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Date:      Sat, 4 Oct 2014 15:59:16 +0000 (UTC)
From:      Ian Lepore <ian@FreeBSD.org>
To:        src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org
Subject:   svn commit: r272528 - in head/sys: kern sys
Message-ID:  <201410041559.s94FxGXn024427@svn.freebsd.org>
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Author: ian
Date: Sat Oct  4 15:59:15 2014
New Revision: 272528
URL: https://svnweb.freebsd.org/changeset/base/272528

Log:
  Make kevent(2) periodic timer events more reliably periodic.  The event
  callout is now scheduled using the C_ABSOLUTE flag, and the absolute time
  of each event is calculated as the time the previous event was scheduled
  for plus the interval.  This ensures that latency in processing a given
  event doesn't perturb the arrival time of any subsequent events.
  
  Reviewed by:	jhb

Modified:
  head/sys/kern/kern_event.c
  head/sys/sys/event.h

Modified: head/sys/kern/kern_event.c
==============================================================================
--- head/sys/kern/kern_event.c	Sat Oct  4 15:42:52 2014	(r272527)
+++ head/sys/kern/kern_event.c	Sat Oct  4 15:59:15 2014	(r272528)
@@ -569,9 +569,10 @@ filt_timerexpire(void *knx)
 
 	if ((kn->kn_flags & EV_ONESHOT) != EV_ONESHOT) {
 		calloutp = (struct callout *)kn->kn_hook;
-		callout_reset_sbt_on(calloutp,
-		    timer2sbintime(kn->kn_sdata, kn->kn_sfflags), 0,
-		    filt_timerexpire, kn, PCPU_GET(cpuid), 0);
+		*kn->kn_ptr.p_nexttime += timer2sbintime(kn->kn_sdata, 
+		    kn->kn_sfflags);
+		callout_reset_sbt_on(calloutp, *kn->kn_ptr.p_nexttime, 0,
+		    filt_timerexpire, kn, PCPU_GET(cpuid), C_ABSOLUTE);
 	}
 }
 
@@ -607,11 +608,13 @@ filt_timerattach(struct knote *kn)
 
 	kn->kn_flags |= EV_CLEAR;		/* automatically set */
 	kn->kn_status &= ~KN_DETACHED;		/* knlist_add clears it */
+	kn->kn_ptr.p_nexttime = malloc(sizeof(sbintime_t), M_KQUEUE, M_WAITOK);
 	calloutp = malloc(sizeof(*calloutp), M_KQUEUE, M_WAITOK);
 	callout_init(calloutp, CALLOUT_MPSAFE);
 	kn->kn_hook = calloutp;
-	callout_reset_sbt_on(calloutp, to, 0,
-	    filt_timerexpire, kn, PCPU_GET(cpuid), 0);
+	*kn->kn_ptr.p_nexttime = to + sbinuptime();
+	callout_reset_sbt_on(calloutp, *kn->kn_ptr.p_nexttime, 0,
+	    filt_timerexpire, kn, PCPU_GET(cpuid), C_ABSOLUTE);
 
 	return (0);
 }
@@ -625,6 +628,7 @@ filt_timerdetach(struct knote *kn)
 	calloutp = (struct callout *)kn->kn_hook;
 	callout_drain(calloutp);
 	free(calloutp, M_KQUEUE);
+	free(kn->kn_ptr.p_nexttime, M_KQUEUE);
 	old = atomic_fetch_sub_explicit(&kq_ncallouts, 1, memory_order_relaxed);
 	KASSERT(old > 0, ("Number of callouts cannot become negative"));
 	kn->kn_status |= KN_DETACHED;	/* knlist_remove sets it */

Modified: head/sys/sys/event.h
==============================================================================
--- head/sys/sys/event.h	Sat Oct  4 15:42:52 2014	(r272527)
+++ head/sys/sys/event.h	Sat Oct  4 15:59:15 2014	(r272528)
@@ -221,6 +221,7 @@ struct knote {
 		struct		proc *p_proc;	/* proc pointer */
 		struct		aiocblist *p_aio;	/* AIO job pointer */
 		struct		aioliojob *p_lio;	/* LIO job pointer */
+		sbintime_t	*p_nexttime;	/* next timer event fires at */
 		void		*p_v;		/* generic other pointer */
 	} kn_ptr;
 	struct			filterops *kn_fop;

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