kernel - Fix rare IPIQ freezes

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

/*
 * Copyright (c) 2003-2010 The DragonFly Project.  All rights reserved.
 *
 * This code is derived from software contributed to The DragonFly Project
 * by Matthew Dillon <dillon@backplane.com>
 *
 * 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.
 */

/*
 * Each cpu in a system has its own self-contained light weight kernel
 * thread scheduler, which means that generally speaking we only need
 * to use a critical section to avoid problems.  Foreign thread 
 * scheduling is queued via (async) IPIs.
 */

#include <sys/param.h>
#include <sys/systm.h>
#include <sys/kernel.h>
#include <sys/proc.h>
#include <sys/rtprio.h>
#include <sys/kinfo.h>
#include <sys/queue.h>
#include <sys/sysctl.h>
#include <sys/kthread.h>
#include <machine/cpu.h>
#include <sys/lock.h>
#include <sys/caps.h>
#include <sys/spinlock.h>
#include <sys/ktr.h>

#include <sys/thread2.h>
#include <sys/spinlock2.h>
#include <sys/mplock2.h>

#include <sys/dsched.h>

#include <vm/vm.h>
#include <vm/vm_param.h>
#include <vm/vm_kern.h>
#include <vm/vm_object.h>
#include <vm/vm_page.h>
#include <vm/vm_map.h>
#include <vm/vm_pager.h>
#include <vm/vm_extern.h>

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

#if !defined(KTR_CTXSW)
#define KTR_CTXSW KTR_ALL
#endif
KTR_INFO_MASTER(ctxsw);
KTR_INFO(KTR_CTXSW, ctxsw, sw, 0, "#cpu[%d].td = %p",
         sizeof(int) + sizeof(struct thread *));
KTR_INFO(KTR_CTXSW, ctxsw, pre, 1, "#cpu[%d].td = %p",
         sizeof(int) + sizeof(struct thread *));
KTR_INFO(KTR_CTXSW, ctxsw, newtd, 2, "#threads[%p].name = %s",
         sizeof (struct thread *) + sizeof(char *));
KTR_INFO(KTR_CTXSW, ctxsw, deadtd, 3, "#threads[%p].name = <dead>", sizeof (struct thread *));

static MALLOC_DEFINE(M_THREAD, "thread", "lwkt threads");

#ifdef  INVARIANTS
static int panic_on_cscount = 0;
#endif
static __int64_t switch_count = 0;
static __int64_t preempt_hit = 0;
static __int64_t preempt_miss = 0;
static __int64_t preempt_weird = 0;
static __int64_t token_contention_count __debugvar = 0;
static int lwkt_use_spin_port;
static struct objcache *thread_cache;

#ifdef SMP
static void lwkt_schedule_remote(void *arg, int arg2, struct intrframe *frame);
static void lwkt_setcpu_remote(void *arg);
#endif
static void lwkt_fairq_accumulate(globaldata_t gd, thread_t td);

extern void cpu_heavy_restore(void);
extern void cpu_lwkt_restore(void);
extern void cpu_kthread_restore(void);
extern void cpu_idle_restore(void);

/*
 * We can make all thread ports use the spin backend instead of the thread
 * backend.  This should only be set to debug the spin backend.
 */
TUNABLE_INT("lwkt.use_spin_port", &lwkt_use_spin_port);

#ifdef  INVARIANTS
SYSCTL_INT(_lwkt, OID_AUTO, panic_on_cscount, CTLFLAG_RW, &panic_on_cscount, 0,
    "Panic if attempting to switch lwkt's while mastering cpusync");
#endif
SYSCTL_QUAD(_lwkt, OID_AUTO, switch_count, CTLFLAG_RW, &switch_count, 0,
    "Number of switched threads");
SYSCTL_QUAD(_lwkt, OID_AUTO, preempt_hit, CTLFLAG_RW, &preempt_hit, 0, 
    "Successful preemption events");
SYSCTL_QUAD(_lwkt, OID_AUTO, preempt_miss, CTLFLAG_RW, &preempt_miss, 0, 
    "Failed preemption events");
SYSCTL_QUAD(_lwkt, OID_AUTO, preempt_weird, CTLFLAG_RW, &preempt_weird, 0,
    "Number of preempted threads.");
#ifdef  INVARIANTS
SYSCTL_QUAD(_lwkt, OID_AUTO, token_contention_count, CTLFLAG_RW,
        &token_contention_count, 0, "spinning due to token contention");
#endif
static int fairq_enable = 1;
SYSCTL_INT(_lwkt, OID_AUTO, fairq_enable, CTLFLAG_RW,
        &fairq_enable, 0, "Turn on fairq priority accumulators");
static int lwkt_spin_loops = 10;
SYSCTL_INT(_lwkt, OID_AUTO, spin_loops, CTLFLAG_RW,
        &lwkt_spin_loops, 0, "");
static int lwkt_spin_delay = 1;
SYSCTL_INT(_lwkt, OID_AUTO, spin_delay, CTLFLAG_RW,
        &lwkt_spin_delay, 0, "Scheduler spin delay in microseconds 0=auto");
static int lwkt_spin_method = 1;
SYSCTL_INT(_lwkt, OID_AUTO, spin_method, CTLFLAG_RW,
        &lwkt_spin_method, 0, "LWKT scheduler behavior when contended");
static int lwkt_spin_fatal = 0; /* disabled */
SYSCTL_INT(_lwkt, OID_AUTO, spin_fatal, CTLFLAG_RW,
        &lwkt_spin_fatal, 0, "LWKT scheduler spin loops till fatal panic");
static int preempt_enable = 1;
SYSCTL_INT(_lwkt, OID_AUTO, preempt_enable, CTLFLAG_RW,
        &preempt_enable, 0, "Enable preemption");
static int lwkt_cache_threads = 32;
SYSCTL_INT(_lwkt, OID_AUTO, cache_threads, CTLFLAG_RD,
        &lwkt_cache_threads, 0, "thread+kstack cache");

static __cachealign int lwkt_cseq_rindex;
static __cachealign int lwkt_cseq_windex;

/*
 * These helper procedures handle the runq, they can only be called from
 * within a critical section.
 *
 * WARNING!  Prior to SMP being brought up it is possible to enqueue and
 * dequeue threads belonging to other cpus, so be sure to use td->td_gd
 * instead of 'mycpu' when referencing the globaldata structure.   Once
 * SMP live enqueuing and dequeueing only occurs on the current cpu.
 */
static __inline
void
_lwkt_dequeue(thread_t td)
{
    if (td->td_flags & TDF_RUNQ) {
        struct globaldata *gd = td->td_gd;

        td->td_flags &= ~TDF_RUNQ;
        TAILQ_REMOVE(&gd->gd_tdrunq, td, td_threadq);
        gd->gd_fairq_total_pri -= td->td_pri;
        if (TAILQ_FIRST(&gd->gd_tdrunq) == NULL)
                atomic_clear_int(&gd->gd_reqflags, RQF_RUNNING);
    }
}

