linux-kernel - Re: [PATCH v2 2/3] mm: memcontrol: clean up and document effective low/min calculations

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Message-ID: <20200221171024.GA23476@blackbody.suse.cz>
Date:   Fri, 21 Feb 2020 18:10:24 +0100
From:   Michal Koutný <mkoutny@...e.com>
To:     Johannes Weiner <hannes@...xchg.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Roman Gushchin <guro@...com>, Michal Hocko <mhocko@...e.com>,
        Tejun Heo <tj@...nel.org>, linux-mm@...ck.org,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com
Subject: Re: [PATCH v2 2/3] mm: memcontrol: clean up and document effective
 low/min calculations

On Thu, Dec 19, 2019 at 03:07:17PM -0500, Johannes Weiner <hannes@...xchg.org> wrote:
> The effective protection of any given cgroup is a somewhat complicated
> construct that depends on the ancestor's configuration, siblings'
> configurations, as well as current memory utilization in all these
> groups.
I agree with that. It makes it a bit hard to determine the equilibrium
in advance.


> + *    Consider the following example tree:
>   *
> + *        A      A/memory.low = 2G, A/memory.current = 6G
> + *       //\\
> + *      BC  DE   B/memory.low = 3G  B/memory.current = 2G
> + *               C/memory.low = 1G  C/memory.current = 2G
> + *               D/memory.low = 0   D/memory.current = 2G
> + *               E/memory.low = 10G E/memory.current = 0
>   *
> + *    and memory pressure is applied, the following memory
> + *    distribution is expected (approximately*):
>   *
> + *      A/memory.current = 2G
> + *      B/memory.current = 1.3G
> + *      C/memory.current = 0.6G
> + *      D/memory.current = 0
> + *      E/memory.current = 0
>   *
> + *    *assuming equal allocation rate and reclaimability
I think the assumptions for this example don't hold (anymore).
Because reclaim rate depends on the usage above protection, the siblings
won't be reclaimed equally and so the low_usage proportionality will
change over time and the equilibrium distribution is IMO different (I'm
attaching an Octave script to calculate it).

As it depends on the initial usage, I don't think there can be given
such a general example (for overcommit).


> @@ -6272,12 +6262,63 @@ struct cgroup_subsys memory_cgrp_subsys = {
>   * for next usage. This part is intentionally racy, but it's ok,
>   * as memory.low is a best-effort mechanism.
Although it's a different issue but since this updates the docs I'm
mentioning it -- we treat memory.min the same, i.e. it's subject to the
same race, however, it's not meant to be best effort. I didn't look into
outcomes of potential misaccounting but the comment seems to miss impact
on memory.min protection.

> @@ -6292,52 +6333,29 @@ enum mem_cgroup_protection mem_cgroup_protected(struct mem_cgroup *root,
> [...]
> +	if (parent == root) {
> +		memcg->memory.emin = memcg->memory.min;
> +		memcg->memory.elow = memcg->memory.low;
> +		goto out;
>  	}
Shouldn't this condition be 'if (parent == root_mem_cgroup)'? (I.e. 1st
level takes direct input, but 2nd and further levels redistribute only
what they really got from parent.)


Michal


View attachment "script" of type "text/plain" (2762 bytes)