linux-kernel - Re: [PATCH v3 1/2] cgroup: Show # of subsystem CSSes in cgroup.stat

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Message-ID: <e42f41af-e8a9-4544-9194-003d6b0f0ba8@redhat.com>
Date: Thu, 11 Jul 2024 17:00:41 -0400
From: Waiman Long <longman@...hat.com>
To: Johannes Weiner <hannes@...xchg.org>
Cc: Tejun Heo <tj@...nel.org>, Zefan Li <lizefan.x@...edance.com>,
 Jonathan Corbet <corbet@....net>, cgroups@...r.kernel.org,
 linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
 Kamalesh Babulal <kamalesh.babulal@...cle.com>,
 Roman Gushchin <roman.gushchin@...ux.dev>
Subject: Re: [PATCH v3 1/2] cgroup: Show # of subsystem CSSes in cgroup.stat

On 7/11/24 15:59, Johannes Weiner wrote:
> On Thu, Jul 11, 2024 at 03:13:12PM -0400, Waiman Long wrote:
>> On 7/11/24 14:59, Tejun Heo wrote:
>>> On Thu, Jul 11, 2024 at 02:51:38PM -0400, Waiman Long wrote:
>>>> On 7/11/24 14:44, Tejun Heo wrote:
>>>>> Hello,
>>>>>
>>>>> On Thu, Jul 11, 2024 at 01:39:38PM -0400, Waiman Long wrote:
>>>>>> On 7/11/24 13:18, Tejun Heo wrote:
>>>>> ...
>>>>>> Currently, I use the for_each_css() macro for iteration. If you mean
>>>>>> displaying all the possible cgroup subsystems even if they are not enabled
>>>>>> for the current cgroup, I will have to manually do the iteration.
>>>>> Just wrapping it with for_each_subsys() should do, no? for_each_css() won't
>>>>> iterate anything if css doesn't exist for the cgroup.
>>>> OK, I wasn't sure if you were asking to list all the possible cgroup v2
>>>> cgroup subsystems even if they weren't enabled in the current cgroup.
>>>> Apparently, that is the case. I prefer it that way too.
>>> Yeah, I think listing all is better. If the list corresponded directly to
>>> cgroup.controllers, it may make sense to only show enabled ones but we can
>>> have dying ones and implicitly enabled memory and so on, so I think it'd be
>>> cleaner to just list them all.
>> That will means cgroup subsystems that are seldomly used like rdma, misc
>> or even hugetlb will always be shown in all the cgroup.stat output. I
>> actually prefer just showing those that are enabled. As for dying memory
>> cgroups, they will only be shown in its online ancestors. We currently
>> don't know how many level down are each of the dying ones.
> It seems odd to me to not show dead ones after a cgroup has disabled
> the controller again. They still consume memory, after all, and so
> continue to be property of that cgroup afterwards.
>
> Instead of doing for_each_css(), would it make more sense to have
>
> 	struct cgroup {
> 		...
> 		int nr_dying_subsys[CGROUP_SUBSYS_COUNT];

What exactly does new this array for? Is this for copying out 
css->nr_dying_descendants before disabling a controller? The number may 
be out of date when it is used. I would think we should store the actual 
css and clearing it again once the css is ready to be freed.

Anyway, I would suggest doing it as a separate add-on patch if we decide 
to do it instead of adding it to the current patch.

Cheers,
Longman