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

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Message-ID: <7a2e9bd2-a0fd-4429-b22f-6a246ceb6155@redhat.com>
Date: Thu, 11 Jul 2024 17:57:10 -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 17:00, Waiman Long wrote:
> 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.

Alternatively, we could delay the clearing of cgroup->subsys[] entry 
from offline time to until the css is ready to be freed. We do need to 
add check about the CSS_ONLINE flag when we only want to deal with 
online csses.

Cheers,
Longman