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

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Message-ID: <20230314171607.GN2017917@hirez.programming.kicks-ass.net>
Date:   Tue, 14 Mar 2023 18:16:07 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vincent Guittot <vincent.guittot@...aro.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 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.

Except I suppose that NOHZ can bite us here. If the old CPU is idle, the
timestamps can be arbitrarily old. Mooo :/