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    SubjectRe: [PATCH v2] sched/fair: sanitize vruntime of entity being migrated
    On Wed, 15 Mar 2023 at 08:18, Vincent Guittot
    <vincent.guittot@linaro.org> wrote:
    >
    > 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

    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
    >
    >
    > >
    > >

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