linux-kernel - Re: [PATCH] memcg: add support to generate the total count of children from root

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Message-ID: <20201124112612.GV27488@dhcp22.suse.cz>
Date:   Tue, 24 Nov 2020 12:26:12 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Aaron Tomlin <atomlin@...hat.com>
Cc:     linux-kernel@...r.kernel.org, hannes@...xchg.org,
        vdavydov.dev@...il.com, akpm@...ux-foundation.org,
        cgroups@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] memcg: add support to generate the total count of
 children from root

On Tue 24-11-20 10:58:36, Aaron Tomlin wrote:
> Each memory-controlled cgroup is assigned a unique ID and the total
> number of memory cgroups is limited to MEM_CGROUP_ID_MAX.
> 
> This patch provides the ability to determine the number of
> memory cgroups from the root memory cgroup, only.
> A value of 1 (i.e. self count) is returned if there are no children.
> For example, the number of memory cgroups can be established by
> reading the /sys/fs/cgroup/memory/memory.total_cnt file.

Could you add some explanation why is this information useful for
userspace? Who is going to use it and why a simple scripting on top of
cgroupfs is insufficient.

> Signed-off-by: Aaron Tomlin <atomlin@...hat.com>
> ---
>  mm/memcontrol.c | 18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 29459a6ce1c7..a4f7cb40e233 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -4535,6 +4535,19 @@ static int mem_cgroup_oom_control_write(struct cgroup_subsys_state *css,
>  	return 0;
>  }
>  
> +static int mem_cgroup_total_count_read(struct cgroup_subsys_state *css,
> +				      struct cftype *cft)
> +{
> +	struct mem_cgroup *iter, *memcg = mem_cgroup_from_css(css);
> +	int num = 0;
> +
> +	for_each_mem_cgroup_tree(iter, memcg)
> +		num++;
> +
> +	/* Returns 1 (i.e. self count) if no children. */
> +	return num;
> +}
> +
>  #ifdef CONFIG_CGROUP_WRITEBACK
>  
>  #include <trace/events/writeback.h>
> @@ -5050,6 +5063,11 @@ static struct cftype mem_cgroup_legacy_files[] = {
>  		.write_u64 = mem_cgroup_oom_control_write,
>  		.private = MEMFILE_PRIVATE(_OOM_TYPE, OOM_CONTROL),
>  	},
> +	{
> +		.name = "total_cnt",
> +		.flags = CFTYPE_ONLY_ON_ROOT,
> +		.read_u64 = mem_cgroup_total_count_read,
> +	},
>  	{
>  		.name = "pressure_level",
>  	},
> -- 
> 2.26.2
> 

-- 
Michal Hocko
SUSE Labs