linux-kernel - Re: [bug-report] possible s64 overflow in max

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Message-ID: <xm26v8kr7ho0.fsf@google.com>
Date:   Fri, 27 Jan 2023 14:10:23 -0800
From:   Benjamin Segall <bsegall@...gle.com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Roman Kagan <rkagan@...zon.de>,
        Zhang Qiao <zhangqiao22@...wei.com>,
        Waiman Long <longman@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: [bug-report] possible s64 overflow in max_vruntime()

Vincent Guittot <vincent.guittot@...aro.org> writes:

> On Fri, 27 Jan 2023 at 12:44, Peter Zijlstra <peterz@...radead.org> wrote:
>>
>> On Thu, Jan 26, 2023 at 07:31:02PM +0100, Roman Kagan wrote:
>>
>> > > All that only matters for small sleeps anyway.
>> > >
>> > > Something like:
>> > >
>> > >         sleep_time = U64_MAX;
>> > >         if (se->avg.last_update_time)
>> > >           sleep_time = cfs_rq_clock_pelt(cfs_rq) - se->avg.last_update_time;
>> >
>> > Interesting, why not rq_clock_task(rq_of(cfs_rq)) - se->exec_start, as
>> > others were suggesting?  It appears to better match the notion of sleep
>> > wall-time, no?
>>
>> Should also work I suppose. cfs_rq_clock takes throttling into account,
>> but that should hopefully also not be *that* long, so either should
>> work.
>
> yes rq_clock_task(rq_of(cfs_rq)) should be fine too

No, last_update_time is based on cfs_rq_clock_pelt(cfs_rq), and it will
get more and more out of sync as time goes on, every time the cfs_rq
throttles. It won't reset when the throttle is done.

>
> Another thing to take into account is the sleeper credit that the
> waking task deserves so the detection should be done once it has been
> subtracted from vruntime.
>
> Last point, when a nice -20 task runs on a rq, it will take a bit more
> than 2 seconds for the vruntime to be increased by more than 24ms (the
> maximum credit that a waking task can get) so threshold must be
> significantly higher than 2 sec. On the opposite side, the lowest
> possible weight of a cfs rq is 2 which means that the problem appears
> for a sleep longer or equal to 2^54 = 2^63*2/1024. We should use this
> value instead of an arbitrary 200 days