linux-kernel - Re: [PATCH v2] cgroup/cpuset: Change nr_deadline_tasks to an atomic

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Date:   Thu, 2 Nov 2023 14:08:54 -0400
From:   Waiman Long <longman@...hat.com>
To:     Juri Lelli <juri.lelli@...hat.com>
Cc:     Michal Koutný <mkoutny@...e.com>,
        Tejun Heo <tj@...nel.org>, Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Qais Yousef <qyousef@...alina.io>, Hao Luo <haoluo@...gle.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Xia Fukun <xiafukun@...wei.com>
Subject: Re: [PATCH v2] cgroup/cpuset: Change nr_deadline_tasks to an atomic_t
 value

On 11/2/23 09:01, Waiman Long wrote:
>
> On 11/2/23 06:26, Juri Lelli wrote:
>> Hi Waiman,
>>
>> On 01/11/23 13:59, Waiman Long wrote:
>>> On 11/1/23 12:34, Michal Koutný wrote:
>>>> On Tue, Oct 24, 2023 at 10:18:34AM -0400, Waiman Long 
>>>> <longman@...hat.com> wrote:
>>>>> The nr_deadline_tasks field in cpuset structure was introduced by
>>>>> commit 6c24849f5515 ("sched/cpuset: Keep track of SCHED_DEADLINE task
>>>>> in cpusets"). Unlike nr_migrate_dl_tasks which is only modified under
>>>>> cpuset_mutex, nr_deadline_tasks can be updated under two different
>>>>> locks - cpuset_mutex in most cases or css_set_lock in 
>>>>> cgroup_exit(). As
>>>>> a result, data races can happen leading to incorrect 
>>>>> nr_deadline_tasks
>>>>> value.
>>>> The effect is that dl_update_tasks_root_domain() processes tasks
>>>> unnecessarily or that it incorrectly skips dl_add_task_root_domain()?
>>> The effect is that dl_update_tasks_root_domain() may return 
>>> incorrectly or
>>> it is doing unnecessary work. Will update the commit log to reflect 
>>> that.
>>>>> Since it is not practical to somehow take cpuset_mutex in 
>>>>> cgroup_exit(),
>>>>> the easy way out to avoid this possible race condition is by making
>>>>> nr_deadline_tasks an atomic_t value.
>>>> If css_set_lock is useless for this fields and it's going to be 
>>>> atomic,
>>>> could you please add (presumably) a cleanup that moves 
>>>> dec_dl_tasks_cs()
>>>> from under css_set_lock in cgroup_exit() to a (new but specific)
>>>> cpuset_cgrp_subsys.exit() handler?
>>> But css_set_lock is needed for updating other css data. It is true 
>>> that we
>>> can move dec_dl_tasks_cs() outside of the lock. I can do that in the 
>>> next
>>> version.
>> Not sure if you had a chance to check my last question/comment on your
>> previous posting?
>>
>> https://lore.kernel.org/lkml/ZSjfBWgZf15TchA5@localhost.localdomain/
>
> Thanks for the reminder. I look at your comment again. Even though 
> dl_rebuild_rd_accounting() operates on css(es) via 
> css_task_iter_start() and css_task_iter_next(), the css_set_lock is 
> released at the end of it. So it is still possible that a task can 
> call cgroup_exit() after css_task_iter_next() and is being processed 
> by dl_add_task_root_domain(). Is there a helper in the do_exit() path 
> to nullify the dl_task() check. Or maybe we can also check for 
> PF_EXITING in dl_add_task_root_domain() under the pi_lock and do the 
> dl_task() check the under pi_lock to synchronize with 
> dl_add_task_root_domain(). What do you think?
>
> I still believe that it doesn't really matter if we call 
> dec_dl_tasks_cs() inside or outside the css_set_lock.

Just curious. Does the deadline code remove the deadline quota of an 
exiting task?

Regards,
Longman