDF flag will override everything as long as we release.
         */
        atomic_set_int(&lp->lwp_thread->td_mpflags, TDF_MP_DIDYIELD);
        dfly_release_curproc(lp);
}

/*
 * Thread was forcefully migrated to another cpu.  Normally forced migrations
 * are used for iterations and the kernel returns to the original cpu before
 * returning and this is not needed.  However, if the kernel migrates a
 * thread to another cpu and wants to leave it there, it has to call this
 * scheduler helper.
 *
 * Note that the lwkt_migratecpu() function also released the thread, so
 * we don't have to worry about that.
 */
static
void
dfly_changedcpu(struct lwp *lp)
{
        dfly_pcpu_t dd = &dfly_pcpu[lp->lwp_qcpu];
        dfly_pcpu_t rdd = &dfly_pcpu[mycpu->gd_cpuid];

        if (dd != rdd) {
                spin_lock(&dd->spin);
                dfly_changeqcpu_locked(lp, dd, rdd);
                spin_unlock(&dd->spin);
        }
}

/*
 * Called from fork1() when a new child process is being created.
 *
 * Give the child process an initial estcpu that is more batch then
 * its parent and dock the parent for the fork (but do not
 * reschedule the parent).
 *
 * fast
 *
 * XXX lwp should be "spawning" instead of "forking"
 */
static void
dfly_forking(struct lwp *plp, struct lwp *lp)
{
        int estcpu;

        /*
         * Put the child 4 queue slots (out of 32) higher than the parent
         * (less desireable than the parent).
         */
        lp->lwp_estcpu = ESTCPULIM(plp->lwp_estcpu +
                                   ESTCPUPPQ * usched_dfly_forkbias);
        lp->lwp_forked = 1;
        lp->lwp_estfast = 0;

        /*
         * Even though the lp will be scheduled specially the first time
         * due to lp->lwp_forked, it is important to initialize lwp_qcpu
         * to avoid favoring a fixed cpu.
         */
#if 0
        static uint16_t save_cpu;
        lp->lwp_qcpu = ++save_cpu % ncpus;
#else
        lp->lwp_qcpu = plp->lwp_qcpu;
        if (CPUMASK_TESTBIT(lp->lwp_cpumask, lp->lwp_qcpu) == 0)
                lp->lwp_qcpu = BSFCPUMASK(lp->lwp_cpumask);
#endif

        /*
         * Dock the parent a cost for the fork, protecting us from fork
         * bombs.  If the parent is forking quickly this makes both the
         * parent and child more batchy.
         */
        estcpu = plp->lwp_estcpu + ESTCPUPPQ / 16;
        plp->lwp_estcpu = ESTCPULIM(estcpu);
}

/*
 * Called when a lwp is being removed from this scheduler, typically
 * during lwp_exit().  We have to clean out any ULOAD accounting before
 * we can let the lp go.  The dd->spin lock is not needed for uload
 * updates.
 *
 * Scheduler dequeueing has already occurred, no further action in that
 * regard is needed.
 */
static void
dfly_exiting(struct lwp *lp, struct proc *child_proc)
{
        dfly_pcpu_t dd = &dfly_pcpu[lp->lwp_qcpu];

        if (lp->lwp_mpflags & LWP_MP_ULOAD) {
                atomic_clear_int(&lp->lwp_mpflags, LWP_MP_ULOAD);
                atomic_add_long(&dd->uload, -lp->lwp_uload);
                atomic_add_int(&dd->ucount, -1);
        }
}

/*
 * This function cannot block in any way, but spinlocks are ok.
 *
 * Update the uload based on the state of the thread (whether it is going
 * to sleep or running again).  The uload is meant to be a longer-term
 * load and not an instantanious load.
 */
static void
dfly_uload_update(struct lwp *lp)
{
        dfly_pcpu_t dd = &dfly_pcpu[lp->lwp_qcpu];

        if (lp->lwp_thread->td_flags & TDF_RUNQ) {
                if ((lp->lwp_mpflags & LWP_MP_ULOAD) == 0) {
                        spin_lock(&dd->spin);
                        if ((lp->lwp_mpflags & LWP_MP_ULOAD) == 0) {
                                atomic_set_int(&lp->lwp_mpflags,
                                               LWP_MP_ULOAD);
                                atomic_add_long(&dd->uload, lp->lwp_uload);
                                atomic_add_int(&dd->ucount, 1);
                        }
                        spin_unlock(&dd->spin);
                }
        } else if (lp->lwp_slptime > 0) {
                if (lp->lwp_mpflags & LWP_MP_ULOAD) {
                        spin_lock(&dd->spin);
                        if (lp->lwp_mpflags & LWP_MP_ULOAD) {
                                atomic_clear_int(&lp->lwp_mpflags,
                                                 LWP_MP_ULOAD);
                                atomic_add_long(&dd->uload, -lp->lwp_uload);
                                atomic_add_int(&dd->ucount, -1);
                        }
                        spin_unlock(&dd->spin);
                }
        }
}

/*
 * chooseproc() is called when a cpu needs a user process to LWKT schedule,
 * it selects a user process and returns it.  If chklp is non-NULL and chklp
 * has a better or equal priority then the process that would otherwise be
 * chosen, NULL is returned.
 *
 * Until we fix the RUNQ code the chklp test has to be strict or we may
 * bounce between processes trying to acquire the current process designation.
 *
 * Must be called with rdd->spin locked.  The spinlock is left intact through
 * the entire routine.  dd->spin does not have to be locked.
 *
 * If worst is non-zero this function finds the worst thread instead of the
 * best thread (used by the schedulerclock-based rover).
 */
static
struct lwp *
dfly_chooseproc_locked(dfly_pcpu_t rdd, dfly_pcpu_t dd,
                       struct lwp *chklp, int worst)
{
        struct lwp *lp;
        struct rq *q;
        u_int32_t *which;
        u_int32_t pri;
        u_int32_t rtqbits;
        u_int32_t tsqbits;
        u_int32_t idqbits;

