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

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Message-ID: <ZUUPS6aRrkDjOywb@localhost.localdomain>
Date:   Fri, 3 Nov 2023 16:18:35 +0100
From:   Juri Lelli <juri.lelli@...hat.com>
To:     Waiman Long <longman@...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 02/11/23 14:08, Waiman Long wrote:
> On 11/2/23 09:01, Waiman Long wrote:
> > 
> > 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.
> 
> Just curious. Does the deadline code remove the deadline quota of an exiting
> task?

Ah, interesting observation. We do indeed remove a DL tasks bandwidth
from either within task_non_contending (if zerolag time has passed at
the time the task is dying) or a bit later when the inactive timer fires
(check related paths with TASK_DEAD in task_non_contending and
inactive_task_timer). So, maybe we could do the cs subtraction at this
point as well? Maybe it's even more correct I'm now thinking (or maybe it's
just Friday :).