linux-kernel - Re: [PATCH 1/1] sched/fair: Fix the wrong sched_stat

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Message-ID: <20200930081953.GU2628@hirez.programming.kicks-ass.net>
Date:   Wed, 30 Sep 2020 10:19:53 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     qianjun.kernel@...il.com
Cc:     mingo@...hat.com, vincent.guittot@...aro.org,
        juri.lelli@...hat.com, linux-kernel@...r.kernel.org,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de, Yafang Shao <laoar.shao@...il.com>
Subject: Re: [PATCH 1/1] sched/fair: Fix the wrong sched_stat_wait time

On Wed, Sep 30, 2020 at 10:47:12AM +0800, qianjun.kernel@...il.com wrote:
> From: jun qian <qianjun.kernel@...il.com>
> 
> When the sched_schedstat changes from 0 to 1, some sched se maybe
> already in the runqueue, the se->statistics.wait_start will be 0.
> So it will let the (rq_of(cfs_rq)) - se->statistics.wait_start)
> wrong. We need to avoid this scenario.

Is this really the only problem there? Did you do a full audit of that
schedstat nonsense?

> Signed-off-by: jun qian <qianjun.kernel@...il.com>
> Signed-off-by: Yafang Shao <laoar.shao@...il.com>
> ---
>  kernel/sched/fair.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 658aa7a..dd7c3bb 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -908,6 +908,14 @@ static void update_curr_fair(struct rq *rq)

your git-diff is 'funny', it got the function ^ wrong.

>  	if (!schedstat_enabled())
>  		return;
>  
> +	/*
> +	 * When the sched_schedstat changes from 0 to 1, some sched se maybe
> +	 * already in the runqueue, the se->statistics.wait_start will be 0.
> +	 * So it will let the delta wrong. We need to avoid this scenario.
> +	 */
> +	if (unlikely(!schedstat_val(se->statistics.wait_start)))
> +		return;
> +
>  	delta = rq_clock(rq_of(cfs_rq)) - schedstat_val(se->statistics.wait_start);
>  
>  	if (entity_is_task(se)) {
> -- 
> 1.8.3.1
>