        /*
         * Select best or worst process.  Once selected, clear the bit
         * in our local variable (idqbits, tsqbits, or rtqbits) just
         * in case we have to loop.
         */
        rtqbits = rdd->rtqueuebits;
        tsqbits = rdd->queuebits;
        idqbits = rdd->idqueuebits;

loopfar:
        if (worst) {
                if (idqbits) {
                        pri = bsrl(idqbits);
                        idqbits &= ~(1U << pri);
                        q = &rdd->idqueues[pri];
                        which = &rdd->idqueuebits;
                } else if (tsqbits) {
                        pri = bsrl(tsqbits);
                        tsqbits &= ~(1U << pri);
                        q = &rdd->queues[pri];
                        which = &rdd->queuebits;
                } else if (rtqbits) {
                        pri = bsrl(rtqbits);
                        rtqbits &= ~(1U << pri);
                        q = &rdd->rtqueues[pri];
                        which = &rdd->rtqueuebits;
                } else {
                        return (NULL);
                }
                lp = TAILQ_LAST(q, rq);
        } else {
                if (rtqbits) {
                        pri = bsfl(rtqbits);
                        rtqbits &= ~(1U << pri);
                        q = &rdd->rtqueues[pri];
                        which = &rdd->rtqueuebits;
                } else if (tsqbits) {
                        pri = bsfl(tsqbits);
                        tsqbits &= ~(1U << pri);
                        q = &rdd->queues[pri];
                        which = &rdd->queuebits;
                } else if (idqbits) {
                        pri = bsfl(idqbits);
                        idqbits &= ~(1U << pri);
                        q = &rdd->idqueues[pri];
                        which = &rdd->idqueuebits;
                } else {
                        return (NULL);
                }
                lp = TAILQ_FIRST(q);
        }
        KASSERT(lp, ("chooseproc: no lwp on busy queue"));

loopnear:
        /*
         * If the passed lwp <chklp> is reasonably close to the selected
         * lwp <lp>, return NULL (indicating that <chklp> should be kept).
         *
         * Note that we must error on the side of <chklp> to avoid bouncing
         * between threads in the acquire code.
         */
        if (chklp) {
                if (chklp->lwp_priority < lp->lwp_priority + PPQ)
                        return(NULL);
        }

        /*
         * When rdd != dd, we have to make sure that the process we
         * are pulling is allow to run on our cpu.  This alternative
         * path is a bit more expensive but its not considered to be
         * in the critical path.
         */
        if (rdd != dd && CPUMASK_TESTBIT(lp->lwp_cpumask, dd->cpuid) == 0) {
                if (worst)
                        lp = TAILQ_PREV(lp, rq, lwp_procq);
                else
                        lp = TAILQ_NEXT(lp, lwp_procq);
                if (lp)
                        goto loopnear;
                goto loopfar;
        }

        KTR_COND_LOG(usched_chooseproc,
            lp->lwp_proc->p_pid == usched_dfly_pid_debug,
            lp->lwp_proc->p_pid,
            lp->lwp_thread->td_gd->gd_cpuid,
            mycpu->gd_cpuid);

        KASSERT((lp->lwp_mpflags & LWP_MP_ONRUNQ) != 0, ("not on runq6!"));
        atomic_clear_int(&lp->lwp_mpflags, LWP_MP_ONRUNQ);
        TAILQ_REMOVE(q, lp, lwp_procq);
        --rdd->runqcount;
        if (TAILQ_EMPTY(q))
                *which &= ~(1 << pri);

        /*
         * If we are choosing a process from rdd with the intent to
         * move it to dd, lwp_qcpu must be adjusted while rdd's spinlock
         * is still held.
         */
        if (rdd != dd) {
                if (lp->lwp_mpflags & LWP_MP_ULOAD) {
                        atomic_add_long(&rdd->uload, -lp->lwp_uload);
                        atomic_add_int(&rdd->ucount, -1);
                }
                lp->lwp_qcpu = dd->cpuid;
                atomic_add_long(&dd->uload, lp->lwp_uload);
                atomic_add_int(&dd->ucount, 1);
                atomic_set_int(&lp->lwp_mpflags, LWP_MP_ULOAD);
        }
        return lp;
}

/*
 * USED TO PUSH RUNNABLE LWPS TO THE LEAST LOADED CPU.
 *
 * Choose a cpu node to schedule lp on, hopefully nearby its current
 * node.
 *
 * We give the current node a modest advantage for obvious reasons.
 *
 * We also give the node the thread was woken up FROM a slight advantage
 * in order to try to schedule paired threads which synchronize/block waiting
 * for each other fairly close to each other.  Similarly in a network setting
 * this feature will also attempt to place a user process near the kernel
 * protocol thread that is feeding it data.  THIS IS A CRITICAL PART of the
 * algorithm as it heuristically groups synchronizing processes for locality
 * of reference in multi-socket systems.
 *
 * We check against running processes and give a big advantage if there
 * are none running.
 *
 * The caller will normally dfly_setrunqueue() lp on the returned queue.
 *
 * When the topology is known choose a cpu whos group has, in aggregate,
 * has the lowest weighted load.
 */
static
dfly_pcpu_t
dfly_choose_best_queue(struct lwp *lp)
{
        cpumask_t wakemask;
        cpumask_t mask;
        cpu_node_t *cpup;
        cpu_node_t *cpun;
        cpu_node_t *cpub;
        dfly_pcpu_t dd = &dfly_pcpu[lp->lwp_qcpu];
        dfly_pcpu_t rdd;
        int wakecpu;
        int cpuid;
        int n;
        long load;
        long lowest_load;

        /*
         * When the topology is unknown choose a random cpu that is hopefully
         * idle.
         */
        if (dd->cpunode == NULL)
                return (dfly_choose_queue_simple(dd, lp));

        /*
         * Pairing mask
         */
        if ((wakecpu = lp->lwp_thread->td_wakefromcpu) >= 0)
                wakemask = dfly_pcpu[wakecpu].cpumask;
        else
                CPUMASK_ASSZERO(wakemask);

