linux-kernel - Re: [PATCH] sched/fair: Change mode of period_timer to HRTIMER_MODE_ABS_PINNED

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Date:   Thu, 20 Oct 2022 18:39:12 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Ruipeng Qi <qiruipeng@...iang.com>
Cc:     mingo@...hat.com, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com, vschneid@...hat.com,
        linux-kernel@...r.kernel.org, ruipengqi7@...il.com
Subject: Re: [PATCH] sched/fair: Change mode of period_timer to
 HRTIMER_MODE_ABS_PINNED_HARD

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.