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

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Message-ID: <389a8abc-7f0f-7bcc-bc58-f70f045d00a5@redhat.com>
Date:   Wed, 11 Oct 2023 08:54:54 -0400
From:   Waiman Long <longman@...hat.com>
To:     Juri Lelli <juri.lelli@...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 10/11/23 04:14, Juri Lelli wrote:
> On 10/10/23 16:03, Waiman Long wrote:
>> On 10/10/23 15:44, Waiman Long wrote:
>>> On 10/10/23 01:34, Juri Lelli wrote:
>>>> Hi,
>>>>
>>>> On 09/10/23 15:15, Waiman Long 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 in various contexts
>>>>> under different locks. As a result, data races may happen that cause
>>>>> incorrect value to be stored in nr_deadline_tasks leading to incorrect
>>>> Could you please make an example of such data races?
>>> Since update to cs->nr_deadline_tasks is not protected by a single lock,
>>> it is possible that multiple CPUs may try to modify it at the same
>>> time.  It is possible that nr_deadline_tasks++ and nr_deadline_tasks--
>>> can be done in a single instruction like in x86 and hence atomic.
>>> However, operation like "cs->nr_deadline_tasks +=
>>> cs->nr_migrate_dl_tasks" is likely a RMW operation and so is subjected
>>> to racing. It is mostly theoretical, but probably not impossible.
>> Sorry, even increment and decrement operators are not atomic.
>>
>> inc_dl_tasks_cs() is only called from switched_to_dl() in deadline.c which
>> is protected by the rq_lock, but there are multiple rq's. dec_dl_tasks_cs()
>> is called from switched_from_dl() in deadline.c and cgroup_exit() in
>> cgroup.c. The later one is protected by css_set_lock. The other place where
>> nr_deadline_tasks can be changed is in cpuset_attach() protected by
>> cpuset_mutex.
> So, let's see. :)
>
> switched_to_dl(), switched_from_dl() and cpuset_attach() should all be
> protected (for DEADLINE tasks) by cpuset_mutex, see [1] for the former
> two.
Yes, I missed the cpuset_lock() call.
> What leaves me perplexed is indeed cgroup_exit(), which seems to operate
> under css_set_lock as you say. I however wonder why is that not racy
> already wrt, say, cpuset_attach() which AFAIU uses css information w/o
> holding css_set_lock?

The css_set_lock protects changes made to css_set. Looking at 
cgroup_migrate_execute(), css_set_lock is taken when the tasks are 
actually moving from one css_set to another one. cpuset_attach() is 
called just to update the CPU and node affinity and cpuset_mutex is 
taken to ensure stability of the CPU and node masks. There is no change 
to css_set and so css_set_lock isn't needed.

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.

Cheers,
Longman


>
> Thanks,
> Juri
>
> 1 - https://elixir.bootlin.com/linux/latest/source/kernel/sched/core.c#L7688
>