svn commit: r271604 - head/sys/kern

[Alexander Motin]

Author: mav
Date: Sun Sep 14 22:13:19 2014
New Revision: 271604
URL: http://svnweb.freebsd.org/changeset/base/271604

Log:
  Add couple memory barries to serialize tdq_cpu_idle and tdq_load accesses.
  
  This change fixes transient performance drops in some of my benchmarks,
  vanishing as soon as I am trying to collect any stats from the scheduler.
  It looks like reordered access to those variables sometimes caused loss of
  IPI_PREEMPT, that delayed thread execution until some later interrupt.
  
  MFC after:	3 days

Modified:
  head/sys/kern/sched_ule.c

Modified: head/sys/kern/sched_ule.c
==============================================================================
--- head/sys/kern/sched_ule.c	Sun Sep 14 22:10:35 2014	(r271603)
+++ head/sys/kern/sched_ule.c	Sun Sep 14 22:13:19 2014	(r271604)
@@ -1037,6 +1037,7 @@ tdq_notify(struct tdq *tdq, struct threa
 	ctd = pcpu_find(cpu)->pc_curthread;
 	if (!sched_shouldpreempt(pri, ctd->td_priority, 1))
 		return;
+	mb();
 	if (TD_IS_IDLETHREAD(ctd)) {
 		/*
 		 * If the MD code has an idle wakeup routine try that before
@@ -2640,6 +2641,7 @@ sched_idletd(void *dummy)
 
 		/* Run main MD idle handler. */
 		tdq->tdq_cpu_idle = 1;
+		mb();
 		cpu_idle(switchcnt * 4 > sched_idlespinthresh);
 		tdq->tdq_cpu_idle = 0;