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

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Message-ID: <rzzosab2z64ae5kemem6evu5qsggef2mcjz3yw2ieysoxzsvvp@26mlfo2qidml>
Date:   Wed, 1 Nov 2023 17:34:54 +0100
From:   Michal Koutný <mkoutny@...e.com>
To:     Waiman Long <longman@...hat.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 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()?

> 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?

Thanks,
Michal

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