/*
 * Priority enqueue.
 *
 * NOTE: There are a limited number of lwkt threads runnable since user
 *       processes only schedule one at a time per cpu.
 */
static __inline
void
_lwkt_enqueue(thread_t td)
{
    thread_t xtd;

    if ((td->td_flags & (TDF_RUNQ|TDF_MIGRATING|TDF_BLOCKQ)) == 0) {
        struct globaldata *gd = td->td_gd;

        td->td_flags |= TDF_RUNQ;
        xtd = TAILQ_FIRST(&gd->gd_tdrunq);
        if (xtd == NULL) {
                TAILQ_INSERT_TAIL(&gd->gd_tdrunq, td, td_threadq);
                atomic_set_int(&gd->gd_reqflags, RQF_RUNNING);
        } else {
                while (xtd && xtd->td_pri > td->td_pri)
                        xtd = TAILQ_NEXT(xtd, td_threadq);
                if (xtd)
                        TAILQ_INSERT_BEFORE(xtd, td, td_threadq);
                else
                        TAILQ_INSERT_TAIL(&gd->gd_tdrunq, td, td_threadq);
        }
        gd->gd_fairq_total_pri += td->td_pri;
    }
}

static __boolean_t
_lwkt_thread_ctor(void *obj, void *privdata, int ocflags)
{
        struct thread *td = (struct thread *)obj;

        td->td_kstack = NULL;
        td->td_kstack_size = 0;
        td->td_flags = TDF_ALLOCATED_THREAD;
        return (1);
}

static void
_lwkt_thread_dtor(void *obj, void *privdata)
{
        struct thread *td = (struct thread *)obj;

        KASSERT(td->td_flags & TDF_ALLOCATED_THREAD,
            ("_lwkt_thread_dtor: not allocated from objcache"));
        KASSERT((td->td_flags & TDF_ALLOCATED_STACK) && td->td_kstack &&
                td->td_kstack_size > 0,
            ("_lwkt_thread_dtor: corrupted stack"));
        kmem_free(&kernel_map, (vm_offset_t)td->td_kstack, td->td_kstack_size);
}

/*
 * Initialize the lwkt s/system.
 *
 * Nominally cache up to 32 thread + kstack structures.
 */
void
lwkt_init(void)
{
    TUNABLE_INT("lwkt.cache_threads", &lwkt_cache_threads);
    thread_cache = objcache_create_mbacked(
                                M_THREAD, sizeof(struct thread),
                                NULL, lwkt_cache_threads,
                                _lwkt_thread_ctor, _lwkt_thread_dtor, NULL);
}

/*
 * Schedule a thread to run.  As the current thread we can always safely
 * schedule ourselves, and a shortcut procedure is provided for that
 * function.
 *
 * (non-blocking, self contained on a per cpu basis)
 */
void
lwkt_schedule_self(thread_t td)
{
    KKASSERT((td->td_flags & TDF_MIGRATING) == 0);
    crit_enter_quick(td);
    KASSERT(td != &td->td_gd->gd_idlethread,
            ("lwkt_schedule_self(): scheduling gd_idlethread is illegal!"));
    KKASSERT(td->td_lwp == NULL || (td->td_lwp->lwp_flag & LWP_ONRUNQ) == 0);
    _lwkt_enqueue(td);
    crit_exit_quick(td);
}

/*
 * Deschedule a thread.
 *
 * (non-blocking, self contained on a per cpu basis)
 */
void
lwkt_deschedule_self(thread_t td)
{
    crit_enter_quick(td);
    _lwkt_dequeue(td);
    crit_exit_quick(td);
}

/*
 * LWKTs operate on a per-cpu basis
 *
 * WARNING!  Called from early boot, 'mycpu' may not work yet.
 */
void
lwkt_gdinit(struct globaldata *gd)
{
    TAILQ_INIT(&gd->gd_tdrunq);
    TAILQ_INIT(&gd->gd_tdallq);
}

/*
 * Create a new thread.  The thread must be associated with a process context
 * or LWKT start address before it can be scheduled.  If the target cpu is
 * -1 the thread will be created on the current cpu.
 *
 * If you intend to create a thread without a process context this function
 * does everything except load the startup and switcher function.
 */
thread_t
lwkt_alloc_thread(struct thread *td, int stksize, int cpu, int flags)
{
    globaldata_t gd = mycpu;
    void *stack;

    /*
     * If static thread storage is not supplied allocate a thread.  Reuse
     * a cached free thread if possible.  gd_freetd is used to keep an exiting
     * thread intact through the exit.
     */
    if (td == NULL) {
        crit_enter_gd(gd);
        if ((td = gd->gd_freetd) != NULL) {
            KKASSERT((td->td_flags & (TDF_RUNNING|TDF_PREEMPT_LOCK|
                                      TDF_RUNQ)) == 0);
            gd->gd_freetd = NULL;
        } else {
            td = objcache_get(thread_cache, M_WAITOK);
            KKASSERT((td->td_flags & (TDF_RUNNING|TDF_PREEMPT_LOCK|
                                      TDF_RUNQ)) == 0);
        }
        crit_exit_gd(gd);
        KASSERT((td->td_flags &
                 (TDF_ALLOCATED_THREAD|TDF_RUNNING)) == TDF_ALLOCATED_THREAD,
                ("lwkt_alloc_thread: corrupted td flags 0x%X", td->td_flags));
        flags |= td->td_flags & (TDF_ALLOCATED_THREAD|TDF_ALLOCATED_STACK);
    }

    /*
     * Try to reuse cached stack.
     */
    if ((stack = td->td_kstack) != NULL && td->td_kstack_size != stksize) {
        if (flags & TDF_ALLOCATED_STACK) {
            kmem_free(&kernel_map, (vm_offset_t)stack, td->td_kstack_size);
            stack = NULL;
        }
    }
    if (stack == NULL) {
        stack = (void *)kmem_alloc_stack(&kernel_map, stksize);
        flags |= TDF_ALLOCATED_STACK;
    }
    if (cpu < 0)
        lwkt_init_thread(td, stack, stksize, flags, gd);
    else
        lwkt_init_thread(td, stack, stksize, flags, globaldata_find(cpu));
    return(td);
}

/*
 * Initialize a preexisting thread structure.  This function is used by
 * lwkt_alloc_thread() and also used to initialize the per-cpu idlethread.
 *
 * All threads start out in a critical section at a priority of
 * TDPRI_KERN_DAEMON.  Higher level code will modify the priority as
 * appropriate.  This function may send an IPI message when the 
 * requested cpu is not the current cpu and consequently gd_tdallq may
 * not be initialized synchronously from the point of view of the originating
 * cpu.
 *
 * NOTE! we have to be careful in regards to creating threads for other cpus
 * if SMP has not yet been activated.
 */
#ifdef SMP

static void
lwkt_init_thread_remote(void *arg)
{
    thread_t td = arg;

    /*
     * Protected by critical section held by IPI dispatch
     */
    TAILQ_INSERT_TAIL(&td->td_gd->gd_tdallq, td, td_allq);
}

#endif

/*
 * lwkt core thread structural initialization.
 *
 * NOTE: All threads are initialized as mpsafe threads.
 */
void
lwkt_init_thread(thread_t td, void *stack, int stksize, int flags,
                struct globaldata *gd)
{
    globaldata_t mygd = mycpu;

    bzero(td, sizeof(struct thread));
    td->td_kstack = stack;
    td->td_kstack_size = stksize;
    td->td_flags = flags;
    td->td_gd = gd;
    td->td_pri = TDPRI_KERN_DAEMON;
    td->td_critcount = 1;
    td->td_toks_stop = &td->td_toks_base;
    if (lwkt_use_spin_port)
        lwkt_initport_spin(&td->td_msgport);
    else
        lwkt_initport_thread(&td->td_msgport, td);
    pmap_init_thread(td);
#ifdef SMP
    /*
     * Normally initializing a thread for a remote cpu requires sending an
     * IPI.  However, the idlethread is setup before the other cpus are
     * activated so we have to treat it as a special case.  XXX manipulation
     * of gd_tdallq requires the BGL.
     */
    if (gd == mygd || td == &gd->gd_idlethread) {
        crit_enter_gd(mygd);
        TAILQ_INSERT_TAIL(&gd->gd_tdallq, td, td_allq);
        crit_exit_gd(mygd);
    } else {
        lwkt_send_ipiq(gd, lwkt_init_thread_remote, td);
    }
#else
    crit_enter_gd(mygd);
    TAILQ_INSERT_TAIL(&gd->gd_tdallq, td, td_allq);
    crit_exit_gd(mygd);
#endif

    dsched_new_thread(td);
}

void
lwkt_set_comm(thread_t td, const char *ctl, ...)
{
    __va_list va;

