* Re: [PATCH] sched/fair: Change mode of period_timer to HRTIMER_MODE_ABS_PINNED_HARD
[not found] <20221020091742.2962-1-qiruipeng@lixiang.com>
@ 2022-10-20 16:39 ` Peter Zijlstra
2022-10-31 10:35 ` Valentin Schneider
1 sibling, 0 replies; 2+ messages in thread
From: Peter Zijlstra @ 2022-10-20 16:39 UTC (permalink / raw)
To: Ruipeng Qi
Cc: mingo, juri.lelli, vincent.guittot, dietmar.eggemann, rostedt,
bsegall, mgorman, bristot, vschneid, linux-kernel, ruipengqi7
On Thu, Oct 20, 2022 at 05:17:46PM +0800, Ruipeng Qi wrote:
> Previous mode of period_timer is HRTIMER_MODE_ABS_PINNED,which means
> timer callback function will be executed in soft irq context.
>
> When one task group runs out of quota in a period,but holding lock of
> softirq_ctrl,the task will be rescheduled later,and then a system stall
> occurs:
> - next period comes,
> - __do_softirq can't acquire lock of softirq_ctrl,
> - __do_softirq will not be invoked,
> - callback of period_timer will not be invoked,
> - task group will not get any quota in any new period.
> - a system stall occurs.
>
> Changing mode of period_timer to HRTIMER_MODE_ABS_PINNED_HARD,which mean
> timer callback function executed in hard irq context fix this problem.
What I'm missing here is a statement about how the runtime complexity of
that timer function is sufficiently bounded to run in hardirq context on
RT kernels.
^ permalink raw reply [flat|nested] 2+ messages in thread* Re: [PATCH] sched/fair: Change mode of period_timer to HRTIMER_MODE_ABS_PINNED_HARD
[not found] <20221020091742.2962-1-qiruipeng@lixiang.com>
2022-10-20 16:39 ` [PATCH] sched/fair: Change mode of period_timer to HRTIMER_MODE_ABS_PINNED_HARD Peter Zijlstra
@ 2022-10-31 10:35 ` Valentin Schneider
1 sibling, 0 replies; 2+ messages in thread
From: Valentin Schneider @ 2022-10-31 10:35 UTC (permalink / raw)
To: Ruipeng Qi, mingo
Cc: peterz, juri.lelli, vincent.guittot, dietmar.eggemann, rostedt,
bsegall, mgorman, bristot, linux-kernel, ruipengqi7, Ruipeng Qi
On 20/10/22 17:17, Ruipeng Qi wrote:
> Previous mode of period_timer is HRTIMER_MODE_ABS_PINNED,which means
> timer callback function will be executed in soft irq context.
>
> When one task group runs out of quota in a period,but holding lock of
> softirq_ctrl,the task will be rescheduled later,and then a system stall
> occurs:
> - next period comes,
> - __do_softirq can't acquire lock of softirq_ctrl,
> - __do_softirq will not be invoked,
> - callback of period_timer will not be invoked,
> - task group will not get any quota in any new period.
> - a system stall occurs.
>
> Changing mode of period_timer to HRTIMER_MODE_ABS_PINNED_HARD,which mean
> timer callback function executed in hard irq context fix this problem.
>
What you want here is some form of PI between the task doing the timer
soft expiry and the one holding the softirq_ctrl.lock. This doesn't happen
ATM because of should_wake_ksoftirqd() - if the lock is held then we won't
wake ksoftirqd, so we won't get a PI.
If you look at the RT stack, this commit gives you that:
c1d68f0da449 ("softirq: Use a dedicated thread for timer wakeups.")
(from linux-6.1.y-rt-rebase)
^ permalink raw reply [flat|nested] 2+ messages in thread