linux-kernel - Re: [PATCH v2] sched/fair: sanitize vruntime of entity being migrated

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Message-ID: <20230309130524.GA273121@hirez.programming.kicks-ass.net>
Date:   Thu, 9 Mar 2023 14:05:24 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Zhang Qiao <zhangqiao22@...wei.com>
Cc:     linux-kernel@...r.kernel.org, mingo@...hat.com,
        juri.lelli@...hat.com, vincent.guittot@...aro.org,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de, bristot@...hat.com, vschneid@...hat.com,
        rkagan@...zon.de
Subject: Re: [PATCH v2] sched/fair: sanitize vruntime of entity being migrated

On Mon, Mar 06, 2023 at 09:24:18PM +0800, Zhang Qiao wrote:
> Commit 829c1651e9c4 ("sched/fair: sanitize vruntime of
> entity being placed") fix an overflowing bug, but ignore
> a case that se->exec_start is reset after a migration.
> 
> For fixing this case, we reset the vruntime of a long
> sleeping task in migrate_task_rq_fair().
> 

> @@ -7635,7 +7653,23 @@ static void migrate_task_rq_fair(struct task_struct *p, int new_cpu)
>  	if (READ_ONCE(p->__state) == TASK_WAKING) {
>  		struct cfs_rq *cfs_rq = cfs_rq_of(se);
>  
> -		se->vruntime -= u64_u32_load(cfs_rq->min_vruntime);
> +		/*
> +		 * We determine whether a task sleeps for long by checking
> +		 * se->exec_start, and if it is, we sanitize its vruntime at
> +		 * place_entity(). However, after a migration, this detection
> +		 * method fails due to se->exec_start being reset.
> +		 *
> +		 * For fixing this case, we add the same check here. For a task
> +		 * which has slept for a long time, its vruntime should be reset
> +		 * to cfs_rq->min_vruntime with a sleep credit. Because waking
> +		 * task's vruntime will be added to cfs_rq->min_vruntime when
> +		 * enqueue, we only need to reset the se->vruntime of waking task
> +		 * to a credit here.
> +		 */
> +		if (entity_is_long_sleep(se))
> +			se->vruntime = -sched_sleeper_credit(se);
> +		else
> +			se->vruntime -= u64_u32_load(cfs_rq->min_vruntime);
>  	}

*groan*, that again...

Can't we simply do:

https://lkml.kernel.org/r/68832dfbb60fda030540b5f4e39c5801942689b1.1648228023.git.tim.c.chen@linux.intel.com

?