From: bsegall@google.com
To: Vincent Guittot <vincent.guittot@linaro.org>
Cc: mingo@redhat.com, peterz@infradead.org, juri.lelli@redhat.com,
dietmar.eggemann@arm.com, rostedt@goodmis.org,
bsegall@google.com, mgorman@suse.de,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH] sched/fair : prevent unlimited runtime on throttled group
Date: Tue, 14 Jan 2020 10:29:43 -0800 [thread overview]
Message-ID: <xm26blr5oprc.fsf@bsegall-linux.svl.corp.google.com> (raw)
In-Reply-To: <1579011236-31256-1-git-send-email-vincent.guittot@linaro.org> (Vincent Guittot's message of "Tue, 14 Jan 2020 15:13:56 +0100")
Vincent Guittot <vincent.guittot@linaro.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.
>
> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
> ---
> kernel/sched/core.c | 9 ++++++++-
> 1 file changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index e7b08d52db93..d0acc67336c0 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -7062,8 +7062,15 @@ void sched_move_task(struct task_struct *tsk)
>
> if (queued)
> enqueue_task(rq, tsk, queue_flags);
> - if (running)
> + if (running) {
> set_next_task(rq, tsk);
> + /*
> + * After changing group, the running task may have joined a
> + * throttled one but it's still the running task. Trigger a
> + * resched to make sure that task can still run.
> + */
> + resched_curr(rq);
> + }
>
> task_rq_unlock(rq, tsk, &rf);
> }
next prev parent reply other threads:[~2020-01-14 18:29 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-01-14 14:13 [PATCH] sched/fair : prevent unlimited runtime on throttled group Vincent Guittot
2020-01-14 18:29 ` bsegall [this message]
2020-01-20 9:46 ` Peter Zijlstra
2020-01-21 18:26 ` bsegall
2020-01-29 11:32 ` [tip: sched/core] sched/fair: Prevent " tip-bot2 for Vincent Guittot
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