Messages in this thread Patch in this message | | | Date | Sat, 16 Jul 2022 00:21:46 +0800 | Subject | Re: [External] Re: [PATCH v2 07/10] sched/fair: use update_load_avg() to attach/detach entity load_avg | From | Chengming Zhou <> |
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On 2022/7/15 19:18, Dietmar Eggemann wrote: > On 13/07/2022 06:04, Chengming Zhou wrote: >> Since update_load_avg() support DO_ATTACH and DO_DETACH now, we can >> use update_load_avg() to implement attach/detach entity load_avg. >> >> Another advantage of using update_load_avg() is that it will check >> last_update_time before attach or detach, instead of unconditional >> attach/detach in the current code. >> >> This way can avoid some corner problematic cases of load tracking, >> like twice attach problem, detach unattached NEW task problem. > > This explanation is somewhat hard to follow for me. Since both issues > have been fixed already (you mention this further below) you're saying > that with you change you don't reintroduce them?
Sorry for this not very clear explanation.
Yes, both issues have been fixed already, what I want to say is that bugfix brings its own problem and limitation mentioned below.
So I want to use another way to solve these problems better.
> >> 1. switch to fair class (twice attach problem) >> >> p->sched_class = fair_class; --> p.se->avg.last_update_time = 0 >> if (queued) >> enqueue_task(p); >> ... >> enqueue_entity() >> update_load_avg(UPDATE_TG | DO_ATTACH) >> if (!se->avg.last_update_time && (flags & DO_ATTACH)) --> true >> attach_entity_load_avg() --> attached, will set last_update_time >> check_class_changed() >> switched_from() (!fair) >> switched_to() (fair) >> switched_to_fair() >> attach_entity_load_avg() --> unconditional attach again! >> >> 2. change cgroup of NEW task (detach unattached task problem) >> >> sched_move_group(p) >> if (queued) >> dequeue_task() >> task_move_group_fair() >> detach_task_cfs_rq() >> detach_entity_load_avg() --> detach unattached NEW task >> set_task_rq() >> attach_task_cfs_rq() >> attach_entity_load_avg() >> if (queued) >> enqueue_task() >> >> These problems have been fixed in commit 7dc603c9028e >> ("sched/fair: Fix PELT integrity for new tasks"), which also >> bring its own problems. >> >> First, it add a new task state TASK_NEW and an unnessary limitation >> that we would fail when change the cgroup of TASK_NEW tasks.
This is the limitation that bugfix has brought.
We can't change cgroup or switch to fair for task with last_update_time=0 if we don't have conditional detach/attach.
So we have to:
1. !fair task also need to set last_update_time. 2. cpu_cgroup_can_attach() have to wait for TASK_NEW to fully attached.
>> >> Second, it attach entity load_avg in post_init_entity_util_avg(), >> in which we only set sched_avg last_update_time for !fair tasks, >> will cause PELT integrity problem when switched_to_fair(). > > I guess those PELT integrity problems are less severe since we have the > enqueue_task_fair() before the switched_to_fair() for enqueued tasks. So > we always decay the time the task spend outside fair.
enqueue_task_fair() enqueue_entity() update_load_avg() if (se->avg.last_update_time && !(flags & SKIP_AGE_LOAD)) --> true __update_load_avg_se(now, cfs_rq, se); --> (1)
We can see above, for queued !fair task, (1) will deay the delta time (now - se.avg.last_update_time) even for a NEW !fair task.
> > Looks to me that you want to replace this by your `freeze PELT when > outside fair` model.
Yes, want to freeze PELT for two !fair cases:
1. !fair task hasn't been fair before: will still have its init load_avg when switch to fair.
2. !fair task has been switched_from_fair(): will still keep its lastest load_avg when switch to fair.
> >> This patch make update_load_avg() the only location of attach/detach, >> and can handle these corner cases like change cgroup of NEW tasks, >> by checking last_update_time before attach/detach. > > [...] > >> @@ -11527,9 +11522,7 @@ static void detach_entity_cfs_rq(struct sched_entity *se) >> struct cfs_rq *cfs_rq = cfs_rq_of(se); >> >> /* Catch up with the cfs_rq and remove our load when we leave */ >> - update_load_avg(cfs_rq, se, 0); >> - detach_entity_load_avg(cfs_rq, se); >> - update_tg_load_avg(cfs_rq); >> + update_load_avg(cfs_rq, se, UPDATE_TG | DO_DETACH); > > IMHO, the DO_[DE|AT]TACH comments in update_load_avg() would have to be > updated in this case.
Correct, will do.
Thanks.
> > [...]
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