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

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Message-ID: <0efbedff-3456-4e6a-8d2d-79b89a18864d@redhat.com>
Date: Thu, 25 Jul 2024 16:05:42 -0400
From: Waiman Long <longman@...hat.com>
To: Michal Koutný <mkoutny@...e.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 7/25/24 09:15, Michal Koutný wrote:
> Hello.
>
> On Wed, Jul 10, 2024 at 10:51:53PM GMT, Waiman Long <longman@...hat.com> wrote:
>> 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 as long as it is not zero.  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.
> This implementation means every onlining/offlining (only additionally)
> contends in root's css updates (even when stats aren't ever read).
>
> 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. 
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.
>
>> --- a/kernel/cgroup/cgroup.c
>> +++ b/kernel/cgroup/cgroup.c
>> @@ -3669,12 +3669,36 @@ static int cgroup_events_show(struct seq_file *seq, void *v)
>>   static int cgroup_stat_show(struct seq_file *seq, void *v)
>>   {
>>   	struct cgroup *cgroup = seq_css(seq)->cgroup;
>> +	struct cgroup_subsys_state *css;
>> +	int ssid;
>>   
>>   	seq_printf(seq, "nr_descendants %d\n",
>>   		   cgroup->nr_descendants);
>>   	seq_printf(seq, "nr_dying_descendants %d\n",
>>   		   cgroup->nr_dying_descendants);
>>   
>> +	/*
>> +	 * Show the number of live and dying csses associated with each of
>> +	 * non-inhibited cgroup subsystems bound to cgroup v2 if non-zero.
>> +	 *
>> +	 * Without proper lock protection, racing is possible. So the
>> +	 * numbers may not be consistent when that happens.
>> +	 */
>> +	rcu_read_lock();
>> +	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.

Cheers,
Longman