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

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Message-ID: <d0918be2-8d4e-427d-ac98-32aecffe3a3b@bytedance.com>
Date: Mon, 22 Jul 2024 14:04:36 +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 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.

Thanks.