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

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Message-ID: <20240711195946.GA1094169@cmpxchg.org>
Date: Thu, 11 Jul 2024 15:59:46 -0400
From: Johannes Weiner <hannes@...xchg.org>
To: Waiman Long <longman@...hat.com>
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 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];
		...
	}

and just always print them all, regardless of what is, or was,
enabled?