linux-kernel - Re: [PATCH] sched/fair : prevent unlimited runtime on throttled group

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Message-ID: <20200120094625.GL14879@hirez.programming.kicks-ass.net>
Date:   Mon, 20 Jan 2020 10:46:25 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     bsegall@...gle.com
Cc:     Vincent Guittot <vincent.guittot@...aro.org>, mingo@...hat.com,
        juri.lelli@...hat.com, dietmar.eggemann@....com,
        rostedt@...dmis.org, mgorman@...e.de, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched/fair : prevent unlimited runtime on throttled group

On Tue, Jan 14, 2020 at 10:29:43AM -0800, bsegall@...gle.com wrote:
> Vincent Guittot <vincent.guittot@...aro.org> writes:
> 
> > When a running task is moved on a throttled task group and there is no
> > other task enqueued on the CPU, the task can keep running using 100% CPU
> > whatever the allocated bandwidth for the group and although its cfs rq is
> > throttled. Furthermore, the group entity of the cfs_rq and its parents are
> > not enqueued but only set as curr on their respective cfs_rqs.
> >
> > We have the following sequence:
> >
> > sched_move_task
> >   -dequeue_task: dequeue task and group_entities.
> >   -put_prev_task: put task and group entities.
> >   -sched_change_group: move task to new group.
> >   -enqueue_task: enqueue only task but not group entities because cfs_rq is
> >     throttled.
> >   -set_next_task : set task and group_entities as current sched_entity of
> >     their cfs_rq.
> >
> > Another impact is that the root cfs_rq runnable_load_avg at root rq stays
> > null because the group_entities are not enqueued. This situation will stay
> > the same until an "external" event triggers a reschedule. Let trigger it
> > immediately instead.
> 
> Sounds reasonable to me, "moved group" being an explicit resched check
> doesn't sound like a problem in general.

Do I read that as an Ack from you Ben? :-)