        /*
         * When the topology is known choose a cpu whos group has, in
         * aggregate, has the lowest weighted load.
         */
        cpup = root_cpu_node;
        rdd = dd;

        while (cpup) {
                /*
                 * Degenerate case super-root
                 */
                if (cpup->child_no == 1) {
                        cpup = cpup->child_node[0];
                        continue;
                }

                /*
                 * Terminal cpunode
                 */
                if (cpup->child_no == 0) {
                        rdd = &dfly_pcpu[BSFCPUMASK(cpup->members)];
                        break;
                }

                cpub = NULL;
                lowest_load = 0x7FFFFFFFFFFFFFFFLL;

                for (n = 0; n < cpup->child_no; ++n) {
                        /*
                         * Accumulate load information for all cpus
                         * which are members of this node.
                         */
                        int count;

                        cpun = cpup->child_node[n];
                        mask = cpun->members;
                        CPUMASK_ANDMASK(mask, usched_global_cpumask);
                        CPUMASK_ANDMASK(mask, smp_active_mask);
                        CPUMASK_ANDMASK(mask, lp->lwp_cpumask);
                        if (CPUMASK_TESTZERO(mask))
                                continue;

                        load = 0;
                        count = 0;

                        while (CPUMASK_TESTNZERO(mask)) {
                                cpuid = BSFCPUMASK(mask);
                                rdd = &dfly_pcpu[cpuid];

                                if (rdd->uschedcp == NULL &&
                                    rdd->runqcount == 0 &&
                                    rdd->gd->gd_tdrunqcount == 0
                                ) {
                                        load += rdd->uload / 2;
                                        load += rdd->ucount *
                                                usched_dfly_weight3 / 2;
                                } else {
                                        load += rdd->uload;
                                        load += rdd->ucount *
                                                usched_dfly_weight3;
                                }
                                CPUMASK_NANDBIT(mask, cpuid);
                                ++count;
                        }

                        /*
                         * Compensate if the lp is already accounted for in
                         * the aggregate uload for this mask set.  We want
                         * to calculate the loads as if lp were not present,
                         * otherwise the calculation is bogus.
                         */
                        if ((lp->lwp_mpflags & LWP_MP_ULOAD) &&
                            CPUMASK_TESTMASK(dd->cpumask, cpun->members)) {
                                load -= lp->lwp_uload;
                                load -= usched_dfly_weight3;    /* ucount */
                        }

                        load /= count;

                        /*
                         * Advantage the cpu group (lp) is already on.
                         */
                        if (CPUMASK_TESTMASK(cpun->members, dd->cpumask))
                                load -= usched_dfly_weight1;

                        /*
                         * Advantage nodes with more memory
                         */
                        if (usched_dfly_node_mem) {
                                load -= cpun->phys_mem * usched_dfly_weight5 /
                                        usched_dfly_node_mem;
                        }

                        /*
                         * Advantage the cpu group we want to pair (lp) to,
                         * but don't let it go to the exact same cpu as
                         * the wakecpu target.
                         *
                         * We do this by checking whether cpun is a
                         * terminal node or not.  All cpun's at the same
                         * level will either all be terminal or all not
                         * terminal.
                         *
                         * If it is and we match we disadvantage the load.
                         * If it is and we don't match we advantage the load.
                         *
                         * Also note that we are effectively disadvantaging
                         * all-but-one by the same amount, so it won't effect
                         * the weight1 factor for the all-but-one nodes.
                         */
                        if (CPUMASK_TESTMASK(cpun->members, wakemask)) {
                                if (cpun->child_no != 0) {
                                        /* advantage */
                                        load -= usched_dfly_weight2;
                                } else {
                                        if (usched_dfly_features & 0x10)
                                                load += usched_dfly_weight2;
                                        else
                                                load -= usched_dfly_weight2;
                                }
                        }

                        /*
                         * Calculate the best load
                         */
                        if (cpub == NULL || lowest_load > load ||
                            (lowest_load == load &&
                             CPUMASK_TESTMASK(cpun->members, dd->cpumask))
                        ) {
                                lowest_load = load;
                                cpub = cpun;
                        }
                }
                cpup = cpub;
        }
        /* Dispatch this outcast to a proper CPU. */
        if (__predict_false(CPUMASK_TESTBIT(lp->lwp_cpumask, rdd->cpuid) == 0))
                rdd = &dfly_pcpu[BSFCPUMASK(lp->lwp_cpumask)];
        if (usched_dfly_chooser > 0) {
                --usched_dfly_chooser;          /* only N lines */
                kprintf("lp %02d->%02d %s\n",
                        lp->lwp_qcpu, rdd->cpuid, lp->lwp_proc->p_comm);
        }
        return (rdd);
}

/*
 * USED TO PULL RUNNABLE LWPS FROM THE MOST LOADED CPU.
 *
 * Choose the worst queue close to dd's cpu node with a non-empty runq
 * that is NOT dd.
 *
 * This is used by the thread chooser when the current cpu's queues are
 * empty to steal a thread from another cpu's queue.  We want to offload
 * the most heavily-loaded queue.
 *
 * However, we do not want to steal from far-away nodes who themselves
 * have idle cpu's that are more suitable to distribute the far-away
 * thread to.
 */
static
dfly_pcpu_t
dfly_choose_worst_queue(dfly_pcpu_t dd, int forceit)
{
        cpumask_t mask;
        cpu_node_t *cpup;
        cpu_node_t *cpun;
        cpu_node_t *cpub;
        dfly_pcpu_t rdd;
        int cpuid;
        int n;
        long load;
        long highest_load;
#if 0
        int pri;
        int hpri;
#endif

        /*
         * When the topology is unknown choose a random cpu that is hopefully
         * idle.
         */
        if (dd->cpunode == NULL) {
                return (NULL);
        }

        /*
         * When the topology is known choose a cpu whos group has, in
         * aggregate, has the highest weighted load.
         */
        cpup = root_cpu_node;
        rdd = dd;
        while (cpup) {
                /*
                 * Degenerate case super-root
                 */
                if (cpup->child_no == 1) {
                        cpup = cpup->child_node[0];
                        continue;
                }