    __va_start(va, ctl);
    kvsnprintf(td->td_comm, sizeof(td->td_comm), ctl, va);
    __va_end(va);
    KTR_LOG(ctxsw_newtd, td, &td->td_comm[0]);
}

void
lwkt_hold(thread_t td)
{
    atomic_add_int(&td->td_refs, 1);
}

void
lwkt_rele(thread_t td)
{
    KKASSERT(td->td_refs > 0);
    atomic_add_int(&td->td_refs, -1);
}

void
lwkt_wait_free(thread_t td)
{
    while (td->td_refs)
        tsleep(td, 0, "tdreap", hz);
}

void
lwkt_free_thread(thread_t td)
{
    KKASSERT(td->td_refs == 0);
    KKASSERT((td->td_flags & (TDF_RUNNING|TDF_PREEMPT_LOCK|TDF_RUNQ)) == 0);
    if (td->td_flags & TDF_ALLOCATED_THREAD) {
        objcache_put(thread_cache, td);
    } else if (td->td_flags & TDF_ALLOCATED_STACK) {
        /* client-allocated struct with internally allocated stack */
        KASSERT(td->td_kstack && td->td_kstack_size > 0,
            ("lwkt_free_thread: corrupted stack"));
        kmem_free(&kernel_map, (vm_offset_t)td->td_kstack, td->td_kstack_size);
        td->td_kstack = NULL;
        td->td_kstack_size = 0;
    }
    KTR_LOG(ctxsw_deadtd, td);
}


/*
 * Switch to the next runnable lwkt.  If no LWKTs are runnable then 
 * switch to the idlethread.  Switching must occur within a critical
 * section to avoid races with the scheduling queue.
 *
 * We always have full control over our cpu's run queue.  Other cpus
 * that wish to manipulate our queue must use the cpu_*msg() calls to
 * talk to our cpu, so a critical section is all that is needed and
 * the result is very, very fast thread switching.
 *
 * The LWKT scheduler uses a fixed priority model and round-robins at
 * each priority level.  User process scheduling is a totally
 * different beast and LWKT priorities should not be confused with
 * user process priorities.
 *
 * PREEMPTION NOTE: Preemption occurs via lwkt_preempt().  lwkt_switch()
 * is not called by the current thread in the preemption case, only when
 * the preempting thread blocks (in order to return to the original thread).
 *
 * SPECIAL NOTE ON SWITCH ATOMICY: Certain operations such as thread
 * migration and tsleep deschedule the current lwkt thread and call
 * lwkt_switch().  In particular, the target cpu of the migration fully
 * expects the thread to become non-runnable and can deadlock against
 * cpusync operations if we run any IPIs prior to switching the thread out.
 *
 * WE MUST BE VERY CAREFUL NOT TO RUN SPLZ DIRECTLY OR INDIRECTLY IF
 * THE CURRENT THREAD HAS BEEN DESCHEDULED!
 */
void
lwkt_switch(void)
{
    globaldata_t gd = mycpu;
    thread_t td = gd->gd_curthread;
    thread_t ntd;
    thread_t xtd;
    int spinning = lwkt_spin_loops;     /* loops before HLTing */
    int reqflags;
    int cseq;
    int oseq;
    int fatal_count;

    KKASSERT(gd->gd_processing_ipiq == 0);

    /*
     * Switching from within a 'fast' (non thread switched) interrupt or IPI
     * is illegal.  However, we may have to do it anyway if we hit a fatal
     * kernel trap or we have paniced.
     *
     * If this case occurs save and restore the interrupt nesting level.
     */
    if (gd->gd_intr_nesting_level) {
        int savegdnest;
        int savegdtrap;

        if (gd->gd_trap_nesting_level == 0 && panic_cpu_gd != mycpu) {
            panic("lwkt_switch: Attempt to switch from a "
                  "a fast interrupt, ipi, or hard code section, "
                  "td %p\n",
                  td);
        } else {
            savegdnest = gd->gd_intr_nesting_level;
            savegdtrap = gd->gd_trap_nesting_level;
            gd->gd_intr_nesting_level = 0;
            gd->gd_trap_nesting_level = 0;
            if ((td->td_flags & TDF_PANICWARN) == 0) {
                td->td_flags |= TDF_PANICWARN;
                kprintf("Warning: thread switch from interrupt, IPI, "
                        "or hard code section.\n"
                        "thread %p (%s)\n", td, td->td_comm);
                print_backtrace(-1);
            }
            lwkt_switch();
            gd->gd_intr_nesting_level = savegdnest;
            gd->gd_trap_nesting_level = savegdtrap;
            return;
        }
    }

    /*
     * Passive release (used to transition from user to kernel mode
     * when we block or switch rather then when we enter the kernel).
     * This function is NOT called if we are switching into a preemption
     * or returning from a preemption.  Typically this causes us to lose
     * our current process designation (if we have one) and become a true
     * LWKT thread, and may also hand the current process designation to
     * another process and schedule thread.
     */
    if (td->td_release)
            td->td_release(td);

    crit_enter_gd(gd);
    if (TD_TOKS_HELD(td))
            lwkt_relalltokens(td);

    /*
     * We had better not be holding any spin locks, but don't get into an
     * endless panic loop.
     */
    KASSERT(gd->gd_spinlocks_wr == 0 || panicstr != NULL, 
            ("lwkt_switch: still holding %d exclusive spinlocks!",
             gd->gd_spinlocks_wr));


#ifdef SMP
#ifdef  INVARIANTS
    if (td->td_cscount) {
        kprintf("Diagnostic: attempt to switch while mastering cpusync: %p\n",
                td);
        if (panic_on_cscount)
            panic("switching while mastering cpusync");
    }
#endif
#endif

    /*
     * If we had preempted another thread on this cpu, resume the preempted
     * thread.  This occurs transparently, whether the preempted thread
     * was scheduled or not (it may have been preempted after descheduling
     * itself).
     *
     * We have to setup the MP lock for the original thread after backing
     * out the adjustment that was made to curthread when the original
     * was preempted.
     */
    if ((ntd = td->td_preempted) != NULL) {
        KKASSERT(ntd->td_flags & TDF_PREEMPT_LOCK);
        ntd->td_flags |= TDF_PREEMPT_DONE;

        /*
         * The interrupt may have woken a thread up, we need to properly
         * set the reschedule flag if the originally interrupted thread is
         * at a lower priority.
         */
        if (TAILQ_FIRST(&gd->gd_tdrunq) &&
            TAILQ_FIRST(&gd->gd_tdrunq)->td_pri > ntd->td_pri) {
            need_lwkt_resched();
        }
        /* YYY release mp lock on switchback if original doesn't need it */
        goto havethread_preempted;
    }

    /*
     * Implement round-robin fairq with priority insertion.  The priority
     * insertion is handled by _lwkt_enqueue()
     *
     * If we cannot obtain ownership of the tokens we cannot immediately
     * schedule the target thread.
     *
     * Reminder: Again, we cannot afford to run any IPIs in this path if
     * the current thread has been descheduled.
     */
    for (;;) {
        /*
         * Clear RQF_AST_LWKT_RESCHED (we handle the reschedule request)
         * and set RQF_WAKEUP (prevent unnecessary IPIs from being
         * received).
         */
        for (;;) {
            reqflags = gd->gd_reqflags;
            if (atomic_cmpset_int(&gd->gd_reqflags, reqflags,
                                  (reqflags & ~RQF_AST_LWKT_RESCHED) |
                                  RQF_WAKEUP)) {
                break;
            }
        }

