linux-kernel - Re: [PATCH 1/2] cgroup/cpuset: Make cpuset hotplug processing synchronous

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Date: Tue, 2 Apr 2024 11:30:11 -0400
From: Waiman Long <longman@...hat.com>
To: Michal Koutný <mkoutny@...e.com>
Cc: Tejun Heo <tj@...nel.org>, Zefan Li <lizefan.x@...edance.com>,
 Johannes Weiner <hannes@...xchg.org>, Thomas Gleixner <tglx@...utronix.de>,
 Peter Zijlstra <peterz@...radead.org>, "Rafael J. Wysocki"
 <rafael@...nel.org>, Len Brown <len.brown@...el.com>,
 Pavel Machek <pavel@....cz>, Shuah Khan <shuah@...nel.org>,
 linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
 linux-pm@...r.kernel.org, linux-kselftest@...r.kernel.org,
 Frederic Weisbecker <frederic@...nel.org>,
 "Paul E. McKenney" <paulmck@...nel.org>, Ingo Molnar <mingo@...nel.org>,
 Valentin Schneider <vschneid@...hat.com>,
 Anna-Maria Behnsen <anna-maria@...utronix.de>, Alex Shi <alexs@...nel.org>,
 Vincent Guittot <vincent.guittot@...aro.org>,
 Barry Song <song.bao.hua@...ilicon.com>
Subject: Re: [PATCH 1/2] cgroup/cpuset: Make cpuset hotplug processing
 synchronous

On 4/2/24 10:13, Michal Koutný wrote:
> Hello Waiman.
>
> (I have no opinion on the overall locking reworks, only the bits about
> v1 migrations caught my attention.)
>
> On Mon, Apr 01, 2024 at 10:58:57AM -0400, Waiman Long <longman@...hat.com> wrote:
> ...
>> @@ -4383,12 +4377,20 @@ hotplug_update_tasks_legacy(struct cpuset *cs,
>>   	/*
>>   	 * Move tasks to the nearest ancestor with execution resources,
>>   	 * This is full cgroup operation which will also call back into
>> -	 * cpuset. Should be done outside any lock.
>> +	 * cpuset. Execute it asynchronously using workqueue.
>                     ...to avoid deadlock on cpus_read_lock
>
> Is this the reason?
> Also, what happens with the tasks in the window till the migration
> happens?
> Is it handled gracefully that their cpu is gone?

Yes, there is a potential that a cpus_read_lock() may be called leading 
to deadlock. So unless we reverse the current cgroup_mutex --> 
cpu_hotplug_lock ordering, it is not safe to call 
cgroup_transfer_tasks() directly.

>
>
>> -	if (is_empty) {
>> -		mutex_unlock(&cpuset_mutex);
>> -		remove_tasks_in_empty_cpuset(cs);
>> -		mutex_lock(&cpuset_mutex);
>> +	if (is_empty && css_tryget_online(&cs->css)) {
>> +		struct cpuset_remove_tasks_struct *s;
>> +
>> +		s = kzalloc(sizeof(*s), GFP_KERNEL);
> Is there a benefit of having a work for each cpuset?
> Instead of traversing whole top_cpuset once in the async work.

We could do that too. It's just that we have the repeat the iteration 
process once the workfn is invoked, but that has the advantage of not 
needing to do memory allocation. I am OK with either way. Let's see what 
other folks think about that.

Cheers,
Longman