linux-kernel - Re: [PATCH v2] mm: memcg: link page counters to root if use

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Date:   Thu, 29 Oct 2020 16:39:21 +0100
From:   Michal Koutný <mkoutny@...e.com>
To:     Roman Gushchin <guro@...com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, kernel-team@...com, ltp@...ts.linux.it,
        Richard Palethorpe <rpalethorpe@...e.com>,
        stable@...r.kernel.org
Subject: Re: [PATCH v2] mm: memcg: link page counters to root if
 use_hierarchy is false

Hi.

On Mon, Oct 26, 2020 at 04:13:26PM -0700, Roman Gushchin <guro@...com> wrote:
> Please note, that in the non-hierarchical mode all objcgs are always
> reparented to the root memory cgroup, even if the hierarchy has more
> than 1 level. This patch doesn't change it.
> 
> The patch also doesn't affect how the hierarchical mode is working,
> which is the only sane and truly supported mode now.
I agree with the patch and you can add
Reviewed-by: Michal Koutný <mkoutny@...e.com>

However, it effectively switches any users of root.use_hierarchy=0 (if there
are any, watching the counters of root memcg) into root.use_hierarchy=1.
So I'd show them the warning even with a single level of cgroups, i.e.
add this hunk

@@ -5356,12 +5356,11 @@
 		page_counter_init(&memcg->kmem, &root_mem_cgroup->kmem);
 		page_counter_init(&memcg->tcpmem, &root_mem_cgroup->tcpmem);
 		/*
-		 * Deeper hierachy with use_hierarchy == false doesn't make
+		 * Hierachy with use_hierarchy == false doesn't make
 		 * much sense so let cgroup subsystem know about this
 		 * unfortunate state in our controller.
 		 */
-		if (parent != root_mem_cgroup)
-			memory_cgrp_subsys.broken_hierarchy = true;
+		memory_cgrp_subsys.broken_hierarchy = true;
 	}
 
 	/* The following stuff does not apply to the root */

What do you think?

Michal

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