Date: Wed, 17 Sep 2014 14:07:41 +0000 (UTC) From: Alexander Motin <mav@FreeBSD.org> To: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-stable@freebsd.org, svn-src-stable-9@freebsd.org Subject: svn commit: r271708 - stable/9/sys/kern Message-ID: <201409171407.s8HE7f1N071682@svn.freebsd.org>
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Author: mav Date: Wed Sep 17 14:07:40 2014 New Revision: 271708 URL: http://svnweb.freebsd.org/changeset/base/271708 Log: MFC r271604, r271616: Add couple memory barriers to order 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. Modified: stable/9/sys/kern/sched_ule.c Directory Properties: stable/9/ (props changed) stable/9/sys/ (props changed) Modified: stable/9/sys/kern/sched_ule.c ============================================================================== --- stable/9/sys/kern/sched_ule.c Wed Sep 17 14:06:21 2014 (r271707) +++ stable/9/sys/kern/sched_ule.c Wed Sep 17 14:07:40 2014 (r271708) @@ -1006,6 +1006,14 @@ tdq_notify(struct tdq *tdq, struct threa ctd = pcpu_find(cpu)->pc_curthread; if (!sched_shouldpreempt(pri, ctd->td_priority, 1)) return; + + /* + * Make sure that tdq_load updated before calling this function + * is globally visible before we read tdq_cpu_idle. Idle thread + * accesses both of them without locks, and the order is important. + */ + mb(); + if (TD_IS_IDLETHREAD(ctd)) { /* * If the MD code has an idle wakeup routine try that before @@ -2607,6 +2615,12 @@ sched_idletd(void *dummy) /* Run main MD idle handler. */ tdq->tdq_cpu_idle = 1; + /* + * Make sure that tdq_cpu_idle update is globally visible + * before cpu_idle() read tdq_load. The order is important + * to avoid race with tdq_notify. + */ + mb(); cpu_idle(switchcnt * 4 > sched_idlespinthresh); tdq->tdq_cpu_idle = 0;
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