nohz: Fix spurious warning when hrtimer and clockevent get out of sync
PATCH 1/1 nohz: Fix spurious warning when hrtimer and clockevent get out of sync
The sanity check ensuring that the tick expiry cache (ts->next_tick) is actually in sync with the hardware clock (dev->next_event) makes the wrong assumption that the clock can't be programmed later than the hrtimer deadline.
In fact the clock hardware can be programmed later on some conditions such as:
* The hrtimer deadline is already in the past.
* The hrtimer deadline is earlier than the minimum delay supported by the hardware.
Such conditions can be met when we program the tick, for example if the last jiffies update hasn't been seen by the current CPU yet, we may program the hrtimer to a deadline that is earlier than ktime_get() because last_jiffies_update is our timestamp base to compute the next tick.
As a result, we can randomly observe such warning:
WARNING: CPU: 5 PID: 0 at kernel/time/tick-sched.c:794 tick_nohz_stop_sched_tick kernel/time/tick-sched.c:791 [inline] Call Trace: tick_nohz_irq_exit tick_irq_exit irq_exit exiting_irq smp_call_function_interrupt smp_call_function_single_interrupt call_function_single_interrupt
Therefore, let's rather make sure that the tick expiry cache is sync'ed with the tick hrtimer deadline, against which it is not supposed to drift away. The clock hardware instead has its own will and can't be used as a reliable comparison point.
--- kernel/time/tick-sched.c | 7 +++++-- 1 file changed, 5 insertions(+), 2 deletions(-)
kernel/time/tick-sched.c +5 -2
@@ -768,7 +768,8 @@ static ktime_t tick_nohz_stop_sched_tick(struct tick_sched *ts,/* Skip reprogram of event if its not changed */ if (ts->tick_stopped && (expires == ts->next_tick)) {/* Sanity check: make sure clockevent is actually programmed */ -if (likely (dev->next_event <= ts->next_tick)) +if (tick ==KTIME_MAX || + ts->next_tick ==hrtimer_get_expires (&ts->sched_timer)) goto out;WARN_ON_ONCE (1 );
@@ -806,8 +807,10 @@ static ktime_t tick_nohz_stop_sched_tick(struct tick_sched *ts, goto out; } +hrtimer_set_expires (&ts->sched_timer, tick); +if (ts->nohz_mode ==NOHZ_MODE_HIGHRES ) -hrtimer_start (&ts->sched_timer, tick,HRTIMER_MODE_ABS_PINNED ); +hrtimer_start_expires (&ts->sched_timer,HRTIMER_MODE_ABS_PINNED );else tick_program_event (tick,1 ); out:
Signature · 1 line
2.7.4
tip-bot for Frederic Weisbecker
merge botnohz: Fix spurious warning when hrtimer and clockevent get out of sync
Commit-ID: d4af6d933ccffd24286528f04d5c39e702c8580f Gitweb: http://git.kernel.org/tip/d4af6d933ccffd24286528f04d5c39e702c8580f Author: Frederic Weisbecker <[email protected]> AuthorDate: Tue, 13 Jun 2017 06:04:14 +0200 Committer: Ingo Molnar <[email protected]> CommitDate: Tue, 13 Jun 2017 08:45:43 +0200
nohz: Fix spurious warning when hrtimer and clockevent get out of sync
The sanity check ensuring that the tick expiry cache (ts->next_tick) is actually in sync with the hardware clock (dev->next_event) makes the wrong assumption that the clock can't be programmed later than the hrtimer deadline.
In fact the clock hardware can be programmed later on some conditions such as:
* The hrtimer deadline is already in the past.
* The hrtimer deadline is earlier than the minimum delay supported by the hardware.
Such conditions can be met when we program the tick, for example if the last jiffies update hasn't been seen by the current CPU yet, we may program the hrtimer to a deadline that is earlier than ktime_get() because last_jiffies_update is our timestamp base to compute the next tick.
As a result, we can randomly observe such warning:
WARNING: CPU: 5 PID: 0 at kernel/time/tick-sched.c:794 tick_nohz_stop_sched_tick kernel/time/tick-sched.c:791 [inline] Call Trace: tick_nohz_irq_exit tick_irq_exit irq_exit exiting_irq smp_call_function_interrupt smp_call_function_single_interrupt call_function_single_interrupt
Therefore, let's rather make sure that the tick expiry cache is sync'ed with the tick hrtimer deadline, against which it is not supposed to drift away. The clock hardware instead has its own will and can't be used as a reliable comparison point.
--- kernel/time/tick-sched.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-)
kernel/time/tick-sched.c +4 -2
@@ -768,7 +768,7 @@ static ktime_t tick_nohz_stop_sched_tick(struct tick_sched *ts,/* Skip reprogram of event if its not changed */ if (ts->tick_stopped && (expires == ts->next_tick)) {/* Sanity check: make sure clockevent is actually programmed */ -if (likely (dev->next_event <= ts->next_tick)) +if (tick ==KTIME_MAX || ts->next_tick ==hrtimer_get_expires (&ts->sched_timer)) goto out;WARN_ON_ONCE (1 );
@@ -806,8 +806,10 @@ static ktime_t tick_nohz_stop_sched_tick(struct tick_sched *ts, goto out; } +hrtimer_set_expires (&ts->sched_timer, tick); +if (ts->nohz_mode ==NOHZ_MODE_HIGHRES ) -hrtimer_start (&ts->sched_timer, tick,HRTIMER_MODE_ABS_PINNED ); +hrtimer_start_expires (&ts->sched_timer,HRTIMER_MODE_ABS_PINNED );else tick_program_event (tick,1 ); out: