Re: [PATCH] cgroup: add cgroup.stat interface with basic hierarchy stats

[Tejun Heo <tj@kernel.org>]

Hello,

On Thu, Jul 27, 2017 at 05:14:20PM +0100, Roman Gushchin wrote:
> Add a cgroup.stat interface to the base cgroup control files
> with the following metrics:
> 
> nr_descendants		total number of descendant cgroups
> nr_dying_descendants	total number of dying descendant cgroups
> max_descendant_depth	maximum descent depth below the current cgroup

Yeah, this'd be great to have.  Some comments below.

> +  cgroup.stat
> +	A read-only flat-keyed file with the following entries:
> +
> +	  nr_descendants
> +		Total number of descendant cgroups.
> +
> +	  nr_dying_descendants
> +		Total number of dying descendant cgroups.

Can you please go into more detail on what's going on with dying
descendants here?

> +static int cgroup_stats_show(struct seq_file *seq, void *v)
> +{
> +	struct cgroup_subsys_state *css;
> +	unsigned long total = 0;
> +	unsigned long offline = 0;
> +	int max_level = 0;
> +
> +	rcu_read_lock();
> +	css_for_each_descendant_pre(css, seq_css(seq)) {
> +		if (css == seq_css(seq))
> +			continue;
> +		++total;

Let's do post increment for consistency.

> +		if (!(css->flags & CSS_ONLINE))
> +			++offline;
> +		if (css->cgroup->level > max_level)
> +			max_level = css->cgroup->level;
> +	}
> +	rcu_read_unlock();

I wonder whether we want to keep these counters in sync instead of
trying to gather the number on read.  Walking all descendants can get
expensive pretty quickly and things like nr_descendants will be useful
for other purposes too.

Thanks.

-- 
tejun