linux-kernel - Re: [PATCH] cgroup/cpuset: update parent subparts cpumask while holding css refcnt

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Date:   Mon, 10 Jul 2023 11:40:36 -0400
From:   Waiman Long <longman@...hat.com>
To:     Michal Koutný <mkoutny@...e.com>,
        Miaohe Lin <linmiaohe@...wei.com>
Cc:     tj@...nel.org, hannes@...xchg.org, lizefan.x@...edance.com,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] cgroup/cpuset: update parent subparts cpumask while
 holding css refcnt

On 7/10/23 11:11, Michal Koutný wrote:
> Hello.
>
> On Sat, Jul 01, 2023 at 02:50:49PM +0800, Miaohe Lin <linmiaohe@...wei.com> wrote:
>> --- a/kernel/cgroup/cpuset.c
>> +++ b/kernel/cgroup/cpuset.c
>> @@ -1806,9 +1806,12 @@ static int update_cpumask(struct cpuset *cs, struct cpuset *trialcs,
>>   		cpuset_for_each_child(cp, css, parent)
>>   			if (is_partition_valid(cp) &&
>>   			    cpumask_intersects(trialcs->cpus_allowed, cp->cpus_allowed)) {
>> +				if (!css_tryget_online(&cp->css))
>> +					continue;
>>   				rcu_read_unlock();
>>   				update_parent_subparts_cpumask(cp, partcmd_invalidate, NULL, &tmp);
>>   				rcu_read_lock();
>> +				css_put(&cp->css);
> Apologies for a possibly noob question -- why is RCU read lock
> temporarily dropped within the loop?
> (Is it only because of callback_lock or cgroup_file_kn_lock (via
> notify_partition_change()) on PREEMPT_RT?)
>
>
>
> [
> OT question:
> 	cpuset_for_each_child(cp, css, parent)				(1)
> 		if (is_partition_valid(cp) &&
> 		    cpumask_intersects(trialcs->cpus_allowed, cp->cpus_allowed)) {
> 			if (!css_tryget_online(&cp->css))
> 				continue;
> 			rcu_read_unlock();
> 			update_parent_subparts_cpumask(cp, partcmd_invalidate, NULL, &tmp);
> 			  ...
> 			  update_tasks_cpumask(cp->parent)
> 			    ...
> 			    css_task_iter_start(&cp->parent->css, 0, &it);	(2)
> 			      ...
> 			rcu_read_lock();
> 			css_put(&cp->css);
> 		}
>
> May this touch each task same number of times as its depth within
> herarchy?

I believe the primary reason is because update_parent_subparts_cpumask() 
can potential run for quite a while. So we don't want to hold the 
rcu_read_lock for too long. There may also be a potential that 
schedule() may be called.

Cheers,
Longman