        /*
         * Hotpath - pull the head of the run queue and attempt to schedule
         * it.  Fairq exhaustion moves the task to the end of the list.  If
         * no threads are runnable we switch to the idle thread.
         */
        for (;;) {
            ntd = TAILQ_FIRST(&gd->gd_tdrunq);

            if (ntd == NULL) {
                /*
                 * Runq is empty, switch to idle and clear RQF_WAKEUP
                 * to allow it to halt.
                 */
                ntd = &gd->gd_idlethread;
#ifdef SMP
                if (gd->gd_trap_nesting_level == 0 && panicstr == NULL)
                    ASSERT_NO_TOKENS_HELD(ntd);
#endif
                cpu_time.cp_msg[0] = 0;
                cpu_time.cp_stallpc = 0;
                atomic_clear_int(&gd->gd_reqflags, RQF_WAKEUP);
                goto haveidle;
            }

            if (ntd->td_fairq_accum >= 0)
                    break;

            /*splz_check(); cannot do this here, see above */
            lwkt_fairq_accumulate(gd, ntd);
            TAILQ_REMOVE(&gd->gd_tdrunq, ntd, td_threadq);
            TAILQ_INSERT_TAIL(&gd->gd_tdrunq, ntd, td_threadq);
        }

        /*
         * Hotpath - schedule ntd.  Leaves RQF_WAKEUP set to prevent
         *           unwanted decontention IPIs.
         *
         * NOTE: For UP there is no mplock and lwkt_getalltokens()
         *           always succeeds.
         */
        if (TD_TOKS_NOT_HELD(ntd) || lwkt_getalltokens(ntd))
            goto havethread;

        /*
         * Coldpath (SMP only since tokens always succeed on UP)
         *
         * We had some contention on the thread we wanted to schedule.
         * What we do now is try to find a thread that we can schedule
         * in its stead until decontention reschedules on our cpu.
         *
         * The coldpath scan does NOT rearrange threads in the run list
         * and it also ignores the accumulator.
         *
         * We do not immediately schedule a user priority thread, instead
         * we record it in xtd and continue looking for kernel threads.
         * A cpu can only have one user priority thread (normally) so just
         * record the first one.
         *
         * NOTE: This scan will also include threads whos fairq's were
         *       accumulated in the first loop.
         */
        ++token_contention_count;
        xtd = NULL;
        while ((ntd = TAILQ_NEXT(ntd, td_threadq)) != NULL) {
            /*
             * Try to switch to this thread.  If the thread is running at
             * user priority we clear WAKEUP to allow decontention IPIs
             * (since this thread is simply running until the one we wanted
             * decontends), and we make sure that LWKT_RESCHED is not set.
             *
             * Otherwise for kernel threads we leave WAKEUP set to avoid
             * unnecessary decontention IPIs.
             */
            if (ntd->td_pri < TDPRI_KERN_LPSCHED) {
                if (xtd == NULL)
                    xtd = ntd;
                continue;
            }

            /*
             * Do not let the fairq get too negative.  Even though we are
             * ignoring it atm once the scheduler decontends a very negative
             * thread will get moved to the end of the queue.
             */
            if (TD_TOKS_NOT_HELD(ntd) || lwkt_getalltokens(ntd)) {
                if (ntd->td_fairq_accum < -TDFAIRQ_MAX(gd))
                    ntd->td_fairq_accum = -TDFAIRQ_MAX(gd);
                goto havethread;
            }

            /*
             * Well fubar, this thread is contended as well, loop
             */
            /* */
        }

        /*
         * We exhausted the run list but we may have recorded a user
         * thread to try.  We have three choices based on
         * lwkt.decontention_method.
         *
         * (0) Atomically clear RQF_WAKEUP in order to receive decontention
         *     IPIs (to interrupt the user process) and test
         *     RQF_AST_LWKT_RESCHED at the same time.
         *
         *     This results in significant decontention IPI traffic but may
         *     be more responsive.
         *
         * (1) Leave RQF_WAKEUP set so we do not receive a decontention IPI.
         *     An automatic LWKT reschedule will occur on the next hardclock
         *     (typically 100hz).
         *
         *     This results in no decontention IPI traffic but may be less
         *     responsive.  This is the default.
         *
         * (2) Refuse to schedule the user process at this time.
         *
         *     This is highly experimental and should not be used under
         *     normal circumstances.  This can cause a user process to
         *     get starved out in situations where kernel threads are
         *     fighting each other for tokens.
         */
        if (xtd) {
            ntd = xtd;

            switch(lwkt_spin_method) {
            case 0:
                for (;;) {
                    reqflags = gd->gd_reqflags;
                    if (atomic_cmpset_int(&gd->gd_reqflags,
                                          reqflags,
                                          reqflags & ~RQF_WAKEUP)) {
                        break;
                    }
                }
                break;
            case 1:
                reqflags = gd->gd_reqflags;
                break;
            default:
                goto skip;
                break;
            }
            if ((reqflags & RQF_AST_LWKT_RESCHED) == 0 &&
                (TD_TOKS_NOT_HELD(ntd) || lwkt_getalltokens(ntd))
            ) {
                if (ntd->td_fairq_accum < -TDFAIRQ_MAX(gd))
                    ntd->td_fairq_accum = -TDFAIRQ_MAX(gd);
                goto havethread;
            }

skip:
            /*
             * Make sure RQF_WAKEUP is set if we failed to schedule the
             * user thread to prevent the idle thread from halting.
             */
            atomic_set_int(&gd->gd_reqflags, RQF_WAKEUP);
        }

        /*
         * We exhausted the run list, meaning that all runnable threads
         * are contended.
         */
        cpu_pause();
        ntd = &gd->gd_idlethread;
#ifdef SMP
        if (gd->gd_trap_nesting_level == 0 && panicstr == NULL)
            ASSERT_NO_TOKENS_HELD(ntd);
        /* contention case, do not clear contention mask */
#endif

        /*
         * Ok, we might want to spin a few times as some tokens are held for
         * very short periods of time and IPI overhead is 1uS or worse
         * (meaning it is usually better to spin).  Regardless we have to
         * call splz_check() to be sure to service any interrupts blocked
         * by our critical section, otherwise we could livelock e.g. IPIs.
         *
         * The IPI mechanic is really a last resort.  In nearly all other
         * cases RQF_WAKEUP is left set to prevent decontention IPIs.
         *
         * When we decide not to spin we clear RQF_WAKEUP and switch to
         * the idle thread.  Clearing RQF_WEAKEUP allows the idle thread
         * to halt and decontended tokens will issue an IPI to us.  The
         * idle thread will check for pending reschedules already set
         * (RQF_AST_LWKT_RESCHED) before actually halting so we don't have
         * to here.
         *
         * Also, if TDF_RUNQ is not set the current thread is trying to
         * deschedule, possibly in an atomic fashion.  We cannot afford to
         * stay here.
         */
        if (spinning <= 0 || (td->td_flags & TDF_RUNQ) == 0) {
            atomic_clear_int(&gd->gd_reqflags, RQF_WAKEUP);
            goto haveidle;
        }
        --spinning;

        /*
         * When spinning a delay is required both to avoid livelocks from
         * token order reversals (a thread may be trying to acquire multiple
         * tokens), and also to reduce cpu cache management traffic.
         *
         * In order to scale to a large number of CPUs we use a time slot
         * resequencer to force contending cpus into non-contending
         * time-slots.  The scheduler may still contend with the lock holder
         * but will not (generally) contend with all the other cpus trying
         * trying to get the same token.
         *
         * The resequencer uses a FIFO counter mechanic.  The owner of the
         * rindex at the head of the FIFO is allowed to pull itself off
         * the FIFO and fetchadd is used to enter into the FIFO.  This bit
         * of code is VERY cache friendly and forces all spinning schedulers
         * into their own time slots.
         *
         * This code has been tested to 48-cpus and caps the cache
         * contention load at ~1uS intervals regardless of the number of
         * cpus.  Scaling beyond 64 cpus might require additional smarts
         * (such as separate FIFOs for specific token cases).
         *
         * WARNING!  We can't call splz_check() or anything else here as
         *           it could cause a deadlock.
         */
#if defined(INVARIANTS) && defined(__amd64__)
        if ((read_rflags() & PSL_I) == 0) {
                cpu_enable_intr();
                panic("lwkt_switch() called with interrupts disabled");
        }
#endif
        cseq = atomic_fetchadd_int(&lwkt_cseq_windex, 1);
        fatal_count = lwkt_spin_fatal;
        while ((oseq = lwkt_cseq_rindex) != cseq) {
            cpu_ccfence();
#if !defined(_KERNEL_VIRTUAL)
            if (cpu_mi_feature & CPU_MI_MONITOR) {
                cpu_mmw_pause_int(&lwkt_cseq_rindex, oseq);
            } else
#endif
            {
                DELAY(1);
                cpu_lfence();
            }
            if (fatal_count && --fatal_count == 0)
                panic("lwkt_switch: fatal spin wait");
        }
        cseq = lwkt_spin_delay; /* don't trust the system operator */
        cpu_ccfence();
        if (cseq < 1)
            cseq = 1;
        if (cseq > 1000)
            cseq = 1000;
        DELAY(cseq);
        atomic_add_int(&lwkt_cseq_rindex, 1);
        splz_check();   /* ok, we already checked that td is still scheduled */
        /* highest level for(;;) loop */
    }

havethread:
    /*
     * We must always decrement td_fairq_accum on non-idle threads just
     * in case a thread never gets a tick due to being in a continuous
     * critical section.  The page-zeroing code does this, for example.
     *
     * If the thread we came up with is a higher or equal priority verses
     * the thread at the head of the queue we move our thread to the
     * front.  This way we can always check the front of the queue.
     *
     * Clear gd_idle_repeat when doing a normal switch to a non-idle
     * thread.
     */
    ++gd->gd_cnt.v_swtch;
    --ntd->td_fairq_accum;
    ntd->td_wmesg = NULL;
    xtd = TAILQ_FIRST(&gd->gd_tdrunq);
    if (ntd != xtd && ntd->td_pri >= xtd->td_pri) {
        TAILQ_REMOVE(&gd->gd_tdrunq, ntd, td_threadq);
        TAILQ_INSERT_HEAD(&gd->gd_tdrunq, ntd, td_threadq);
    }
    gd->gd_idle_repeat = 0;

havethread_preempted:
    /*
     * If the new target does not need the MP lock and we are holding it,
     * release the MP lock.  If the new target requires the MP lock we have
     * already acquired it for the target.
     */
    ;
haveidle:
    KASSERT(ntd->td_critcount,
            ("priority problem in lwkt_switch %d %d",
            td->td_critcount, ntd->td_critcount));

    if (td != ntd) {
        /*
         * Execute the actual thread switch operation.  This function
         * returns to the current thread and returns the previous thread
         * (which may be different from the thread we switched to).
         *
         * We are responsible for marking ntd as TDF_RUNNING.
         */
        ++switch_count;
        KTR_LOG(ctxsw_sw, gd->gd_cpuid, ntd);
        ntd->td_flags |= TDF_RUNNING;
        lwkt_switch_return(td->td_switch(ntd));
        /* ntd invalid, td_switch() can return a different thread_t */
    }
    /* NOTE: current cpu may have changed after switch */
    crit_exit_quick(td);
}

/*
 * Called by assembly in the td_switch (thread restore path) for thread
 * bootstrap cases which do not 'return' to lwkt_switch().
 */
void
lwkt_switch_return(thread_t otd)
{
#ifdef SMP
        globaldata_t rgd;

        /*
         * Check if otd was migrating.  Now that we are on ntd we can finish
         * up the migration.  This is a bit messy but it is the only place
         * where td is known to be fully descheduled.
         *
         * We can only activate the migration if otd was migrating but not
         * held on the cpu due to a preemption chain.  We still have to
         * clear TDF_RUNNING on the old thread either way.
         *
         * We are responsible for clearing the previously running thread's
         * TDF_RUNNING.
         */
        if ((rgd = otd->td_migrate_gd) != NULL &&
            (otd->td_flags & TDF_PREEMPT_LOCK) == 0) {
                KKASSERT((otd->td_flags & (TDF_MIGRATING | TDF_RUNNING)) ==
                         (TDF_MIGRATING | TDF_RUNNING));
                otd->td_migrate_gd = NULL;
                otd->td_flags &= ~TDF_RUNNING;
                lwkt_send_ipiq(rgd, lwkt_setcpu_remote, otd);
        } else {
                otd->td_flags &= ~TDF_RUNNING;
        }
#else
        otd->td_flags &= ~TDF_RUNNING;
#endif
}

/*
 * Request that the target thread preempt the current thread.  Preemption
 * only works under a specific set of conditions:
 *
 *      - We are not preempting ourselves
 *      - The target thread is owned by the current cpu
 *      - We are not currently being preempted
 *      - The target is not currently being preempted
 *      - We are not holding any spin locks
 *      - The target thread is not holding any tokens
 *      - We are able to satisfy the target's MP lock requirements (if any).
 *
 * THE CALLER OF LWKT_PREEMPT() MUST BE IN A CRITICAL SECTION.  Typically
 * this is called via lwkt_schedule() through the td_preemptable callback.
 * critcount is the managed critical priority that we should ignore in order
 * to determine whether preemption is possible (aka usually just the crit
 * priority of lwkt_schedule() itself).
 *
 * XXX at the moment we run the target thread in a critical section during
 * the preemption in order to prevent the target from taking interrupts
 * that *WE* can't.  Preemption is strictly limited to interrupt threads
 * and interrupt-like threads, outside of a critical section, and the
 * preempted source thread will be resumed the instant the target blocks
 * whether or not the source is scheduled (i.e. preemption is supposed to
 * be as transparent as possible).
 */
void
lwkt_preempt(thread_t ntd, int critcount)
{
    struct globaldata *gd = mycpu;
    thread_t xtd;
    thread_t td;
    int save_gd_intr_nesting_level;

    /*
     * The caller has put us in a critical section.  We can only preempt
     * if the caller of the caller was not in a critical section (basically
     * a local interrupt), as determined by the 'critcount' parameter.  We
     * also can't preempt if the caller is holding any spinlocks (even if
     * he isn't in a critical section).  This also handles the tokens test.
     *
     * YYY The target thread must be in a critical section (else it must
     * inherit our critical section?  I dunno yet).
     *
     * Set need_lwkt_resched() unconditionally for now YYY.
     */
    KASSERT(ntd->td_critcount, ("BADCRIT0 %d", ntd->td_pri));

    if (preempt_enable == 0) {
        ++preempt_miss;
        return;
    }

    td = gd->gd_curthread;
    if (ntd->td_pri <= td->td_pri) {
        ++preempt_miss;
        return;
    }
    if (td->td_critcount > critcount) {
        ++preempt_miss;
        need_lwkt_resched();
        return;
    }
#ifdef SMP
    if (ntd->td_gd != gd) {
        ++preempt_miss;
        need_lwkt_resched();
        return;
    }
#endif
    /*
     * We don't have to check spinlocks here as they will also bump
     * td_critcount.
     *
     * Do not try to preempt if the target thread is holding any tokens.
     * We could try to acquire the tokens but this case is so rare there
     * is no need to support it.
     */
    KKASSERT(gd->gd_spinlocks_wr == 0);

    if (TD_TOKS_HELD(ntd)) {
        ++preempt_miss;
        need_lwkt_resched();
        return;
    }
    if (td == ntd || ((td->td_flags | ntd->td_flags) & TDF_PREEMPT_LOCK)) {
        ++preempt_weird;
        need_lwkt_resched();
        return;
    }
    if (ntd->td_preempted) {
        ++preempt_hit;
        need_lwkt_resched();
        return;
    }
    KKASSERT(gd->gd_processing_ipiq == 0);

