MIME-Version: 1.0
References: <CAKfTPtAf5RrzZRSHtfK+r3QvnFQ-oM3+rJ-z5SB8T4+nUv1aQw@mail.gmail.com>
 <20230309142825.GB273121@hirez.programming.kicks-ass.net> <ZAnvCGdlOrWbIC/o@hirez.programming.kicks-ass.net>
 <CAKfTPtADUas2QHZCQyu0ad-JTKRQ=PcsB=o7+PuJNVxHwAzkCQ@mail.gmail.com>
 <ZAs+zV0o9ShO7nLT@vingu-book> <02a08042-e7c4-464d-bc20-9ec4ccdab1ff@arm.com>
 <8c093661-7431-00d8-d703-b8f7a7c8e747@arm.com> <CAKfTPtBw9SJxVBcN1qff7jRzE81kXSjbc-rXD6goEBFiXEwbyg@mail.gmail.com>
 <20230314120726.GG1845660@hirez.programming.kicks-ass.net>
 <CAKfTPtBHocw4N-YMHeqfMj78Ro=aF8sJPanxVCN=tM70hr6r8g@mail.gmail.com> <20230314171607.GN2017917@hirez.programming.kicks-ass.net>
In-Reply-To: <20230314171607.GN2017917@hirez.programming.kicks-ass.net>
From:   Vincent Guittot <vincent.guittot@linaro.org>
Date:   Wed, 15 Mar 2023 08:18:26 +0100
Message-ID: <CAKfTPtBurhAxcykDWQHoSZ0aiokgK4jhamdh-F29643cL1jVsw@mail.gmail.com>
Subject: Re: [PATCH v2] sched/fair: sanitize vruntime of entity being migrated
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Zhang Qiao <zhangqiao22@huawei.com>,
        linux-kernel@vger.kernel.org, mingo@redhat.com,
        juri.lelli@redhat.com, rostedt@goodmis.org, bsegall@google.com,
        mgorman@suse.de, bristot@redhat.com, vschneid@redhat.com,
        rkagan@amazon.de
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Tue, 14 Mar 2023 at 18:16, Peter Zijlstra <peterz@infradead.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

>
> 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


>
>