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

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Message-ID: <ZSjfBWgZf15TchA5@localhost.localdomain>
Date:   Fri, 13 Oct 2023 08:09:09 +0200
From:   Juri Lelli <juri.lelli@...hat.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,
        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] cgroup/cpuset: Change nr_deadline_tasks to an atomic_t
 value

On 12/10/23 12:35, Waiman Long wrote:
> On 10/11/23 08:54, Waiman Long wrote:

...

> > We can argue that there can be racing between cgroup_exit() and the
> > iteration of tasks in cpuset_attach() or cpuset_can_attach(). An
> > rcu_read_lock() is probably needed. I am stilling investigating that.
> 
> Cgroup has a rather complex task migration and iteration scheme. According
> to the following comments in include/linux/cgroup-defs.h:
> 
>         /*
>          * Lists running through all tasks using this cgroup group.
>          * mg_tasks lists tasks which belong to this cset but are in the
>          * process of being migrated out or in.  Protected by
>          * css_set_lock, but, during migration, once tasks are moved to
>          * mg_tasks, it can be read safely while holding cgroup_mutex.
>          */
>         struct list_head tasks;
>         struct list_head mg_tasks;
>         struct list_head dying_tasks;
> 
> I haven't fully figured out how that protection works yet. Assuming that is
> the case, task iteration in cpuset_attach() should be fine since
> cgroup_mutex is indeed held when it is invoked. That protection, however,
> does not applied to nr_deadline_tasks. It may be too costly to acquire
> cpuset_mutex before updating nr_deadline_tasks in cgroup_exit(). So changing
> it to an atomic_t should be the easy way out of the potential racing
> problem.

My biggest perplexity is/was still about dl_rebuild_rd_accounting() and
cgroup_exit(); I wonder if the latter, operating outside cpuset_mutex
guard, might still be racy wrt the former (even if we change to
atomic_t).

However, looking again at it, dl_rebuild_rd_accounting() operates on
css(es) via css_task_iter_start(), which grabs css_set_lock. So maybe we
are OK already also for this case?

Apologies for being pedantic, but we fought already several times with
races around these bits and now I'm probably over-suspicious. :)

Thanks,
Juri