linux-kernel - Re: [PATCH] sched: refill quota for current period when refilling timer activates

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Date:   Mon, 14 May 2018 14:05:14 +0300
From:   Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
To:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org
Cc:     Ben Segall <bsegall@...gle.com>
Subject: Re: [PATCH] sched: refill quota for current period when refilling
 timer activates

On 14.05.2018 13:58, Konstantin Khlebnikov wrote:
> Period timer deactivates if task group has no activity during past period,
> i.e. if were no throttle and runtime from global pool weren't consumed.
> When timer activates back global pool contains unpredictable amount of
> expired runtime allocated long ago. In some cases this works fine and
> task could use it until next refill. But series short execution slices
> could drain all that leftovers because each switch expires local poll.
> In this case task group will be throttled until quota refill.

Attached piece of code allows to reproduce problem.

run
sched-burst 10 10000000 1000000000

in cgroup with

cpu.cfs_period_us = 100000
cpu.cfs_quota_us = 1100000

Series of sched_yield() drains stale runtime from global pool before refilling.

> 
> This patch refills quota right at the moment of timer's activation.
> 
> Signed-off-by: Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
> ---
>   kernel/sched/fair.c |    2 ++
>   1 file changed, 2 insertions(+)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 79f574dba096..b8d73ed17ff6 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5165,6 +5165,8 @@ void start_cfs_bandwidth(struct cfs_bandwidth *cfs_b)
>   
>   	if (!cfs_b->period_active) {
>   		cfs_b->period_active = 1;
> +		/* refill quota for current period after inactivity */
> +		__refill_cfs_bandwidth_runtime(cfs_b);
>   		hrtimer_forward_now(&cfs_b->period_timer, cfs_b->period);
>   		hrtimer_start_expires(&cfs_b->period_timer, HRTIMER_MODE_ABS_PINNED);
>   	}
> 

View attachment "sched-burst.c" of type "text/x-csrc" (2071 bytes)