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

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Message-ID: <e4e3a63d-a5f2-4c1d-a9da-4ddbc3693450@bytedance.com>
Date: Mon, 22 Jul 2024 15:46:24 +0800
From: Chuyi Zhou <zhouchuyi@...edance.com>
To: Zhang Qiao <zhangqiao22@...wei.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

Hello,

在 2024/7/22 15:16, Zhang Qiao 写道:
> 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.
> 

IIUC, cpus_read_lock/cpus_read_unlock can be called in 
cpu_cgroup_css_released() right? But cfs bandwidth destroy maybe run in 
a rcu callback since task group list is protected by RCU so we could not
get the lock. Did I miss something important?