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

[Waiman Long]

On 7/25/24 09:15, Michal Koutný wrote:
> Hello.
>
> On Wed, Jul 10, 2024 at 10:51:53PM GMT, Waiman Long <longman@xxxxxxxxxx> 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