Re: [RFC] tick/nohz: schedule TIMER_SOFTIRQ immediately for expired timers

[Wanpeng Li]

2017-05-26 20:55 GMT+08:00 Frederic Weisbecker <fweisbec@xxxxxxxxx>:
> On Wed, May 24, 2017 at 03:39:24PM +0300, Octavian Purdila wrote:
>> Currently, when detecting expired timers in tick_nohz_stop_sched_tick
>> we just schedule a new hrtimer and let its handler to schedule
>> TIMER_SOFITRQ.
>>
>> This can lead to indefinite timer stalls if the system is busy with a
>> stream of interrupts that have the period shorter or about the same
>> with the value of the minimum delay of the current clocksource driver:
>>
>> -> idle
>> -> IRQ (time = x)
>>    -> irq_exit -> tick_nohz_irq_exit -> tick_nohz_stop_sched_tick
>>       -> tick_nohz_restart
>>          -> cancel hrtimer (set clocksource event to x + max_delay)
>>          -> set clocksource to x + min_delay
>> ...
>> -> IRQ (time = y, where y < x + min_delay)
>>    -> irq_exit -> tick_nohz_irq_exit -> tick_nohz_stop_sched_tick
>>       -> tick_nohz_restart
>>          -> cancel hrtimer (set clocksource event to x + max_delay)
>>          -> set clocksource to y + min_delay
>>
>> So, instead of prodding the hrtimer interrupt, schedule TIMER_SOFTIRQ
>> since we know that timers are ready. The timers will run either at the
>> next interrupt or from ksoftirq so no hrtimer interrupt is
>> needed. This also avoids spurious programming of the clocksource in
>> this scenario.
>>
>> Signed-off-by: Octavian Purdila <octavian.purdila@xxxxxxx>
>> ---
>>  kernel/time/tick-sched.c | 9 ++++++++-
>>  1 file changed, 8 insertions(+), 1 deletion(-)
>>
>> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
>> index 3bcb61b..0a30278 100644
>> --- a/kernel/time/tick-sched.c
>> +++ b/kernel/time/tick-sched.c
>> @@ -727,9 +727,16 @@ static ktime_t tick_nohz_stop_sched_tick(struct tick_sched *ts,
>>                *
>>                * Only once we exit the idle loop will we re-enable the tick,
>>                * see tick_nohz_idle_exit().
>> +              *
>> +              * Also, make sure we schedule TIMER_SOFTIRQ now instead of
>> +              * relying on the hrtimer interrupt to do it to avoid
>> +              * postponing processing of expired timers. If we have a
>> +              * constant stream of interrupts with a period shorter than
>> +              * the minimum delay of the current clocksource we can end up
>> +              * postponing the timers indefinitely.
>>                */
>>               if (delta == 0) {
>> -                     tick_nohz_restart(ts, now);
>> +                     raise_softirq(TIMER_SOFTIRQ);
>>                       goto out;
>>               }
>>       }
>
> Nice catch! And the patch should work but that actually restores a behaviour we've
> removed some time ago. We wanted to get rid of that softirq raise.
>
> So discussing this with Thomas, here is an alternate solution. That tick restart looks
> like an unnecessary special case. In fact the normal path ending with hrtimer_start()
> in tick_nohz_stop_sched_tick() should work if the delta deadline is 0. And if
> we do so, we benefit from the optimization path:
>
>         if (ts->tick_stopped && (expires == dev->next_event))
>
> ...which avoids the cancel/reprog game and therefore should fix that issue.
>
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index ed18ca5..58c257c 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -713,8 +713,6 @@ static ktime_t tick_nohz_stop_sched_tick(struct tick_sched *ts,
>          */
>         delta = next_tick - basemono;
>         if (delta <= (u64)TICK_NSEC) {
> -               tick = 0;
> -
>                 /*
>                  * Tell the timer code that the base is not idle, i.e. undo
>                  * the effect of get_next_timer_interrupt():
> @@ -724,23 +722,8 @@ static ktime_t tick_nohz_stop_sched_tick(struct tick_sched *ts,
>                  * We've not stopped the tick yet, and there's a timer in the
>                  * next period, so no point in stopping it either, bail.
>                  */
> -               if (!ts->tick_stopped)
> -                       goto out;
> -
> -               /*
> -                * If, OTOH, we did stop it, but there's a pending (expired)
> -                * timer reprogram the timer hardware to fire now.
> -                *
> -                * We will not restart the tick proper, just prod the timer
> -                * hardware into firing an interrupt to process the pending
> -                * timers. Just like tick_irq_exit() will not restart the tick
> -                * for 'normal' interrupts.
> -                *
> -                * Only once we exit the idle loop will we re-enable the tick,
> -                * see tick_nohz_idle_exit().
> -                */
> -               if (delta == 0) {
> -                       tick_nohz_restart(ts, now);
> +               if (!ts->tick_stopped) {
> +                       tick = 0;
>                         goto out;
>                 }
>         }
>

Reviewed-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>