    /*
     * Since we are able to preempt the current thread, there is no need to
     * call need_lwkt_resched().
     *
     * We must temporarily clear gd_intr_nesting_level around the switch
     * since switchouts from the target thread are allowed (they will just
     * return to our thread), and since the target thread has its own stack.
     *
     * A preemption must switch back to the original thread, assert the
     * case.
     */
    ++preempt_hit;
    ntd->td_preempted = td;
    td->td_flags |= TDF_PREEMPT_LOCK;
    KTR_LOG(ctxsw_pre, gd->gd_cpuid, ntd);
    save_gd_intr_nesting_level = gd->gd_intr_nesting_level;
    gd->gd_intr_nesting_level = 0;
    ntd->td_flags |= TDF_RUNNING;
    xtd = td->td_switch(ntd);
    KKASSERT(xtd == ntd);
    lwkt_switch_return(xtd);
    gd->gd_intr_nesting_level = save_gd_intr_nesting_level;

    KKASSERT(ntd->td_preempted && (td->td_flags & TDF_PREEMPT_DONE));
    ntd->td_preempted = NULL;
    td->td_flags &= ~(TDF_PREEMPT_LOCK|TDF_PREEMPT_DONE);
}

/*
 * Conditionally call splz() if gd_reqflags indicates work is pending.
 * This will work inside a critical section but not inside a hard code
 * section.
 *
 * (self contained on a per cpu basis)
 */
void
splz_check(void)
{
    globaldata_t gd = mycpu;
    thread_t td = gd->gd_curthread;

    if ((gd->gd_reqflags & RQF_IDLECHECK_MASK) &&
        gd->gd_intr_nesting_level == 0 &&
        td->td_nest_count < 2)
    {
        splz();
    }
}

/*
 * This version is integrated into crit_exit, reqflags has already
 * been tested but td_critcount has not.
 *
 * We only want to execute the splz() on the 1->0 transition of
 * critcount and not in a hard code section or if too deeply nested.
 */
void
lwkt_maybe_splz(thread_t td)
{
    globaldata_t gd = td->td_gd;

    if (td->td_critcount == 0 &&
        gd->gd_intr_nesting_level == 0 &&
        td->td_nest_count < 2)
    {
        splz();
    }
}

/*
 * This function is used to negotiate a passive release of the current
 * process/lwp designation with the user scheduler, allowing the user
 * scheduler to schedule another user thread.  The related kernel thread
 * (curthread) continues running in the released state.
 */
void
lwkt_passive_release(struct thread *td)
{
    struct lwp *lp = td->td_lwp;

    td->td_release = NULL;
    lwkt_setpri_self(TDPRI_KERN_USER);
    lp->lwp_proc->p_usched->release_curproc(lp);
}


/*
 * This implements a normal yield.  This routine is virtually a nop if
 * there is nothing to yield to but it will always run any pending interrupts
 * if called from a critical section.
 *
 * This yield is designed for kernel threads without a user context.
 *
 * (self contained on a per cpu basis)
 */
void
lwkt_yield(void)
{
    globaldata_t gd = mycpu;
    thread_t td = gd->gd_curthread;
    thread_t xtd;

    if ((gd->gd_reqflags & RQF_IDLECHECK_MASK) && td->td_nest_count < 2)
        splz();
    if (td->td_fairq_accum < 0) {
        lwkt_schedule_self(curthread);
        lwkt_switch();
    } else {
        xtd = TAILQ_FIRST(&gd->gd_tdrunq);
        if (xtd && xtd->td_pri > td->td_pri) {
            lwkt_schedule_self(curthread);
            lwkt_switch();
        }
    }
}

/*
 * This yield is designed for kernel threads with a user context.
 *
 * The kernel acting on behalf of the user is potentially cpu-bound,
 * this function will efficiently allow other threads to run and also
 * switch to other processes by releasing.
 *
 * The lwkt_user_yield() function is designed to have very low overhead
 * if no yield is determined to be needed.
 */
void
lwkt_user_yield(void)
{
    globaldata_t gd = mycpu;
    thread_t td = gd->gd_curthread;

    /*
     * Always run any pending interrupts in case we are in a critical
     * section.
     */
    if ((gd->gd_reqflags & RQF_IDLECHECK_MASK) && td->td_nest_count < 2)
        splz();

    /*
     * Switch (which forces a release) if another kernel thread needs
     * the cpu, if userland wants us to resched, or if our kernel
     * quantum has run out.
     */
    if (lwkt_resched_wanted() ||
        user_resched_wanted() ||
        td->td_fairq_accum < 0)
    {
        lwkt_switch();
    }

#if 0
    /*
     * Reacquire the current process if we are released.
     *
     * XXX not implemented atm.  The kernel may be holding locks and such,
     *     so we want the thread to continue to receive cpu.
     */
    if (td->td_release == NULL && lp) {
        lp->lwp_proc->p_usched->acquire_curproc(lp);
        td->td_release = lwkt_passive_release;
        lwkt_setpri_self(TDPRI_USER_NORM);
    }
#endif
}

/*
 * Generic schedule.  Possibly schedule threads belonging to other cpus and
 * deal with threads that might be blocked on a wait queue.
 *
 * We have a little helper inline function which does additional work after
 * the thread has been enqueued, including dealing with preemption and
 * setting need_lwkt_resched() (which prevents the kernel from returning
 * to userland until it has processed higher priority threads).
 *
 * It is possible for this routine to be called after a failed _enqueue
 * (due to the target thread migrating, sleeping, or otherwise blocked).
 * We have to check that the thread is actually on the run queue!
 *
 * reschedok is an optimized constant propagated from lwkt_schedule() or
 * lwkt_schedule_noresched().  By default it is non-zero, causing a
 * reschedule to be requested if the target thread has a higher priority.
 * The port messaging code will set MSG_NORESCHED and cause reschedok to
 * be 0, prevented undesired reschedules.
 */
static __inline
void
_lwkt_schedule_post(globaldata_t gd, thread_t ntd, int ccount, int reschedok)
{
    thread_t otd;

    if (ntd->td_flags & TDF_RUNQ) {
        if (ntd->td_preemptable && reschedok) {
            ntd->td_preemptable(ntd, ccount);   /* YYY +token */
        } else if (reschedok) {
            otd = curthread;
            if (ntd->td_pri > otd->td_pri)
                need_lwkt_resched();
        }

        /*
         * Give the thread a little fair share scheduler bump if it
         * has been asleep for a while.  This is primarily to avoid
         * a degenerate case for interrupt threads where accumulator
         * crosses into negative territory unnecessarily.
         */
        if (ntd->td_fairq_lticks != ticks) {
            ntd->td_fairq_lticks = ticks;
            ntd->td_fairq_accum += gd->gd_fairq_total_pri;
            if (ntd->td_fairq_accum > TDFAIRQ_MAX(gd))
                    ntd->td_fairq_accum = TDFAIRQ_MAX(gd);
        }
    }
}

static __inline
void
_lwkt_schedule(thread_t td, int reschedok)
{
    globaldata_t mygd = mycpu;

