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

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Message-ID: <20200220100915.GA14721@cqw-OptiPlex-7050>
Date:   Thu, 20 Feb 2020 18:09:15 +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

On Thu, Feb 20, 2020 at 10:38:02AM +0100, Vincent Guittot wrote:
> On Thu, 20 Feb 2020 at 08:29, <qiwuchen55@...il.com> wrote:
> >
> > From: chenqiwu <chenqiwu@...omi.com>
> >
> > We igonre checking for !se->on_rq condition before dequeue one
> > entity from cfs rq. It must be required in case the entity has
> > been dequeued.
> 
> Do you have a use case that triggers this situation ?
> 
> This is the only way to reach this situation seems to be dequeuing a
> task on a throttled cfs_rq
>
Sorry, I have no use case triggers this situation. It's just found by
reading code.
I agree the situation you mentioned above may have a racy with
dequeue_task_fair() in the following code path:
__schedule
	pick_next_task_fair
		put_prev_entity
			check_cfs_rq_runtime
				throttle_cfs_rq
					dequeue_entity

So this check is worth to be added for dequeue_task_fair().