                /*
                 * Terminal cpunode
                 */
                if (cpup->child_no == 0) {
                        rdd = &dfly_pcpu[BSFCPUMASK(cpup->members)];
                        break;
                }

                cpub = NULL;
                highest_load = -0x7FFFFFFFFFFFFFFFLL;

                for (n = 0; n < cpup->child_no; ++n) {
                        /*
                         * Accumulate load information for all cpus
                         * which are members of this node.
                         */
                        int count;

                        cpun = cpup->child_node[n];
                        mask = cpun->members;
                        CPUMASK_ANDMASK(mask, usched_global_cpumask);
                        CPUMASK_ANDMASK(mask, smp_active_mask);
                        if (CPUMASK_TESTZERO(mask))
                                continue;

                        load = 0;
                        count = 0;

                        while (CPUMASK_TESTNZERO(mask)) {
                                cpuid = BSFCPUMASK(mask);
                                rdd = &dfly_pcpu[cpuid];

                                if (rdd->uschedcp == NULL &&
                                    rdd->runqcount == 0 &&
                                    rdd->gd->gd_tdrunqcount == 0
                                ) {
                                        load += rdd->uload / 2;
                                        load += rdd->ucount *
                                                usched_dfly_weight3 / 2;
                                } else {
                                        load += rdd->uload;
                                        load += rdd->ucount *
                                                usched_dfly_weight3;
                                }
                                CPUMASK_NANDBIT(mask, cpuid);
                                ++count;
                        }
                        load /= count;

                        /*
                         * Advantage the cpu group (dd) is already on.
                         *
                         * When choosing the worst queue we reverse the
                         * sign, but only count half the weight.
                         *
                         * weight1 needs to be high enough to be stable,
                         * but this can also cause it to be too sticky,
                         * so the iterator which rebalances the load sets
                         * forceit to ignore it.
                         */
                        if (forceit == 0 &&
                            CPUMASK_TESTMASK(dd->cpumask, cpun->members)) {
                                load += usched_dfly_weight1 / 2;
                        }

                        /*
                         * Disadvantage nodes with more memory (same sign).
                         */
                        if (usched_dfly_node_mem) {
                                load -= cpun->phys_mem * usched_dfly_weight5 /
                                        usched_dfly_node_mem;
                        }


                        /*
                         * The best candidate is the one with the worst
                         * (highest) load.
                         */
                        if (cpub == NULL || highest_load < load ||
                            (highest_load == load &&
                             CPUMASK_TESTMASK(cpun->members, dd->cpumask))) {
                                highest_load = load;
                                cpub = cpun;
                        }
                }
                cpup = cpub;
        }

        /*
         * We never return our own node (dd), and only return a remote
         * node if it's load is significantly worse than ours (i.e. where
         * stealing a thread would be considered reasonable).
         *
         * This also helps us avoid breaking paired threads apart which
         * can have disastrous effects on performance.
         */
        if (rdd == dd)
                return(NULL);

#if 0
        hpri = 0;
        if (rdd->rtqueuebits && hpri < (pri = bsrl(rdd->rtqueuebits)))
                hpri = pri;
        if (rdd->queuebits && hpri < (pri = bsrl(rdd->queuebits)))
                hpri = pri;
        if (rdd->idqueuebits && hpri < (pri = bsrl(rdd->idqueuebits)))
                hpri = pri;
        hpri *= PPQ;
        if (rdd->uload - hpri < dd->uload + hpri)
                return(NULL);
#endif
        return (rdd);
}

static
dfly_pcpu_t
dfly_choose_queue_simple(dfly_pcpu_t dd, struct lwp *lp)
{
        dfly_pcpu_t rdd;
        cpumask_t tmpmask;
        cpumask_t mask;
        int cpubase;
        int cpuid;

        /*
         * Fallback to the original heuristic, select random cpu,
         * first checking the cpus not currently running a user thread.
         *
         * Use cpuid as the base cpu in our scan, first checking
         * cpuid...(ncpus-1), then 0...(cpuid-1).  This avoid favoring
         * lower-numbered cpus.
         */
        ++dd->scancpu;          /* SMP race ok */
        mask = dfly_rdyprocmask;
        CPUMASK_NANDMASK(mask, dfly_curprocmask);
        CPUMASK_ANDMASK(mask, lp->lwp_cpumask);
        CPUMASK_ANDMASK(mask, smp_active_mask);
        CPUMASK_ANDMASK(mask, usched_global_cpumask);

        cpubase = (int)(dd->scancpu % ncpus);
        CPUMASK_ASSBMASK(tmpmask, cpubase);
        CPUMASK_INVMASK(tmpmask);
        CPUMASK_ANDMASK(tmpmask, mask);
        while (CPUMASK_TESTNZERO(tmpmask)) {
                cpuid = BSFCPUMASK(tmpmask);
                rdd = &dfly_pcpu[cpuid];

                if ((rdd->upri & ~PPQMASK) >= (lp->lwp_priority & ~PPQMASK))
                        goto found;
                CPUMASK_NANDBIT(tmpmask, cpuid);
        }

        CPUMASK_ASSBMASK(tmpmask, cpubase);
        CPUMASK_ANDMASK(tmpmask, mask);
        while (CPUMASK_TESTNZERO(tmpmask)) {
                cpuid = BSFCPUMASK(tmpmask);
                rdd = &dfly_pcpu[cpuid];

                if ((rdd->upri & ~PPQMASK) >= (lp->lwp_priority & ~PPQMASK))
                        goto found;
                CPUMASK_NANDBIT(tmpmask, cpuid);
        }

        /*
         * Then cpus which might have a currently running lp
         */
        mask = dfly_rdyprocmask;
        CPUMASK_ANDMASK(mask, dfly_curprocmask);
        CPUMASK_ANDMASK(mask, lp->lwp_cpumask);
        CPUMASK_ANDMASK(mask, smp_active_mask);
        CPUMASK_ANDMASK(mask, usched_global_cpumask);

