kernel - Add usched_dfly algorith, set as default for now (6)

* Fix an edge case where the pairing could cause flapping.

* Fix an edge case where user processes were interrupting each other
  when they were in the same queue, which could cause a synchronous
  process like a postgres server to lose cpu while holding internal
  locks during a short operation.

diff --git a/sys/kern/usched_dfly.c b/sys/kern/usched_dfly.c
index 276b6a5..dff7d6a 100644 (file)
--- a/sys/kern/usched_dfly.c
+++ b/sys/kern/usched_dfly.c
@@ -211,7 +211,8 @@ SYSCTL_INT(_debug, OID_AUTO, dfly_chooser, CTLFLAG_RW,
 static int usched_dfly_smt = 0;
 static int usched_dfly_cache_coherent = 0;
 static int usched_dfly_weight1 = 25;   /* thread's current cpu */
-static int usched_dfly_weight2 = 15;   /* synchronous peer's current cpu */
+static int usched_dfly_weight2 = 0;    /* synchronous peer's current cpu */
+                                       /* XXX can cause cpu flapping */
 static int usched_dfly_weight3 = 10;   /* number of threads on queue */
 static int usched_dfly_pull_enable = 1;        /* allow pulls */
 #endif
@@ -402,13 +403,18 @@ dfly_acquire_curproc(struct lwp *lp)
                        dd->uschedcp = lp;
                        dd->upri = lp->lwp_priority;
                        KKASSERT(lp->lwp_qcpu == dd->cpuid);
-               } else if (dd->uschedcp && dd->upri > lp->lwp_priority) {
+               } else if (dd->uschedcp && (dd->upri & ~PPQMASK) >
+                                          (lp->lwp_priority & ~PPQMASK)) {
                        /*
                         * We can steal the current cpu's lwp designation
                         * away simply by replacing it.  The other thread
                         * will stall when it tries to return to userland,
                         * possibly rescheduling elsewhere when it calls
                         * setrunqueue.
+                        *
+                        * It is important to do a masked test to avoid the
+                        * edge case where two near-equal-priority threads
+                        * are constantly interrupting each other.
                         */
                        dd->uschedcp = lp;
                        dd->upri = lp->lwp_priority;