linux-kernel - Re: [PATCH-cgroup v4] cgroup: Show # of subsystem CSSes in cgroup.stat

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Message-ID: <qozzqah5blnsvc73jrhfuldsaxwsoluuewvgpukzgcuud4nqgc@xnctlkgk5yjv>
Date: Fri, 26 Jul 2024 10:19:05 +0200
From: Michal Koutný <mkoutny@...e.com>
To: Waiman Long <longman@...hat.com>
Cc: Tejun Heo <tj@...nel.org>, 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 v4] cgroup: Show # of subsystem CSSes in
 cgroup.stat

On Thu, Jul 25, 2024 at 04:05:42PM GMT, Waiman Long <longman@...hat.com> wrote:
> > There's also 'debug' subsys. Have you looked at (extending) that wrt
> > dying csses troubleshooting?
> > It'd be good to document here why you decided against it.
> The config that I used for testing doesn't include CONFIG_CGROUP_DEBUG.

I mean if you enable CONFIG_CGROUP_DEBUG, there is 'debug' controller
that exposes files like debug.csses et al.

> That is why "debug" doesn't show up in the sample outputs. The CSS #
> for the debug subsystem should show up if it is enabled.

So these "debugging" numbers could be implemented via debug subsys. So I
wondered why it's not done this way. That reasoning is missing in the
commit message.

> > > +	for_each_css(css, ssid, cgroup) {
> > > +		if ((BIT(ssid) & cgrp_dfl_inhibit_ss_mask) ||
> > > +		    (cgroup_subsys[ssid]->root !=  &cgrp_dfl_root))
> > > +			continue;
> > Is this taken? (Given cgroup.stat is only on the default hierarchy.)
> 
> I am not sure what you are asking here. Since cgroup.stat is a cgroup v2
> only control file, it won't show subsystems that are bound to cgroup v1.

So, is the if (...) ever true? (The file won't exist on v1.)

Michal

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