        CPUMASK_ASSBMASK(tmpmask, cpubase);
        CPUMASK_INVMASK(tmpmask);
        CPUMASK_ANDMASK(tmpmask, mask);
        while (CPUMASK_TESTNZERO(tmpmask)) {
                cpuid = BSFCPUMASK(tmpmask);
                rdd = &dfly_pcpu[cpuid];

                if ((rdd->upri & ~PPQMASK) > (lp->lwp_priority & ~PPQMASK))
                        goto found;
                CPUMASK_NANDBIT(tmpmask, cpuid);
        }

        CPUMASK_ASSBMASK(tmpmask, cpubase);
        CPUMASK_ANDMASK(tmpmask, mask);
        while (CPUMASK_TESTNZERO(tmpmask)) {
                cpuid = BSFCPUMASK(tmpmask);
                rdd = &dfly_pcpu[cpuid];

                if ((rdd->upri & ~PPQMASK) > (lp->lwp_priority & ~PPQMASK))
                        goto found;
                CPUMASK_NANDBIT(tmpmask, cpuid);
        }

        /*
         * If we cannot find a suitable cpu we round-robin using scancpu.
         * Other cpus will pickup as they release their current lwps or
         * become ready.
         *
         * Avoid a degenerate system lockup case if usched_global_cpumask
         * is set to 0 or otherwise does not cover lwp_cpumask.
         *
         * We only kick the target helper thread in this case, we do not
         * set the user resched flag because
         */
        cpuid = cpubase;
        if (CPUMASK_TESTBIT(lp->lwp_cpumask, cpuid) == 0)
                cpuid = BSFCPUMASK(lp->lwp_cpumask);
        else if (CPUMASK_TESTBIT(usched_global_cpumask, cpuid) == 0)
                cpuid = 0;
        rdd = &dfly_pcpu[cpuid];
found:
        return (rdd);
}

static
void
dfly_need_user_resched_remote(void *dummy)
{
        globaldata_t gd = mycpu;
        dfly_pcpu_t  dd = &dfly_pcpu[gd->gd_cpuid];

        /*
         * Flag reschedule needed
         */
        need_user_resched();

        /*
         * If no user thread is currently running we need to kick the helper
         * on our cpu to recover.  Otherwise the cpu will never schedule
         * anything again.
         *
         * We cannot schedule the process ourselves because this is an
         * IPI callback and we cannot acquire spinlocks in an IPI callback.
         *
         * Call wakeup_mycpu to avoid sending IPIs to other CPUs
         */
        if (dd->uschedcp == NULL && (dd->flags & DFLY_PCPU_RDYMASK)) {
                ATOMIC_CPUMASK_NANDBIT(dfly_rdyprocmask, gd->gd_cpuid);
                dd->flags &= ~DFLY_PCPU_RDYMASK;
                wakeup_mycpu(dd->helper_thread);
        }
}

/*
 * dfly_remrunqueue_locked() removes a given process from the run queue
 * that it is on, clearing the queue busy bit if it becomes empty.
 *
 * Note that user process scheduler is different from the LWKT schedule.
 * The user process scheduler only manages user processes but it uses LWKT
 * underneath, and a user process operating in the kernel will often be
 * 'released' from our management.
 *
 * uload is NOT adjusted here.  It is only adjusted if the lwkt_thread goes
 * to sleep or the lwp is moved to a different runq.
 */
static void
dfly_remrunqueue_locked(dfly_pcpu_t rdd, struct lwp *lp)
{
        struct rq *q;
        u_int32_t *which;
        u_int8_t pri;

        KKASSERT(rdd->runqcount >= 0);

        pri = lp->lwp_rqindex;

        switch(lp->lwp_rqtype) {
        case RTP_PRIO_NORMAL:
                q = &rdd->queues[pri];
                which = &rdd->queuebits;
                break;
        case RTP_PRIO_REALTIME:
        case RTP_PRIO_FIFO:
                q = &rdd->rtqueues[pri];
                which = &rdd->rtqueuebits;
                break;
        case RTP_PRIO_IDLE:
                q = &rdd->idqueues[pri];
                which = &rdd->idqueuebits;
                break;
        default:
                panic("remrunqueue: invalid rtprio type");
                /* NOT REACHED */
        }
        KKASSERT(lp->lwp_mpflags & LWP_MP_ONRUNQ);
        atomic_clear_int(&lp->lwp_mpflags, LWP_MP_ONRUNQ);
        TAILQ_REMOVE(q, lp, lwp_procq);
        --rdd->runqcount;
        if (TAILQ_EMPTY(q)) {
                KASSERT((*which & (1 << pri)) != 0,
                        ("remrunqueue: remove from empty queue"));
                *which &= ~(1 << pri);
        }
}

/*
 * dfly_setrunqueue_locked()
 *
 * Add a process whos rqtype and rqindex had previously been calculated
 * onto the appropriate run queue.   Determine if the addition requires
 * a reschedule on a cpu and return the cpuid or -1.
 *
 * NOTE:          Lower priorities are better priorities.
 *
 * NOTE ON ULOAD: This variable specifies the aggregate load on a cpu, the
 *                sum of the rough lwp_priority for all running and runnable
 *                processes.  Lower priority processes (higher lwp_priority
 *                values) actually DO count as more load, not less, because
 *                these are the programs which require the most care with
 *                regards to cpu selection.
 */
static void
dfly_setrunqueue_locked(dfly_pcpu_t rdd, struct lwp *lp)
{
        u_int32_t *which;
        struct rq *q;
        int pri;

