Messages in this thread | | | Date | Wed, 20 Jul 2022 21:40:55 +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 2022/7/19 18:29, Vincent Guittot wrote: > On Fri, 15 Jul 2022 at 18:21, Chengming Zhou > <zhouchengming@bytedance.com> wrote: >> >> 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. > > As Dietmar said, the commit message is misleading because someone can > think you fix these bugs whereas it's not the case
Hi Vincent, thanks for your review! I will refactor the commit message to avoid this misleading, sorry for my bad English expression.
> >> >> 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. > > But I'm not sure it makes sense to keep these init values. As an > example, the util_avg is set according to the cpu utilization at the > time of the task creation. I would tend to decay them as these init > values become less and less relevant. > > so we should return early in post_init_entity_util_avg() and don't set > util_avg if sched class is not cfs
Yes, this indeed is a problem if we attach this init sched_avg of !fair task. I'm also not sure whether it make sense to keep them to 0 ? Will it cause unfairness problem between cfs_rqs?
> >> >> 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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