linux-kernel - Re: [External] Re: [PATCH v2 07/10] sched/fair: use update_load_avg() to attach/detach entity load

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Message-ID: <952fc252-30e1-3629-42c4-98a55ada12f9@bytedance.com>
Date:   Wed, 20 Jul 2022 21:43:59 +0800
From:   Chengming Zhou <zhouchengming@...edance.com>
To:     Dietmar Eggemann <dietmar.eggemann@....com>, mingo@...hat.com,
        peterz@...radead.org, vincent.guittot@...aro.org,
        rostedt@...dmis.org, bsegall@...gle.com, vschneid@...hat.com
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [External] Re: [PATCH v2 07/10] sched/fair: use update_load_avg()
 to attach/detach entity load_avg

On 2022/7/19 23:02, Dietmar Eggemann wrote:
> On 15/07/2022 18:21, Chengming Zhou 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.
>>
>> So I want to use another way to solve these problems better.
> 
> [...]
> 
>>>> 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.
> 
> I see.
> 
> `cgroup_migrate_execute() -> cpu_cgroup_[can|]_attach()` has to wait for
> `wake_up_new_task() -> WRITE_ONCE(p->__state, TASK_RUNNING)`.
> 
> Just to understand this change better: IMHO, this is still the case for
> fair tasks, right?

Yes, I think so. We could delete this limitation when we have conditional
detach/attach, since nothing will be detached when last_update_time==0.

Thanks!

> 
> `wake_up_new_task() -> post_init_entity_util_avg() ->
> attach_entity_cfs_rq()` has to happen before the fair task can move
> between taskgroups in `cgroup_migrate_execute() -> cpu_cgroup_attach()
> -> sched_move_task() -> sched_change_group() -> task_change_group_fair()`.
> 
> [...]