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

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Message-ID: <xm26d0gvirdg.fsf@bsegall-linux.svl.corp.google.com>
Date:   Fri, 23 Aug 2019 13:00:43 -0700
From:   bsegall@...gle.com
To:     Liangyan <liangyan.peng@...ux.alibaba.com>
Cc:     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

Liangyan <liangyan.peng@...ux.alibaba.com> writes:

> do_sched_cfs_period_timer() will refill cfs_b runtime and call
> distribute_cfs_runtime() to unthrottle cfs_rq, sometimes cfs_b->runtime
> will allocate all quota to one cfs_rq incorrectly.
> This will cause other cfs_rq can't get runtime and will be throttled.
> We find that one throttled cfs_rq has non-negative
> cfs_rq->runtime_remaining and cause an unexpetced cast from s64 to u64
> in snippet: distribute_cfs_runtime() {
> runtime = -cfs_rq->runtime_remaining + 1; }.
> This cast will cause that runtime will be a large number and
> cfs_b->runtime will be subtracted to be zero at last.
>
> This commit prevents cfs_rq to be assgined new runtime if it has been
> throttled to avoid the above incorrect type cast.
>
> Signed-off-by: Liangyan <liangyan.peng@...ux.alibaba.com>

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.

I think this solution is less risky than unthrottling
in this area, so other than that:

Reviewed-by: Ben Segall <bsegall@...gle.com>


> ---
>  kernel/sched/fair.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 81fd8a2a605b..b14d67d28138 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4068,6 +4068,8 @@ static void __account_cfs_rq_runtime(struct cfs_rq *cfs_rq, u64 delta_exec)
>  	if (likely(cfs_rq->runtime_remaining > 0))
>  		return;
>  
> +	if (cfs_rq->throttled)
> +		return;
>  	/*
>  	 * if we're unable to extend our runtime we resched so that the active
>  	 * hierarchy can be throttled