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

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Mar 21, 2023 at 12:13:06PM +0100, Dietmar Eggemann wrote:
> On 21/03/2023 11:49, Peter Zijlstra wrote:
> > On Tue, Mar 21, 2023 at 11:29:13AM +0100, Dietmar Eggemann wrote:
> >> On 21/03/2023 11:02, Peter Zijlstra wrote:
> >>> On Fri, Mar 17, 2023 at 05:08:10PM +0100, Vincent Guittot wrote:
> >>>> Commit 829c1651e9c4 ("sched/fair: sanitize vruntime of entity being placed")
> >>>> fixes an overflowing bug, but ignore a case that se->exec_start is reset
> >>>> after a migration.
> >>>>
> >>>> For fixing this case, we delay the reset of se->exec_start after
> >>>> placing the entity which se->exec_start to detect long sleeping task.
> >>>>
> >>>> In order to take into account a possible divergence between the clock_task
> >>>> of 2 rqs, we increase the threshold to around 104 days.
> >>>>
> >>>>
> >>>> Fixes: 829c1651e9c4 ("sched/fair: sanitize vruntime of entity being placed")
> >>>> Signed-off-by: Zhang Qiao <zhangqiao22@huawei.com>
> >>>> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
> >>>> ---
> >>>
> >>> Blergh, this just isn't going to be nice. I'll go queue this for
> >>> sched/urgent and then we can forget about this for a little while.
> >>>
> >>> Thanks!
> >>
> >> Don't we miss setting `se->exec_start = 0` for fair task in
> >> move_queued_task()? ( ... and __migrate_swap_task())
> >>
> >> https://lkml.kernel.org/r/df2cccda-1550-b06b-aa74-e0f054e9fb9d@arm.com
> > 
> > Ah, I see what you mean now... When I read your and Vincent's replies
> > earlier today I though you mean to avoid the extra ENQUEUE_MIGRATED use,
> > but your actual goal was to capure more sites.
> > 
> > Hmm, we could of course go add more ENQUEUE_MIGRATED, but you're right
> > in that TASK_ON_RQ_MIGRATING already captures that.
> 
> And in case of move_queued_task() this would have to be conditioned on
> SCHED_NORMAL.

I would prefer to not do that -- keep uniform rules. AFAICT the only
other user of ENQUEUE_MIGRATED is deadline and that needs
ENQUEUE_WAKEUP|ENQUEUE_MIGRATED combination to be effective and I don't
think we've added any of those.

> > An alternative is something like the below, that matches
> > deactivate_task(), but still uses ENQUEUE_MIGRATED to pass it down into
> > the class methods.
> > 
> > Hmm?
> > 
> > 
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -2084,6 +2084,9 @@ static inline void dequeue_task(struct r
> >  
> >  void activate_task(struct rq *rq, struct task_struct *p, int flags)
> >  {
> > +	if (task_on_rq_migrating(p))
> > +		flags |= ENQUEUE_MIGRATED;
> > +
> >  	enqueue_task(rq, p, flags);
> >  
> >  	p->on_rq = TASK_ON_RQ_QUEUED;
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -8726,7 +8726,7 @@ static void attach_task(struct rq *rq, s
> >  	lockdep_assert_rq_held(rq);
> >  
> >  	WARN_ON_ONCE(task_rq(p) != rq);
> > -	activate_task(rq, p, ENQUEUE_NOCLOCK | ENQUEUE_MIGRATED);
> > +	activate_task(rq, p, ENQUEUE_NOCLOCK);
> >  	check_preempt_curr(rq, p, 0);
> >  }
> 
> Would work too.

OK, let me fold this in and then we can always tinker with it later if
we're so motivated :-)