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

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Message-ID: <63726aac-2a9b-11f2-6c24-9f33ced68706@redhat.com>
Date:   Thu, 2 Nov 2023 09:01:38 -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 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.

Cheers,
Longman

Cheers,
Longman