        KKASSERT(lp->lwp_qcpu == rdd->cpuid);

        if ((lp->lwp_mpflags & LWP_MP_ULOAD) == 0) {
                atomic_set_int(&lp->lwp_mpflags, LWP_MP_ULOAD);
                atomic_add_long(&rdd->uload, lp->lwp_uload);
                atomic_add_int(&rdd->ucount, 1);
        }

        pri = lp->lwp_rqindex;

        switch(lp->lwp_rqtype) {
        case RTP_PRIO_NORMAL:
                q = &rdd->queues[pri];
                which = &rdd->queuebits;
                break;
        case RTP_PRIO_REALTIME:
        case RTP_PRIO_FIFO:
                q = &rdd->rtqueues[pri];
                which = &rdd->rtqueuebits;
                break;
        case RTP_PRIO_IDLE:
                q = &rdd->idqueues[pri];
                which = &rdd->idqueuebits;
                break;
        default:
                panic("remrunqueue: invalid rtprio type");
                /* NOT REACHED */
        }

        /*
         * Place us on the selected queue.  Determine if we should be
         * placed at the head of the queue or at the end.
         *
         * We are placed at the tail if our round-robin count has expired,
         * or is about to expire and the system thinks its a good place to
         * round-robin, or there is already a next thread on the queue
         * (it might be trying to pick up where it left off and we don't
         * want to interfere).
         */
        KKASSERT((lp->lwp_mpflags & LWP_MP_ONRUNQ) == 0);
        atomic_set_int(&lp->lwp_mpflags, LWP_MP_ONRUNQ);
        ++rdd->runqcount;

        if (lp->lwp_rrcount >= usched_dfly_rrinterval ||
            (lp->lwp_rrcount >= usched_dfly_rrinterval / 2 &&
             (lp->lwp_thread->td_mpflags & TDF_MP_BATCH_DEMARC))
        ) {
                /*
                 * Place on tail
                 */
                atomic_clear_int(&lp->lwp_thread->td_mpflags,
                                 TDF_MP_BATCH_DEMARC);
                lp->lwp_rrcount = 0;
                TAILQ_INSERT_TAIL(q, lp, lwp_procq);
        } else {
                /*
                 * Retain rrcount and place on head.  Count is retained
                 * even if the queue is empty.
                 */
                TAILQ_INSERT_HEAD(q, lp, lwp_procq);
        }
        *which |= 1 << pri;
}

/*
 * For SMP systems a user scheduler helper thread is created for each
 * cpu and is used to allow one cpu to wakeup another for the purposes of
 * scheduling userland threads from setrunqueue().
 *
 * UP systems do not need the helper since there is only one cpu.
 *
 * We can't use the idle thread for this because we might block.
 * Additionally, doing things this way allows us to HLT idle cpus
 * on MP systems.
 */
static void
dfly_helper_thread(void *dummy)
{
    globaldata_t gd;
    dfly_pcpu_t dd;
    dfly_pcpu_t rdd;
    struct lwp *nlp;
    cpumask_t mask;
    int cpuid;

    gd = mycpu;
    cpuid = gd->gd_cpuid;       /* doesn't change */
    mask = gd->gd_cpumask;      /* doesn't change */
    dd = &dfly_pcpu[cpuid];

    /*
     * Initial interlock, make sure all dfly_pcpu[] structures have
     * been initialized before proceeding.
     */
    lockmgr(&usched_dfly_config_lk, LK_SHARED);
    lockmgr(&usched_dfly_config_lk, LK_RELEASE);

    /*
     * Since we only want to be woken up only when no user processes
     * are scheduled on a cpu, run at an ultra low priority.
     */
    lwkt_setpri_self(TDPRI_USER_SCHEDULER);

    for (;;) {
        /*
         * We use the LWKT deschedule-interlock trick to avoid racing
         * dfly_rdyprocmask.  This means we cannot block through to the
         * manual lwkt_switch() call we make below.
         */
        crit_enter_gd(gd);
        tsleep_interlock(dd->helper_thread, 0);

        spin_lock(&dd->spin);
        if ((dd->flags & DFLY_PCPU_RDYMASK) == 0) {
                ATOMIC_CPUMASK_ORMASK(dfly_rdyprocmask, mask);
                dd->flags |= DFLY_PCPU_RDYMASK;
        }
        clear_user_resched();   /* This satisfied the reschedule request */
#if 0
        dd->rrcount = 0;        /* Reset the round-robin counter */
#endif

        if (dd->runqcount || dd->uschedcp != NULL) {
                /*
                 * Threads are available.  A thread may or may not be
                 * currently scheduled.  Get the best thread already queued
                 * to this cpu.
                 */
                nlp = dfly_chooseproc_locked(dd, dd, dd->uschedcp, 0);
                if (nlp) {
                        if ((dd->flags & DFLY_PCPU_CURMASK) == 0) {
                                ATOMIC_CPUMASK_ORMASK(dfly_curprocmask, mask);
                                dd->flags |= DFLY_PCPU_CURMASK;
                        }
                        dd->upri = nlp->lwp_priority;
                        dd->uschedcp = nlp;
#if 0
                        dd->rrcount = 0;        /* reset round robin */
#endif
                        spin_unlock(&dd->spin);
                        lwkt_acquire(nlp->lwp_thread);
                        lwkt_schedule(nlp->lwp_thread);
                } else {
                        /*
                         * This situation should not occur because we had
                         * at least one thread available.
                         */
                        spin_unlock(&dd->spin);
                }
        } else if (usched_dfly_features & 0x01) {
                /*
                 * This cpu is devoid of runnable threads, steal a thread
                 * from another cpu.  Since we're stealing, might as well
                 * load balance at the same time.
                 *
                 * We choose the highest-loaded thread from the worst queue.
                 *
                 * NOTE! This function only returns a non-NULL rdd when
                 *       another cpu's queue is obviously overloaded.  We
                 *       do not want to perform the type of rebalancing
                 *       the schedclock does here because it would result
                 *       in insane process pulling when 'steady' state is
                 *       partially unbalanced (e.g. 6 runnables and only
                 *       4 cores).
                 */
                rdd = dfly_choose_worst_queue(dd, 0);
                if (rdd && dd->uload + usched_dfly_weight6 < rdd->uload &&
                    spin_trylock(&rdd->spin)) {
                        nlp = dfly_chooseproc_locked(rdd, dd, NULL, 1);
                        spin_unlock(&rdd->spin);
                } else {
                        nlp = NULL;
                }
                if (nlp) {
                        if ((dd->flags & DFLY_PCPU_CURMASK) == 0) {
                                ATOMIC_CPUMASK_ORMASK(dfly_curprocmask, mask);
                                dd->flags |= DFLY_PCPU_CURMASK;
                        }
                        dd->upri = nlp->lwp_priority;
                        dd->uschedcp = nlp;
#if 0
                        dd->rrcount = 0;        /* reset round robin */
#endif
                        spin_unlock(&dd->spin);
                        lwkt_acquire(nlp->lwp_thread);
                        lwkt_schedule(nlp->lwp_thread);
                } else {
                        /*
                         * Leave the thread on our run queue.  Another
                         * scheduler will try to pull it later.
                         */
                        spin_unlock(&dd->spin);
                }
        } else {
                /*
                 * devoid of runnable threads and not allowed to steal
                 * any.
                 */
                spin_unlock(&dd->spin);
        }

