linux-kernel - Re: [PATCH] sched/schedutil: Fix deadlock between cpuset and cpu hotplug when using schedutil

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Date:   Tue, 12 Jul 2022 22:49:57 -0400
From:   Waiman Long <longman@...hat.com>
To:     Tejun Heo <tj@...nel.org>, Qais Yousef <qais.yousef@....com>
Cc:     Xuewen Yan <xuewen.yan@...soc.com>, rafael@...nel.org,
        viresh.kumar@...aro.org, mingo@...hat.com, peterz@...radead.org,
        juri.lelli@...hat.com, vincent.guittot@...aro.org,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de, bristot@...hat.com, linux-kernel@...r.kernel.org,
        ke.wang@...soc.com, xuewyan@...mail.com, linux-pm@...r.kernel.org
Subject: Re: [PATCH] sched/schedutil: Fix deadlock between cpuset and cpu
 hotplug when using schedutil


On 7/11/22 16:58, Tejun Heo wrote:
> (cc'ing Waiman)
>
> On Mon, Jul 11, 2022 at 06:46:29PM +0100, Qais Yousef wrote:
>> Have you tried running with PROVE_LOCKDEP enabled? It'll help print a useful
>> output about the DEADLOCK. But your explanation was good and clear to me.
> I don't think lockdep would be able to track CPU1 -> CPU2 dependency here
> unfortunately.

That is the case AFAIK. Lockdep only track individually the locks taken 
by each task.


>> AFAIU:
>>
>>
>> CPU0                                     CPU1                                   CPU2
>>
>> // attach task to a different
>> // cpuset cgroup via sysfs
>> __acquire(cgroup_threadgroup_rwsem)
>>
>>                                           // pring up CPU2 online
>>                                           __acquire(cpu_hotplug_lock)
>>                                           // wait for CPU2 to come online
>>                                                                                  // bringup cpu online
>>                                                                                  // call cpufreq_online() which tries to create sugov kthread
>> __acquire(cpu_hotplug_lock)                                                     copy_process()
>>                                                                                     cgroup_can_fork()
>>                                                                                        cgroup_css_set_fork()
>>                                                                                        __acquire(cgroup_threadgroup_rwsem)
>> // blocks forever                        // blocks forever                            // blocks forever
>>
>>
>> Is this a correct summary of the problem?
>>
>> The locks are held in reverse order and we end up with a DEADLOCK.
>>
>> I believe the same happens on offline it's just the path to hold the
>> cgroup_threadgroup_rwsem on CPU2 is different.
>>
>> This will be a tricky one. Your proposed patch might fix it for this case, but
>> if there's anything else that creates a kthread when a cpu goes online/offline
>> then we'll hit the same problem again.
>>
>> I haven't reviewed your patch to be honest, but I think worth seeing first if
>> there's something that can be done at the 'right level' first.
>>
>> Needs head scratching from my side at least. This is the not the first type of
>> locking issue between hotplug and cpuset :-/
> Well, the only thing I can think of is always grabbing cpus_read_lock()
> before grabbing threadgroup_rwsem. Waiman, what do you think?

That is a possible solution as cpus_read_lock() is rather lightweight. 
It is a good practice to acquire it first.

Cheers,
Longman

>
> Thanks.
>