linux-kernel - Re: [PATCH] sched/fair: don't assign runtime for throttled cfs

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Message-ID: <xm26k1azn7yd.fsf@bsegall-linux.svl.corp.google.com>
Date:   Mon, 26 Aug 2019 10:38:02 -0700
From:   bsegall@...gle.com
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     Liangyan <liangyan.peng@...ux.alibaba.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org, shanpeic@...ux.alibaba.com,
        xlpang@...ux.alibaba.com
Subject: Re: [PATCH] sched/fair: don't assign runtime for throttled cfs_rq

Valentin Schneider <valentin.schneider@....com> writes:

> On 23/08/2019 21:00, bsegall@...gle.com wrote:
> [...]
>> Could you mention in the message that this a throttled cfs_rq can have
>> account_cfs_rq_runtime called on it because it is throttled before
>> idle_balance, and the idle_balance calls update_rq_clock to add time
>> that is accounted to the task.
>> 
>
> Mayhaps even a comment for the extra condition.
>
>> I think this solution is less risky than unthrottling
>> in this area, so other than that:
>> 
>> Reviewed-by: Ben Segall <bsegall@...gle.com>
>> 
>
> If you don't mind squashing this in:
>
> -----8<-----
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index b1d9cec9b1ed..b47b0bcf56bc 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4630,6 +4630,10 @@ static u64 distribute_cfs_runtime(struct cfs_bandwidth *cfs_b, u64 remaining)
>  		if (!cfs_rq_throttled(cfs_rq))
>  			goto next;
>  
> +		/* By the above check, this should never be true */
> +		WARN_ON(cfs_rq->runtime_remaining > 0);
> +
> +		/* Pick the minimum amount to return to a positive quota state */
>  		runtime = -cfs_rq->runtime_remaining + 1;
>  		if (runtime > remaining)
>  			runtime = remaining;
> ----->8-----
>
> I'm not adamant about the extra comment, but the WARN_ON would be nice IMO.
>
>
> @Ben, do you reckon we want to strap
>
> Cc: <stable@...r.kernel.org>
> Fixes: ec12cb7f31e2 ("sched: Accumulate per-cfs_rq cpu usage and charge against bandwidth")
>
> to the thing? AFAICT the pick_next_task_fair() + idle_balance() dance you
> described should still be possible on that commit.

I'm not sure about stable policy in general, but it seems reasonable.
The WARN_ON might want to be WARN_ON_ONCE, and it seems fine to have it
or not.

>
>
> Other than that,
>
> Reviewed-by: Valentin Schneider <valentin.schneider@....com>
>
> [...]