Re: [PATCH v3] sched/fair: sanitize vruntime of entity being placed

From: Vincent Guittot <vincent.guittot@linaro.org>
To: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Roman Kagan <rkagan@amazon.de>,
	Peter Zijlstra <peterz@infradead.org>,
	linux-kernel@vger.kernel.org,
	Valentin Schneider <vschneid@redhat.com>,
	Zhang Qiao <zhangqiao22@huawei.com>,
	Ben Segall <bsegall@google.com>, Waiman Long <longman@redhat.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Mel Gorman <mgorman@suse.de>,
	Daniel Bristot de Oliveira <bristot@redhat.com>,
	Ingo Molnar <mingo@redhat.com>,
	Juri Lelli <juri.lelli@redhat.com>
Subject: Re: [PATCH v3] sched/fair: sanitize vruntime of entity being placed
Date: Mon, 27 Feb 2023 18:15:15 +0100	[thread overview]
Message-ID: <CAKfTPtCvy0qbEYJ3A935Cf0t_NPg=0B8-HagTwxmE+0hA1gfSw@mail.gmail.com> (raw)
In-Reply-To: <ccd80042-39ef-f9ea-c2cd-eac5bd0e8095@arm.com>

On Mon, 27 Feb 2023 at 18:00, Dietmar Eggemann <dietmar.eggemann@arm.com> wrote:
>
> On 27/02/2023 15:37, Vincent Guittot wrote:
> > On Mon, 27 Feb 2023 at 09:43, Roman Kagan <rkagan@amazon.de> wrote:
> >>
> >> On Tue, Feb 21, 2023 at 06:26:11PM +0100, Vincent Guittot wrote:
> >>> On Tue, 21 Feb 2023 at 17:57, Roman Kagan <rkagan@amazon.de> wrote:
> >>>> What scares me, though, is that I've got a message from the test robot
> >>>> that this commit drammatically affected hackbench results, see the quote
> >>>> below.  I expected the commit not to affect any benchmarks.
> >>>>
> >>>> Any idea what could have caused this change?
> >>>
> >>> Hmm, It's most probably because se->exec_start is reset after a
> >>> migration and the condition becomes true for newly migrated task
> >>> whereas its vruntime should be after min_vruntime.
> >>>
> >>> We have missed this condition
> >>
> >> Makes sense to me.
> >>
> >> But what would then be the reliable way to detect a sched_entity which
> >> has slept for long and risks overflowing in .vruntime comparison?
> >
> > For now I don't have a better idea than adding the same check in
> > migrate_task_rq_fair()
>
> Don't we have the issue that we could have a non-up-to-date rq clock in
> migrate? No rq lock held in `!task_on_rq_migrating(p)`.

yes the rq clock may be not up to date but that would also mean that
the cfs was idle and as a result its min_vruntime has not moved
forward and we don't have a problem of possible overflow

>
> Also deferring `se->exec_start = 0` from `migrate` into `enqueue ->
> place entity` doesn't seem to work since the rq clocks of different CPUs
> are not in sync.

yes

>