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* [PATCH] hrtimer: Reset hrtimer cpu base proper on CPU hotplug
@ 2018-01-26 13:54 Thomas Gleixner
  2018-01-26 22:09 ` Paul E. McKenney
  2018-01-27 14:31 ` [tip:timers/urgent] " tip-bot for Thomas Gleixner
  0 siblings, 2 replies; 9+ messages in thread
From: Thomas Gleixner @ 2018-01-26 13:54 UTC (permalink / raw)
  To: LKML
  Cc: Paul E. McKenney, Sebastian Sewior, Anna-Maria Gleixner,
	Peter Zijlstra, Ingo Molnar

The hrtimer interrupt code contains a hang detection and mitigation
mechanism, which prevents that a long delayed hrtimer interrupt causes a
continous retriggering of interrupts which prevent the system from making
progress. If a hang is detected then the timer hardware is programmed with
a certain delay into the future and a flag is set in the hrtimer cpu base
which prevents newly enqueued timers from reprogramming the timer hardware
prior to the chosen delay. The subsequent hrtimer interrupt after the delay
clears the flag and resumes normal operation.

If such a hang happens in the last hrtimer interrupt before a CPU is
unplugged then the hang_detected flag is set and stays that way when the
CPU is plugged in again. At that point the timer hardware is not armed and
it cannot be armed because the hang_detected flag is still active, so
nothing clears that flag. As a consequence the CPU does not receive hrtimer
interrupts and no timers expire on that CPU which results in RCU stalls and
other malfunctions.

Clear the flag along with some other less critical members of the hrtimer
cpu base to ensure starting from a clean state when a CPU is plugged in.

Thanks to Paul, Sebastian and Anna-Maria for their help to get down to the
root cause of that hard to reproduce heisenbug. Once understood it's
trivial and certainly justifies a brown paperbag.

Fixes: 41d2e4949377 ("hrtimer: Tune hrtimer_interrupt hang logic")
Reported-by: Paul E. McKenney <>
Signed-off-by: Thomas Gleixner <>
 kernel/time/hrtimer.c |    3 +++
 1 file changed, 3 insertions(+)

--- a/kernel/time/hrtimer.c
+++ b/kernel/time/hrtimer.c
@@ -655,7 +655,9 @@ static void hrtimer_reprogram(struct hrt
 static inline void hrtimer_init_hres(struct hrtimer_cpu_base *base)
 	base->expires_next = KTIME_MAX;
+	base->hang_detected = 0;
 	base->hres_active = 0;
+	base->next_timer = NULL;
@@ -1589,6 +1591,7 @@ int hrtimers_prepare_cpu(unsigned int cp
+	cpu_base->active_bases = 0;
 	cpu_base->cpu = cpu;
 	return 0;

^ permalink raw reply	[flat|nested] 9+ messages in thread

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Thread overview: 9+ messages (download: mbox.gz / follow: Atom feed)
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2018-01-26 13:54 [PATCH] hrtimer: Reset hrtimer cpu base proper on CPU hotplug Thomas Gleixner
2018-01-26 22:09 ` Paul E. McKenney
2018-01-28  0:53   ` Paul E. McKenney
2018-01-29  8:20   ` Sebastian Sewior
2018-01-29  9:57     ` Paul E. McKenney
2018-01-29 23:43       ` Paul E. McKenney
2018-01-30 21:03         ` Thomas Gleixner
2018-01-31  0:52           ` Paul E. McKenney
2018-01-27 14:31 ` [tip:timers/urgent] " tip-bot for Thomas Gleixner

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