        /*
         * We're descheduled unless someone scheduled us.  Switch away.
         * Exiting the critical section will cause splz() to be called
         * for us if interrupts and such are pending.
         */
        crit_exit_gd(gd);
        tsleep(dd->helper_thread, PINTERLOCKED, "schslp", 0);
    }
}

#if 0
static int
sysctl_usched_dfly_stick_to_level(SYSCTL_HANDLER_ARGS)
{
        int error, new_val;

        new_val = usched_dfly_stick_to_level;

        error = sysctl_handle_int(oidp, &new_val, 0, req);
        if (error != 0 || req->newptr == NULL)
                return (error);
        if (new_val > cpu_topology_levels_number - 1 || new_val < 0)
                return (EINVAL);
        usched_dfly_stick_to_level = new_val;
        return (0);
}
#endif

/*
 * Setup the queues and scheduler helpers (scheduler helpers are SMP only).
 * Note that curprocmask bit 0 has already been cleared by rqinit() and
 * we should not mess with it further.
 */
static void
usched_dfly_cpu_init(void)
{
        int i;
        int j;
        int smt_not_supported = 0;
        int cache_coherent_not_supported = 0;

        if (bootverbose)
                kprintf("Start usched_dfly helpers on cpus:\n");

        sysctl_ctx_init(&usched_dfly_sysctl_ctx);
        usched_dfly_sysctl_tree =
                SYSCTL_ADD_NODE(&usched_dfly_sysctl_ctx,
                                SYSCTL_STATIC_CHILDREN(_kern), OID_AUTO,
                                "usched_dfly", CTLFLAG_RD, 0, "");

        usched_dfly_node_mem = get_highest_node_memory();

        lockmgr(&usched_dfly_config_lk, LK_EXCLUSIVE);

        for (i = 0; i < ncpus; ++i) {
                dfly_pcpu_t dd = &dfly_pcpu[i];
                cpumask_t mask;

                CPUMASK_ASSBIT(mask, i);
                if (CPUMASK_TESTMASK(mask, smp_active_mask) == 0)
                    continue;

                spin_init(&dd->spin, "uschedcpuinit");
                dd->cpunode = get_cpu_node_by_cpuid(i);
                dd->cpuid = i;
                dd->gd = globaldata_find(i);
                CPUMASK_ASSBIT(dd->cpumask, i);
                for (j = 0; j < NQS; j++) {
                        TAILQ_INIT(&dd->queues[j]);
                        TAILQ_INIT(&dd->rtqueues[j]);
                        TAILQ_INIT(&dd->idqueues[j]);
                }
                ATOMIC_CPUMASK_NANDBIT(dfly_curprocmask, 0);
                if (i == 0)
                        dd->flags &= ~DFLY_PCPU_CURMASK;

                if (dd->cpunode == NULL) {
                        smt_not_supported = 1;
                        cache_coherent_not_supported = 1;
                        if (bootverbose)
                                kprintf ("    cpu%d - WARNING: No CPU NODE "
                                         "found for cpu\n", i);
                } else {
                        switch (dd->cpunode->type) {
                        case THREAD_LEVEL:
                                if (bootverbose)
                                        kprintf ("    cpu%d - HyperThreading "
                                                 "available. Core siblings: ",
                                                 i);
                                break;
                        case CORE_LEVEL:
                                smt_not_supported = 1;

                                if (bootverbose)
                                        kprintf ("    cpu%d - No HT available, "
                                                 "multi-core/physical "
                                                 "cpu. Physical siblings: ",
                                                 i);
                                break;
                        case CHIP_LEVEL:
                                smt_not_supported = 1;

                                if (bootverbose)
                                        kprintf ("    cpu%d - No HT available, "
                                                 "single-core/physical cpu. "
                                                 "Package siblings: ",
                                                 i);
                                break;
                        default:
                                /* Let's go for safe defaults here */
                                smt_not_supported = 1;
                                cache_coherent_not_supported = 1;
                                if (bootverbose)
                                        kprintf ("    cpu%d - Unknown cpunode->"
                                                 "type=%u. siblings: ",
                                                 i,
                                                 (u_int)dd->cpunode->type);
                                break;
                        }

                        if (bootverbose) {
                                if (dd->cpunode->parent_node != NULL) {
                                        kprint_cpuset(&dd->cpunode->
                                                        parent_node->members);
                                        kprintf("\n");
                                } else {
                                        kprintf(" no siblings\n");
                                }
                        }
                }

                lwkt_create(dfly_helper_thread, NULL, &dd->helper_thread, NULL,
                            0, i, "usched %d", i);

                /*
                 * Allow user scheduling on the target cpu.  cpu #0 has already
                 * been enabled in rqinit().
                 */
                if (i) {
                        ATOMIC_CPUMASK_NANDMASK(dfly_curprocmask, mask);
                        dd->flags &= ~DFLY_PCPU_CURMASK;
                }
                if ((dd->flags & DFLY_PCPU_RDYMASK) == 0) {
                        ATOMIC_CPUMASK_ORMASK(dfly_rdyprocmask, mask);
                        dd->flags |= DFLY_PCPU_RDYMASK;
                }
                dd->upri = PRIBASE_NULL;

