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Date:   Thu, 12 Oct 2023 12:35:21 -0400
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Subject: Re: [PATCH] cgroup/cpuset: Change nr_deadline_tasks to an atomic_t
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From:   Waiman Long <longman@redhat.com>
To:     Juri Lelli <juri.lelli@redhat.com>
Cc:     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>
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On 10/11/23 08:54, Waiman Long wrote:
>
> 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.

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.

I can update the commit log with these new analysis if you have no 
further objection to this change.

Cheers,
Longman