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 [1] added in [2], but the
periodic flushing started in mem_cgroup_css_online is not. Skip it.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/mm/memcontrol.c?h=v6.8-rc1#n753
[2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=7d7ef0a4686abe43cd76a141b340a348f45ecdf2

Fixes: aa48e47e3906 ("memcg: infrastructure to flush memcg stats")
Reported-by: Minchan Kim <minchan@google.com>
Signed-off-by: T.J. Mercier <tjmercier@google.com>
---
 mm/memcontrol.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index e4c8735e7c85..bad8f9dfc9ab 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -5586,7 +5586,7 @@ static int mem_cgroup_css_online(struct cgroup_subsys_state *css)
 	if (alloc_shrinker_info(memcg))
 		goto offline_kmem;
 
-	if (unlikely(mem_cgroup_is_root(memcg)))
+	if (unlikely(mem_cgroup_is_root(memcg)) && !mem_cgroup_disabled())
 		queue_delayed_work(system_unbound_wq, &stats_flush_dwork,
 				   FLUSH_TIME);
 	lru_gen_online_memcg(memcg);
-- 
2.43.0.429.g432eaa2c6b-goog

On Fri, Jan 26, 2024 at 08:33:52PM +0000, T.J. Mercier 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 [1] added in [2], but the
> periodic flushing started in mem_cgroup_css_online is not. Skip it.
> 
> [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/mm/memcontrol.c?h=v6.8-rc1#n753
> [2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=7d7ef0a4686abe43cd76a141b340a348f45ecdf2
> 
> Fixes: aa48e47e3906 ("memcg: infrastructure to flush memcg stats")
> Reported-by: Minchan Kim <minchan@google.com>
> Signed-off-by: T.J. Mercier <tjmercier@google.com>

With what Shakeel pointed out resolved:

Acked-by: Johannes Weiner <hannes@cmpxchg.org>

On Fri, Jan 26, 2024 at 1:06 PM Johannes Weiner <hannes@cmpxchg.org> wrote:
>
> On Fri, Jan 26, 2024 at 08:33:52PM +0000, T.J. Mercier 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 [1] added in [2], but the
> > periodic flushing started in mem_cgroup_css_online is not. Skip it.
> >
> > [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/mm/memcontrol.c?h=v6.8-rc1#n753
> > [2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=7d7ef0a4686abe43cd76a141b340a348f45ecdf2
> >
> > Fixes: aa48e47e3906 ("memcg: infrastructure to flush memcg stats")
> > Reported-by: Minchan Kim <minchan@google.com>
> > Signed-off-by: T.J. Mercier <tjmercier@google.com>
>
> With what Shakeel pointed out resolved:
>
> Acked-by: Johannes Weiner <hannes@cmpxchg.org>

Thanks Shakeel and Johannes. v2 is here, hopefully fixed up correctly:
https://lore.kernel.org/all/20240126211927.1171338-1-tjmercier@google.com/

On Fri, Jan 26, 2024 at 12:34 PM T.J. Mercier <tjmercier@google.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 [1] added in [2], but the

Remove [1] reference and instead of [2] add the actual commit reference.

> periodic flushing started in mem_cgroup_css_online is not. Skip it.
>
> [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/mm/memcontrol.c?h=v6.8-rc1#n753
> [2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=7d7ef0a4686abe43cd76a141b340a348f45ecdf2
>
> Fixes: aa48e47e3906 ("memcg: infrastructure to flush memcg stats")
> Reported-by: Minchan Kim <minchan@google.com>
> Signed-off-by: T.J. Mercier <tjmercier@google.com>

Acked-by: Shakeel Butt <shakeelb@google.com>