linux-kernel - [GIT PULL] nohz fix

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Date:   Sun, 15 Jan 2017 11:03:35 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Frédéric Weisbecker <fweisbec@...il.com>
Subject: [GIT PULL] nohz fix

Linus,

Please pull the latest timers-urgent-for-linus git tree from:

   git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git timers-urgent-for-linus

   # HEAD: 24b91e360ef521a2808771633d76ebc68bd5604b nohz: Fix collision between tick and other hrtimers

This fixes an old NOHZ race where we incorrectly calculate the next timer 
interrupt in certain circumstances where hrtimers are pending, that can cause
hard to reproduce stalled-values artifacts in /proc/stat.

 Thanks,

	Ingo

------------------>
Frederic Weisbecker (1):
      nohz: Fix collision between tick and other hrtimers


 kernel/time/tick-sched.c | 9 +++++++--
 kernel/time/tick-sched.h | 2 ++
 2 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 2c115fdab397..74e0388cc88d 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -767,7 +767,7 @@ static ktime_t tick_nohz_stop_sched_tick(struct tick_sched *ts,
 	tick = expires;
 
 	/* Skip reprogram of event if its not changed */
-	if (ts->tick_stopped && (expires == dev->next_event))
+	if (ts->tick_stopped && (expires == ts->next_tick))
 		goto out;
 
 	/*
@@ -787,6 +787,8 @@ static ktime_t tick_nohz_stop_sched_tick(struct tick_sched *ts,
 		trace_tick_stop(1, TICK_DEP_MASK_NONE);
 	}
 
+	ts->next_tick = tick;
+
 	/*
 	 * If the expiration time == KTIME_MAX, then we simply stop
 	 * the tick timer.
@@ -802,7 +804,10 @@ static ktime_t tick_nohz_stop_sched_tick(struct tick_sched *ts,
 	else
 		tick_program_event(tick, 1);
 out:
-	/* Update the estimated sleep length */
+	/*
+	 * Update the estimated sleep length until the next timer
+	 * (not only the tick).
+	 */
 	ts->sleep_length = ktime_sub(dev->next_event, now);
 	return tick;
 }
diff --git a/kernel/time/tick-sched.h b/kernel/time/tick-sched.h
index bf38226e5c17..075444e3d48e 100644
--- a/kernel/time/tick-sched.h
+++ b/kernel/time/tick-sched.h
@@ -27,6 +27,7 @@ enum tick_nohz_mode {
  *			timer is modified for nohz sleeps. This is necessary
  *			to resume the tick timer operation in the timeline
  *			when the CPU returns from nohz sleep.
+ * @next_tick:		Next tick to be fired when in dynticks mode.
  * @tick_stopped:	Indicator that the idle tick has been stopped
  * @idle_jiffies:	jiffies at the entry to idle for idle time accounting
  * @idle_calls:		Total number of idle calls
@@ -44,6 +45,7 @@ struct tick_sched {
 	unsigned long			check_clocks;
 	enum tick_nohz_mode		nohz_mode;
 	ktime_t				last_tick;
+	ktime_t				next_tick;
 	int				inidle;
 	int				tick_stopped;
 	unsigned long			idle_jiffies;