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

From: Dietmar Eggemann <dietmar.eggemann@arm.com>
To: Vincent Guittot <vincent.guittot@linaro.org>,
	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 18:00:09 +0100	[thread overview]
Message-ID: <ccd80042-39ef-f9ea-c2cd-eac5bd0e8095@arm.com> (raw)
In-Reply-To: <CAKfTPtAq3yBYBxpR=RO8zxrQduOymqkdAEhigjfCuGfsY1uHsg@mail.gmail.com>

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)`.

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.