linux-kernel - Re: [PATCH] sched/fair: add !se->on

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Message-ID: <20200220144251.GA15604@cqw-OptiPlex-7050>
Date:   Thu, 20 Feb 2020 22:42:51 +0800
From:   chenqiwu <qiwuchen55@...il.com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        chenqiwu <chenqiwu@...omi.com>
Subject: Re: [PATCH] sched/fair: add !se->on_rq check before dequeue entity

> 
> Hmm i have been too quick in my reply. I wanted to say:
> AFAICT, there is no other way to dequeue a task from a cfs_rq for
> which the group entity is not enqueued

But we should notice the potential racy pathes called by deactivate_task().
For example:
One path is dequeue a task from its cfs_rq called by schedule():
__schedule
	deactivate_task
		dequeue_task
			dequeue_task_fair

Another path is trying to migrate the same task to a CPU on the preferred node:
numa_migrate_preferred
	task_numa_migrate	
		migrate_swap
			stop_two_cpus
				migrate_swap_stop
					__migrate_swap_task
						deactivate_task
							dequeue_task_fair

There could be a racy if the task is dequeued form its cfs_rq in parallel.