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

[Peter Zijlstra]

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@xxxxxxxxxx>
> >> Signed-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> >> ---
> > 
> > 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@xxxxxxx

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.

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);
 }