        }

        /* usched_dfly sysctl configurable parameters */

        SYSCTL_ADD_INT(&usched_dfly_sysctl_ctx,
                       SYSCTL_CHILDREN(usched_dfly_sysctl_tree),
                       OID_AUTO, "rrinterval", CTLFLAG_RW,
                       &usched_dfly_rrinterval, 0, "");
        SYSCTL_ADD_INT(&usched_dfly_sysctl_ctx,
                       SYSCTL_CHILDREN(usched_dfly_sysctl_tree),
                       OID_AUTO, "decay", CTLFLAG_RW,
                       &usched_dfly_decay, 0, "Extra decay when not running");

        /* Add enable/disable option for SMT scheduling if supported */
        if (smt_not_supported) {
                usched_dfly_smt = 0;
                SYSCTL_ADD_STRING(&usched_dfly_sysctl_ctx,
                                  SYSCTL_CHILDREN(usched_dfly_sysctl_tree),
                                  OID_AUTO, "smt", CTLFLAG_RD,
                                  "NOT SUPPORTED", 0, "SMT NOT SUPPORTED");
        } else {
                usched_dfly_smt = 1;
                SYSCTL_ADD_INT(&usched_dfly_sysctl_ctx,
                               SYSCTL_CHILDREN(usched_dfly_sysctl_tree),
                               OID_AUTO, "smt", CTLFLAG_RW,
                               &usched_dfly_smt, 0, "Enable SMT scheduling");
        }

        /*
         * Add enable/disable option for cache coherent scheduling
         * if supported
         */
        if (cache_coherent_not_supported) {
                usched_dfly_cache_coherent = 0;
                SYSCTL_ADD_STRING(&usched_dfly_sysctl_ctx,
                                  SYSCTL_CHILDREN(usched_dfly_sysctl_tree),
                                  OID_AUTO, "cache_coherent", CTLFLAG_RD,
                                  "NOT SUPPORTED", 0,
                                  "Cache coherence NOT SUPPORTED");
        } else {
                usched_dfly_cache_coherent = 1;
                SYSCTL_ADD_INT(&usched_dfly_sysctl_ctx,
                               SYSCTL_CHILDREN(usched_dfly_sysctl_tree),
                               OID_AUTO, "cache_coherent", CTLFLAG_RW,
                               &usched_dfly_cache_coherent, 0,
                               "Enable/Disable cache coherent scheduling");

                SYSCTL_ADD_INT(&usched_dfly_sysctl_ctx,
                               SYSCTL_CHILDREN(usched_dfly_sysctl_tree),
                               OID_AUTO, "weight1", CTLFLAG_RW,
                               &usched_dfly_weight1, 200,
                               "Weight selection for current cpu");

                SYSCTL_ADD_INT(&usched_dfly_sysctl_ctx,
                               SYSCTL_CHILDREN(usched_dfly_sysctl_tree),
                               OID_AUTO, "weight2", CTLFLAG_RW,
                               &usched_dfly_weight2, 180,
                               "Weight selection for wakefrom cpu");

                SYSCTL_ADD_INT(&usched_dfly_sysctl_ctx,
                               SYSCTL_CHILDREN(usched_dfly_sysctl_tree),
                               OID_AUTO, "weight3", CTLFLAG_RW,
                               &usched_dfly_weight3, 40,
                               "Weight selection for num threads on queue");

                SYSCTL_ADD_INT(&usched_dfly_sysctl_ctx,
                               SYSCTL_CHILDREN(usched_dfly_sysctl_tree),
                               OID_AUTO, "weight4", CTLFLAG_RW,
                               &usched_dfly_weight4, 160,
                               "Availability of other idle cpus");

                SYSCTL_ADD_INT(&usched_dfly_sysctl_ctx,
                               SYSCTL_CHILDREN(usched_dfly_sysctl_tree),
                               OID_AUTO, "weight5", CTLFLAG_RW,
                               &usched_dfly_weight5, 50,
                               "Memory attached to node");

                SYSCTL_ADD_INT(&usched_dfly_sysctl_ctx,
                               SYSCTL_CHILDREN(usched_dfly_sysctl_tree),
                               OID_AUTO, "weight6", CTLFLAG_RW,
                               &usched_dfly_weight6, 150,
                               "Transfer weight");

                SYSCTL_ADD_INT(&usched_dfly_sysctl_ctx,
                               SYSCTL_CHILDREN(usched_dfly_sysctl_tree),
                               OID_AUTO, "fast_resched", CTLFLAG_RW,
                               &usched_dfly_fast_resched, 0,
                               "Availability of other idle cpus");

                SYSCTL_ADD_INT(&usched_dfly_sysctl_ctx,
                               SYSCTL_CHILDREN(usched_dfly_sysctl_tree),
                               OID_AUTO, "features", CTLFLAG_RW,
                               &usched_dfly_features, 0x8F,
                               "Allow pulls into empty queues");

                SYSCTL_ADD_INT(&usched_dfly_sysctl_ctx,
                               SYSCTL_CHILDREN(usched_dfly_sysctl_tree),
                               OID_AUTO, "swmask", CTLFLAG_RW,
                               &usched_dfly_swmask, ~PPQMASK,
                               "Queue mask to force thread switch");

#if 0
                SYSCTL_ADD_PROC(&usched_dfly_sysctl_ctx,
                                SYSCTL_CHILDREN(usched_dfly_sysctl_tree),
                                OID_AUTO, "stick_to_level",
                                CTLTYPE_INT | CTLFLAG_RW,
                                NULL, sizeof usched_dfly_stick_to_level,
                                sysctl_usched_dfly_stick_to_level, "I",
                                "Stick a process to this level. See sysctl"
                                "paremter hw.cpu_topology.level_description");
#endif
        }
        lockmgr(&usched_dfly_config_lk, LK_RELEASE);
}

SYSINIT(uschedtd, SI_BOOT2_USCHED, SI_ORDER_SECOND,
        usched_dfly_cpu_init, NULL);