linux-kernel - Re: [PATCH v2] mm: memcg: Don't periodically flush stats when memcg is disabled

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Date: Thu, 1 Feb 2024 15:26:12 +0100
From: Michal Koutný <mkoutny@...e.com>
To: "T.J. Mercier" <tjmercier@...gle.com>
Cc: Johannes Weiner <hannes@...xchg.org>, Michal Hocko <mhocko@...nel.org>, 
	Roman Gushchin <roman.gushchin@...ux.dev>, Shakeel Butt <shakeelb@...gle.com>, 
	Muchun Song <muchun.song@...ux.dev>, Andrew Morton <akpm@...ux-foundation.org>, android-mm@...gle.com, 
	Minchan Kim <minchan@...gle.com>, cgroups@...r.kernel.org, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] mm: memcg: Don't periodically flush stats when memcg
 is disabled

On Fri, Jan 26, 2024 at 09:19:25PM +0000, "T.J. Mercier" <tjmercier@...gle.com> wrote:
> The root memcg is onlined even when memcg is disabled. When it's onlined
> a 2 second periodic stat flush is started, but no stat flushing is
> required when memcg is disabled because there can be no child memcgs.
> Most calls to flush memcg stats are avoided when memcg is disabled as a
> result of the mem_cgroup_disabled check added in 7d7ef0a4686a
> ("mm: memcg: restore subtree stats flushing"), but the periodic flushing
> started in mem_cgroup_css_online is not. Skip it.

Have you tried
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -6099,6 +6099,9 @@ int __init cgroup_init(void)
        cgroup_unlock();

        for_each_subsys(ss, ssid) {
+               if (!cgroup_ssid_enabled(ssid))
+                       continue;
+
                if (ss->early_init) {
                        struct cgroup_subsys_state *css =
                                init_css_set.subsys[ss->id];
@@ -6118,9 +6121,6 @@ int __init cgroup_init(void)
                 * disabled flag and cftype registration needs kmalloc,
                 * both of which aren't available during early_init.
                 */
-               if (!cgroup_ssid_enabled(ssid))
-                       continue;
-
                if (cgroup1_ssid_disabled(ssid))
                        pr_info("Disabling %s control group subsystem in v1 mounts\n",
                                ss->legacy_name);
?
I'm asking about a try because I'm not sure whether this does not blow
up due to something missing. But I think disabled controllers would not
need to be (root-)onlined at all.

Thanks,
Michal

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