    KASSERT(td != &td->td_gd->gd_idlethread,
            ("lwkt_schedule(): scheduling gd_idlethread is illegal!"));
    KKASSERT((td->td_flags & TDF_MIGRATING) == 0);
    crit_enter_gd(mygd);
    KKASSERT(td->td_lwp == NULL || (td->td_lwp->lwp_flag & LWP_ONRUNQ) == 0);
    if (td == mygd->gd_curthread) {
        _lwkt_enqueue(td);
    } else {
        /*
         * If we own the thread, there is no race (since we are in a
         * critical section).  If we do not own the thread there might
         * be a race but the target cpu will deal with it.
         */
#ifdef SMP
        if (td->td_gd == mygd) {
            _lwkt_enqueue(td);
            _lwkt_schedule_post(mygd, td, 1, reschedok);
        } else {
            lwkt_send_ipiq3(td->td_gd, lwkt_schedule_remote, td, 0);
        }
#else
        _lwkt_enqueue(td);
        _lwkt_schedule_post(mygd, td, 1, reschedok);
#endif
    }
    crit_exit_gd(mygd);
}

void
lwkt_schedule(thread_t td)
{
    _lwkt_schedule(td, 1);
}

void
lwkt_schedule_noresched(thread_t td)
{
    _lwkt_schedule(td, 0);
}

#ifdef SMP

/*
 * When scheduled remotely if frame != NULL the IPIQ is being
 * run via doreti or an interrupt then preemption can be allowed.
 *
 * To allow preemption we have to drop the critical section so only
 * one is present in _lwkt_schedule_post.
 */
static void
lwkt_schedule_remote(void *arg, int arg2, struct intrframe *frame)
{
    thread_t td = curthread;
    thread_t ntd = arg;

    if (frame && ntd->td_preemptable) {
        crit_exit_noyield(td);
        _lwkt_schedule(ntd, 1);
        crit_enter_quick(td);
    } else {
        _lwkt_schedule(ntd, 1);
    }
}

/*
 * Thread migration using a 'Pull' method.  The thread may or may not be
 * the current thread.  It MUST be descheduled and in a stable state.
 * lwkt_giveaway() must be called on the cpu owning the thread.
 *
 * At any point after lwkt_giveaway() is called, the target cpu may
 * 'pull' the thread by calling lwkt_acquire().
 *
 * We have to make sure the thread is not sitting on a per-cpu tsleep
 * queue or it will blow up when it moves to another cpu.
 *
 * MPSAFE - must be called under very specific conditions.
 */
void
lwkt_giveaway(thread_t td)
{
    globaldata_t gd = mycpu;

    crit_enter_gd(gd);
    if (td->td_flags & TDF_TSLEEPQ)
        tsleep_remove(td);
    KKASSERT(td->td_gd == gd);
    TAILQ_REMOVE(&gd->gd_tdallq, td, td_allq);
    td->td_flags |= TDF_MIGRATING;
    crit_exit_gd(gd);
}

void
lwkt_acquire(thread_t td)
{
    globaldata_t gd;
    globaldata_t mygd;
    int retry = 10000000;

    KKASSERT(td->td_flags & TDF_MIGRATING);
    gd = td->td_gd;
    mygd = mycpu;
    if (gd != mycpu) {
        cpu_lfence();
        KKASSERT((td->td_flags & TDF_RUNQ) == 0);
        crit_enter_gd(mygd);
        DEBUG_PUSH_INFO("lwkt_acquire");
        while (td->td_flags & (TDF_RUNNING|TDF_PREEMPT_LOCK)) {
#ifdef SMP
            lwkt_process_ipiq();
#endif
            cpu_lfence();
            if (--retry == 0) {
                kprintf("lwkt_acquire: stuck: td %p td->td_flags %08x\n",
                        td, td->td_flags);
                retry = 10000000;
            }
        }
        DEBUG_POP_INFO();
        cpu_mfence();
        td->td_gd = mygd;
        TAILQ_INSERT_TAIL(&mygd->gd_tdallq, td, td_allq);
        td->td_flags &= ~TDF_MIGRATING;
        crit_exit_gd(mygd);
    } else {
        crit_enter_gd(mygd);
        TAILQ_INSERT_TAIL(&mygd->gd_tdallq, td, td_allq);
        td->td_flags &= ~TDF_MIGRATING;
        crit_exit_gd(mygd);
    }
}

#endif

/*
 * Generic deschedule.  Descheduling threads other then your own should be
 * done only in carefully controlled circumstances.  Descheduling is 
 * asynchronous.  
 *
 * This function may block if the cpu has run out of messages.
 */
void
lwkt_deschedule(thread_t td)
{
    crit_enter();
#ifdef SMP
    if (td == curthread) {
        _lwkt_dequeue(td);
    } else {
        if (td->td_gd == mycpu) {
            _lwkt_dequeue(td);
        } else {
            lwkt_send_ipiq(td->td_gd, (ipifunc1_t)lwkt_deschedule, td);
        }
    }
#else
    _lwkt_dequeue(td);
#endif
    crit_exit();
}

/*
 * Set the target thread's priority.  This routine does not automatically
 * switch to a higher priority thread, LWKT threads are not designed for
 * continuous priority changes.  Yield if you want to switch.
 */
void
lwkt_setpri(thread_t td, int pri)
{
    KKASSERT(td->td_gd == mycpu);
    if (td->td_pri != pri) {
        KKASSERT(pri >= 0);
        crit_enter();
        if (td->td_flags & TDF_RUNQ) {
            _lwkt_dequeue(td);
            td->td_pri = pri;
            _lwkt_enqueue(td);
        } else {
            td->td_pri = pri;
        }
        crit_exit();
    }
}

/*
 * Set the initial priority for a thread prior to it being scheduled for
 * the first time.  The thread MUST NOT be scheduled before or during
 * this call.  The thread may be assigned to a cpu other then the current
 * cpu.
 *
 * Typically used after a thread has been created with TDF_STOPPREQ,
 * and before the thread is initially scheduled.
 */
void
lwkt_setpri_initial(thread_t td, int pri)
{
    KKASSERT(pri >= 0);
    KKASSERT((td->td_flags & TDF_RUNQ) == 0);
    td->td_pri = pri;
}

void
lwkt_setpri_self(int pri)
{
    thread_t td = curthread;

    KKASSERT(pri >= 0 && pri <= TDPRI_MAX);
    crit_enter();
    if (td->td_flags & TDF_RUNQ) {
        _lwkt_dequeue(td);
        td->td_pri = pri;
        _lwkt_enqueue(td);
    } else {
        td->td_pri = pri;
    }
    crit_exit();
}

/*
 * 1/hz tick (typically 10ms) x TDFAIRQ_SCALE (typ 8) = 80ms full cycle.
 *
 * Example: two competing threads, same priority N.  decrement by (2*N)
 * increment by N*8, each thread will get 4 ticks.
 */
void
lwkt_fairq_schedulerclock(thread_t td)
{
    globaldata_t gd;

    if (fairq_enable) {
        while (td) {
            gd = td->td_gd;
            if (td != &gd->gd_idlethread) {
                td->td_fairq_accum -= gd->gd_fairq_total_pri;
                if (td->td_fairq_accum < -TDFAIRQ_MAX(gd))
                        td->td_fairq_accum = -TDFAIRQ_MAX(gd);
                if (td->td_fairq_accum < 0)
                        need_lwkt_resched();
                td->td_fairq_lticks = ticks;
            }
            td = td->td_preempted;
        }
    }
}

static void
lwkt_fairq_accumulate(globaldata_t gd, thread_t td)
{
        td->td_fairq_accum += td->td_pri * TDFAIRQ_SCALE;
        if (td->td_fairq_accum > TDFAIRQ_MAX(td->td_gd))
                td->td_fairq_accum = TDFAIRQ_MAX(td->td_gd);
}

