Messages in this thread | | | From | Vincent Guittot <> | Date | Wed, 15 Mar 2023 08:18:26 +0100 | Subject | Re: [PATCH v2] sched/fair: sanitize vruntime of entity being migrated |
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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
> >
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