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Date:   Thu, 2 Nov 2023 11:26:39 +0100
From:   Juri Lelli <juri.lelli@redhat.com>
To:     Waiman Long <longman@redhat.com>
Cc:     Michal =?iso-8859-1?Q?Koutn=FD?= <mkoutny@suse.com>,
        Tejun Heo <tj@kernel.org>, Zefan Li <lizefan.x@bytedance.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        linux-kernel@vger.kernel.org, cgroups@vger.kernel.org,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>,
        Qais Yousef <qyousef@layalina.io>, Hao Luo <haoluo@google.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Xia Fukun <xiafukun@huawei.com>
Subject: Re: [PATCH v2] cgroup/cpuset: Change nr_deadline_tasks to an
 atomic_t value
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References: <20231024141834.4073262-1-longman@redhat.com>
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Hi Waiman,

On 01/11/23 13:59, Waiman Long wrote:
> On 11/1/23 12:34, Michal Koutn? wrote:
> > On Tue, Oct 24, 2023 at 10:18:34AM -0400, Waiman Long <longman@redhat.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,
Juri