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

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Message-ID: <8e1b5497-d4ca-50a0-7cb1-ffa098e0a1c2@redhat.com>
Date:   Wed, 1 Nov 2023 13:59:47 -0400
From:   Waiman Long <longman@...hat.com>
To:     Michal Koutný <mkoutny@...e.com>
Cc:     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,
        Juri Lelli <juri.lelli@...hat.com>,
        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 11/1/23 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.

Cheers,
Longman