/*
 * Migrate the current thread to the specified cpu. 
 *
 * This is accomplished by descheduling ourselves from the current cpu
 * and setting td_migrate_gd.  The lwkt_switch() code will detect that the
 * 'old' thread wants to migrate after it has been completely switched out
 * and will complete the migration.
 *
 * TDF_MIGRATING prevents scheduling races while the thread is being migrated.
 *
 * We must be sure to release our current process designation (if a user
 * process) before clearing out any tsleepq we are on because the release
 * code may re-add us.
 *
 * We must be sure to remove ourselves from the current cpu's tsleepq
 * before potentially moving to another queue.  The thread can be on
 * a tsleepq due to a left-over tsleep_interlock().
 */

void
lwkt_setcpu_self(globaldata_t rgd)
{
#ifdef SMP
    thread_t td = curthread;

    if (td->td_gd != rgd) {
        crit_enter_quick(td);

        if (td->td_release)
            td->td_release(td);
        if (td->td_flags & TDF_TSLEEPQ)
            tsleep_remove(td);

        /*
         * Set TDF_MIGRATING to prevent a spurious reschedule while we are
         * trying to deschedule ourselves and switch away, then deschedule
         * ourself, remove us from tdallq, and set td_migrate_gd.  Finally,
         * call lwkt_switch() to complete the operation.
         */
        td->td_flags |= TDF_MIGRATING;
        lwkt_deschedule_self(td);
        TAILQ_REMOVE(&td->td_gd->gd_tdallq, td, td_allq);
        td->td_migrate_gd = rgd;
        lwkt_switch();

        /*
         * We are now on the target cpu
         */
        KKASSERT(rgd == mycpu);
        TAILQ_INSERT_TAIL(&rgd->gd_tdallq, td, td_allq);
        crit_exit_quick(td);
    }
#endif
}

void
lwkt_migratecpu(int cpuid)
{
#ifdef SMP
        globaldata_t rgd;

        rgd = globaldata_find(cpuid);
        lwkt_setcpu_self(rgd);
#endif
}

#ifdef SMP
/*
 * Remote IPI for cpu migration (called while in a critical section so we
 * do not have to enter another one).
 *
 * The thread (td) has already been completely descheduled from the
 * originating cpu and we can simply assert the case.  The thread is
 * assigned to the new cpu and enqueued.
 *
 * The thread will re-add itself to tdallq when it resumes execution.
 */
static void
lwkt_setcpu_remote(void *arg)
{
    thread_t td = arg;
    globaldata_t gd = mycpu;

    KKASSERT((td->td_flags & (TDF_RUNNING|TDF_PREEMPT_LOCK)) == 0);
    td->td_gd = gd;
    cpu_mfence();
    td->td_flags &= ~TDF_MIGRATING;
    KKASSERT(td->td_migrate_gd == NULL);
    KKASSERT(td->td_lwp == NULL || (td->td_lwp->lwp_flag & LWP_ONRUNQ) == 0);
    _lwkt_enqueue(td);
}
#endif

struct lwp *
lwkt_preempted_proc(void)
{
    thread_t td = curthread;
    while (td->td_preempted)
        td = td->td_preempted;
    return(td->td_lwp);
}

/*
 * Create a kernel process/thread/whatever.  It shares it's address space
 * with proc0 - ie: kernel only.
 *
 * NOTE!  By default new threads are created with the MP lock held.  A 
 * thread which does not require the MP lock should release it by calling
 * rel_mplock() at the start of the new thread.
 */
int
lwkt_create(void (*func)(void *), void *arg, struct thread **tdp,
            thread_t template, int tdflags, int cpu, const char *fmt, ...)
{
    thread_t td;
    __va_list ap;

    td = lwkt_alloc_thread(template, LWKT_THREAD_STACK, cpu,
                           tdflags);
    if (tdp)
        *tdp = td;
    cpu_set_thread_handler(td, lwkt_exit, func, arg);

    /*
     * Set up arg0 for 'ps' etc
     */
    __va_start(ap, fmt);
    kvsnprintf(td->td_comm, sizeof(td->td_comm), fmt, ap);
    __va_end(ap);

    /*
     * Schedule the thread to run
     */
    if ((td->td_flags & TDF_STOPREQ) == 0)
        lwkt_schedule(td);
    else
        td->td_flags &= ~TDF_STOPREQ;
    return 0;
}

/*
 * Destroy an LWKT thread.   Warning!  This function is not called when
 * a process exits, cpu_proc_exit() directly calls cpu_thread_exit() and
 * uses a different reaping mechanism.
 */
void
lwkt_exit(void)
{
    thread_t td = curthread;
    thread_t std;
    globaldata_t gd;

    /*
     * Do any cleanup that might block here
     */
    if (td->td_flags & TDF_VERBOSE)
        kprintf("kthread %p %s has exited\n", td, td->td_comm);
    caps_exit(td);
    biosched_done(td);
    dsched_exit_thread(td);

    /*
     * Get us into a critical section to interlock gd_freetd and loop
     * until we can get it freed.
     *
     * We have to cache the current td in gd_freetd because objcache_put()ing
     * it would rip it out from under us while our thread is still active.
     */
    gd = mycpu;
    crit_enter_quick(td);
    while ((std = gd->gd_freetd) != NULL) {
        KKASSERT((std->td_flags & (TDF_RUNNING|TDF_PREEMPT_LOCK)) == 0);
        gd->gd_freetd = NULL;
        objcache_put(thread_cache, std);
    }

    /*
     * Remove thread resources from kernel lists and deschedule us for
     * the last time.  We cannot block after this point or we may end
     * up with a stale td on the tsleepq.
     */
    if (td->td_flags & TDF_TSLEEPQ)
        tsleep_remove(td);
    lwkt_deschedule_self(td);
    lwkt_remove_tdallq(td);
    KKASSERT(td->td_refs == 0);

    /*
     * Final cleanup
     */
    KKASSERT(gd->gd_freetd == NULL);
    if (td->td_flags & TDF_ALLOCATED_THREAD)
        gd->gd_freetd = td;
    cpu_thread_exit();
}

void
lwkt_remove_tdallq(thread_t td)
{
    KKASSERT(td->td_gd == mycpu);
    TAILQ_REMOVE(&td->td_gd->gd_tdallq, td, td_allq);
}

/*
 * Code reduction and branch prediction improvements.  Call/return
 * overhead on modern cpus often degenerates into 0 cycles due to
 * the cpu's branch prediction hardware and return pc cache.  We
 * can take advantage of this by not inlining medium-complexity
 * functions and we can also reduce the branch prediction impact
 * by collapsing perfectly predictable branches into a single
 * procedure instead of duplicating it.
 *
 * Is any of this noticeable?  Probably not, so I'll take the
 * smaller code size.
 */
void
crit_exit_wrapper(__DEBUG_CRIT_ARG__)
{
    _crit_exit(mycpu __DEBUG_CRIT_PASS_ARG__);
}

void
crit_panic(void)
{
    thread_t td = curthread;
    int lcrit = td->td_critcount;

    td->td_critcount = 0;
    panic("td_critcount is/would-go negative! %p %d", td, lcrit);
    /* NOT REACHED */
}

#ifdef SMP

/*
 * Called from debugger/panic on cpus which have been stopped.  We must still
 * process the IPIQ while stopped, even if we were stopped while in a critical
 * section (XXX).
 *
 * If we are dumping also try to process any pending interrupts.  This may
 * or may not work depending on the state of the cpu at the point it was
 * stopped.
 */
void
lwkt_smp_stopped(void)
{
    globaldata_t gd = mycpu;

    crit_enter_gd(gd);
    if (dumping) {
        lwkt_process_ipiq();
        splz();
    } else {
        lwkt_process_ipiq();
    }
    crit_exit_gd(gd);
}

#endif