linux-kernel - Re: [PATCH 1/2] sched/fair: Decrease cfs bandwidth usage in task

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Message-ID: <14c3db2b-5de2-4f1c-b4d4-6183568b8c24@huawei.com>
Date: Mon, 22 Jul 2024 15:16:25 +0800
From: Zhang Qiao <zhangqiao22@...wei.com>
To: Chuyi Zhou <zhouchuyi@...edance.com>, <mingo@...hat.com>,
	<peterz@...radead.org>, <juri.lelli@...hat.com>,
	<vincent.guittot@...aro.org>, <dietmar.eggemann@....com>,
	<rostedt@...dmis.org>, <bsegall@...gle.com>, <mgorman@...e.de>,
	<vschneid@...hat.com>
CC: <chengming.zhou@...ux.dev>, <linux-kernel@...r.kernel.org>,
	<joshdon@...gle.com>
Subject: Re: [PATCH 1/2] sched/fair: Decrease cfs bandwidth usage in
 task_group destruction

hi

在 2024/7/22 14:04, Chuyi Zhou 写道:
> Hello
> 
> 在 2024/7/22 11:47, Zhang Qiao 写道:
>>
>>
>> Hi, Chuyi
>>
>> 在 2024/7/21 20:52, Chuyi Zhou 写道:
>>> The static key __cfs_bandwidth_used is used to indicate whether bandwidth
>>> control is enabled in the system. Currently, it is only decreased when a
>>> task group disables bandwidth control. This is incorrect because if there
>>> was a task group in the past that enabled bandwidth control, the
>>> __cfs_bandwidth_used will never go to zero, even if there are no task_group
>>> using bandwidth control now.
>>>
>>> This patch tries to fix this issue by decrsasing bandwidth usage in
>>> destroy_cfs_bandwidth().
>>>
>>> Signed-off-by: Chuyi Zhou <zhouchuyi@...edance.com>
>>> ---
>>>   kernel/sched/fair.c | 3 +++
>>>   1 file changed, 3 insertions(+)
>>>
>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>>> index b1e07ce90284..7ad50dc31a93 100644
>>> --- a/kernel/sched/fair.c
>>> +++ b/kernel/sched/fair.c
>>> @@ -6447,6 +6447,9 @@ static void destroy_cfs_bandwidth(struct cfs_bandwidth *cfs_b)
>>>       hrtimer_cancel(&cfs_b->period_timer);
>>>       hrtimer_cancel(&cfs_b->slack_timer);
>>>   +    if (cfs_b->quota != RUNTIME_INF)
>>> +        cfs_bandwidth_usage_dec();
>>
>> This calls static_key_slow_dec_cpuslocked, but destroy_cfs_bandwidth
>> isn't holding the hotplug lock [1].
>>
>> For fixing this issue, i also sent a patch, but it be not merged into mainline [2].
>>
>> [1]: https://lore.kernel.org/all/20210712162655.w3j6uczwbfkzazvt@oracle.com/
>> [2]: https://lore.kernel.org/all/20210910094139.184582-1-zhangqiao22@huawei.com/
>>
> 
> Thanks for your information.
> 
> I think maybe cfs_bandwidth_usage_dec() should be moved to other more suitable places where could
> hold hotplug lock(e.g. cpu_cgroup_css_released()). I would do some test to verify it.
> 

The cpu_cgroup_css_released() also doesn't seem to be in the cpu hotplug lock-holding context.


> Thanks.
> 
> 
> 
> 
>