linux-kernel - Re: [PATCH 1/3] sched/core: Fix the bug that task won't enqueue into core tree when update cookie

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Date:   Sun, 3 Jul 2022 22:19:38 +0800
From:   cruzzhao <cruzzhao@...ux.alibaba.com>
To:     mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] sched/core: Fix the bug that task won't enqueue into
 core tree when update cookie

Ping... This bug really affects the performance, and is this bugfix correct?

在 2022/6/28 下午3:57, Cruz Zhao 写道:
> In function sched_core_update_cookie(), a task will enqueue into the
> core tree only when it enqueued before, that is, if an uncookied task
> is cookied, it will not enqueue into the core tree until it enqueue
> again, which will result in unnecessary force idle.
> 
> Here follows the scenario:
>   CPU x and CPU y are a pair of SMT siblings.
>   1. Start task a running on CPU x without sleeping, and task b and
>      task c running on CPU y without sleeping.
>   2. We create a cookie and share it to task a and task b, and then
>      we create another cookie and share it to task c.
>   3. Simpling core_forceidle_sum of task a and b from /proc/PID/sched
> 
> And we will find out that core_forceidle_sum of task a takes 30%
> time of the sampling period, which shouldn't happen as task a and b
> have the same cookie.
> 
> Then we migrate task a to CPU x', migrate task b and c to CPU y', where
> CPU x' and CPU y' are a pair of SMT siblings, and sampling again, we
> will found out that core_forceidle_sum of task a and b are almost zero.
> 
> To solve this problem, we enqueue the task into the core tree if it's
> on rq.
> 
> Fixes: 6e33cad0af49("sched: Trivial core scheduling cookie management")
> Signed-off-by: Cruz Zhao <CruzZhao@...ux.alibaba.com>
> ---
>  kernel/sched/core_sched.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/core_sched.c b/kernel/sched/core_sched.c
> index 38a2cec..ba2466c 100644
> --- a/kernel/sched/core_sched.c
> +++ b/kernel/sched/core_sched.c
> @@ -75,7 +75,7 @@ static unsigned long sched_core_update_cookie(struct task_struct *p,
>  	old_cookie = p->core_cookie;
>  	p->core_cookie = cookie;
>  
> -	if (enqueued)
> +	if (task_on_rq_queued(p))
>  		sched_core_enqueue(rq, p);
>  
>  	/*