linux-kernel - Re: [PATCH-cgroup v7] cgroup: Show # of subsystem CSSes in cgroup.stat

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Message-ID: <4cb4bczascgy4w774k7y5z5yewvl4civpkuhl46g73ckvoubj2@ag2cfvgayfqs>
Date: Wed, 31 Jul 2024 11:41:39 +0200
From: Michal Koutný <mkoutny@...e.com>
To: Tejun Heo <tj@...nel.org>
Cc: Waiman Long <longman@...hat.com>, Zefan Li <lizefan.x@...edance.com>, 
	Johannes Weiner <hannes@...xchg.org>, 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-cgroup v7] cgroup: Show # of subsystem CSSes in
 cgroup.stat

On Tue, Jul 30, 2024 at 02:21:56PM GMT, Tejun Heo <tj@...nel.org> wrote:
> On Mon, Jul 15, 2024 at 11:00:34AM -0400, Waiman Long wrote:
> > Cgroup subsystem state (CSS) is an abstraction in the cgroup layer to
> > help manage different structures in various cgroup subsystems by being
> > an embedded element inside a larger structure like cpuset or mem_cgroup.
> > 
> > The /proc/cgroups file shows the number of cgroups for each of the
> > subsystems.  With cgroup v1, the number of CSSes is the same as the
> > number of cgroups.  That is not the case anymore with cgroup v2. The
> > /proc/cgroups file cannot show the actual number of CSSes for the
> > subsystems that are bound to cgroup v2.
> > 
> > So if a v2 cgroup subsystem is leaking cgroups (usually memory cgroup),
> > we can't tell by looking at /proc/cgroups which cgroup subsystems may
> > be responsible.
> > 
> > As cgroup v2 had deprecated the use of /proc/cgroups, the hierarchical
> > cgroup.stat file is now being extended to show the number of live and
> > dying CSSes associated with all the non-inhibited cgroup subsystems that
> > have been bound to cgroup v2. The number includes CSSes in the current
> > cgroup as well as in all the descendants underneath it.  This will help
> > us pinpoint which subsystems are responsible for the increasing number
> > of dying (nr_dying_descendants) cgroups.
> > 
> > The CSSes dying counts are stored in the cgroup structure itself
> > instead of inside the CSS as suggested by Johannes. This will allow
> > us to accurately track dying counts of cgroup subsystems that have
> > recently been disabled in a cgroup. It is now possible that a zero
> > subsystem number is coupled with a non-zero dying subsystem number.
> > 
> > The cgroup-v2.rst file is updated to discuss this new behavior.
> > 
> > With this patch applied, a sample output from root cgroup.stat file
> > was shown below.
> ...
> 
> Applied to cgroup/for-6.12.

I think the commit message is missing something like this:

| 'debug' controller wasn't used to provide this information because
| the controller is not recommended in productions kernels, also many of
| them won't enable CONFIG_CGROUP_DEBUG by default.
| 
| Similar information could be retrieved with debuggers like drgn but
| that's also not always available (e.g. lockdown) and the additional
| cost of runtime tracking here is deemed marginal.

or a 'Link:' to the discussion ;-)

Thanks,
Michal

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