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

From: Peter Zijlstra <peterz@infradead.org>
To: Zhang Qiao <zhangqiao22@huawei.com>
Cc: linux-kernel@vger.kernel.org, mingo@redhat.com,
	juri.lelli@redhat.com, vincent.guittot@linaro.org,
	dietmar.eggemann@arm.com, rostedt@goodmis.org,
	bsegall@google.com, mgorman@suse.de, bristot@redhat.com,
	vschneid@redhat.com, rkagan@amazon.de
Subject: Re: [PATCH v2] sched/fair: sanitize vruntime of entity being migrated
Date: Thu, 9 Mar 2023 14:05:24 +0100	[thread overview]
Message-ID: <20230309130524.GA273121@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20230306132418.50389-1-zhangqiao22@huawei.com>

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

?