linux-kernel - Re: [PATCH v2] sched/fair: sanitize vruntime of entity being migrated

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Message-ID: <CAKfTPtCra1QV32w1MZQY2SHVDv58m2MT2QwpHu0huJJ3B1wcCA@mail.gmail.com>
Date:   Wed, 15 Mar 2023 09:42:39 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Dietmar Eggemann <dietmar.eggemann@....com>,
        Zhang Qiao <zhangqiao22@...wei.com>,
        linux-kernel@...r.kernel.org, mingo@...hat.com,
        juri.lelli@...hat.com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de, bristot@...hat.com, vschneid@...hat.com,
        rkagan@...zon.de
Subject: Re: [PATCH v2] sched/fair: sanitize vruntime of entity being migrated

On Wed, 15 Mar 2023 at 08:18, Vincent Guittot
<vincent.guittot@...aro.org> wrote:
>
> On Tue, 14 Mar 2023 at 18:16, Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > On Tue, Mar 14, 2023 at 02:24:37PM +0100, Vincent Guittot wrote:
> >
> > > > @@ -7632,11 +7646,8 @@ static void migrate_task_rq_fair(struct task_struct *p, int new_cpu)
> > > >          * min_vruntime -- the latter is done by enqueue_entity() when placing
> > > >          * the task on the new runqueue.
> > > >          */
> > > > -       if (READ_ONCE(p->__state) == TASK_WAKING) {
> > > > -               struct cfs_rq *cfs_rq = cfs_rq_of(se);
> > > > -
> > > > +       if (READ_ONCE(p->__state) == TASK_WAKING || reset_vruntime(cfs_rq, se))
> > >
> > > That's somehow what was proposed in one of the previous proposals but
> > > we can't call rq_clock_task(rq_of(cfs_rq)) because rq lock might not
> > > be hold and rq task clock has not been updated before being used
> >
> > Argh indeed. I spend a lot of time ensuring we didn't take the old rq
> > lock on wakeup -- and then a lot of time cursing about how we don't :-)
> >
> > Now, if we could rely on the rq-clock being no more than 1 tick behind
> > current, this would still be entirely sufficient to catch the long sleep
> > case.
>
> We should also take care when loading rq_clock_task that we are not
> racing with an update especially for a 32bits system like pelt
> last_update_time

We still have this possibility:
https://lore.kernel.org/lkml/ZAiFxWLSb9HDazSI@vingu-book/

which uses pelt last_update_time when migrating and keep using
rq_clock_task in place_entity

>
> >
> > Except I suppose that NOHZ can bite us here. If the old CPU is idle, the
> > timestamps can be arbitrarily old. Mooo :/
>
> That should not be a real problem because if the cpu is idle and the
> rq clock is not updated, the min_vruntime will not move forward so we
> are "safe" in regard to the overflow.
>
> That's what was done in the v2 and v3 of this patch
>
>
> >
> >