linux-kernel - Re: [PATCH] memcg: Optimize exit to user space

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Message-Id: <20250813154038.dee67ecd858b7223d6b5cc6c@linux-foundation.org>
Date: Wed, 13 Aug 2025 15:40:38 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Shakeel Butt <shakeel.butt@...ux.dev>
Cc: Thomas Gleixner <tglx@...utronix.de>, linux-mm@...ck.org, LKML
 <linux-kernel@...r.kernel.org>, Peter Zijlstra <peterz@...radead.org>,
 Johannes Weiner <hannes@...xchg.org>, Michal Hocko <mhocko@...nel.org>,
 Roman Gushchin <roman.gushchin@...ux.dev>, Muchun Song
 <muchun.song@...ux.dev>, cgroups@...r.kernel.org
Subject: Re: [PATCH] memcg: Optimize exit to user space

On Wed, 13 Aug 2025 10:19:03 -0700 Shakeel Butt <shakeel.butt@...ux.dev> wrote:

> On Wed, Aug 13, 2025 at 04:57:55PM +0200, Thomas Gleixner wrote:
> > memcg uses TIF_NOTIFY_RESUME to handle reclaiming on exit to user
> > space. TIF_NOTIFY_RESUME is a multiplexing TIF bit, which is utilized by
> > other entities as well.
> > 
> > This results in a unconditional mem_cgroup_handle_over_high() call for
> > every invocation of resume_user_mode_work(), which is a pointless
> > exercise as most of the time there is no reclaim work to do.
> > 
> > Especially since RSEQ is used by glibc, TIF_NOTIFY_RESUME is raised
> > quite frequently and the empty calls show up in exit path profiling.
> > 
> > Optimize this by doing a quick check of the reclaim condition before
> > invoking it.
> > 
> > Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> > Cc: Johannes Weiner <hannes@...xchg.org>
> > Cc: Michal Hocko <mhocko@...nel.org>
> > Cc: Roman Gushchin <roman.gushchin@...ux.dev>
> > Cc: Shakeel Butt <shakeel.butt@...ux.dev>
> > Cc: Muchun Song <muchun.song@...ux.dev>
> > Cc: Andrew Morton <akpm@...ux-foundation.org>
> 
> Acked-by: Shakeel Butt <shakeel.butt@...ux.dev>
> 
> Since this is seen in profiling data and it is simple enough, I think it
> is worth backporting to stable trees as well.

People will probably do this, but it's a big break of -stable rules.

If it is a regression fix (ie, has a Fixes:) and if it makes a big
difference (ie, comes with impressive quantitative testing results)
then maybe we could push it into -stable anyway...

> In the followup cleanup, we can remove the (!nr_pages) check inside
> __mem_cgroup_handle_over_high() as well.

yup, how about we do that now

--- a/mm/memcontrol.c~memcg-optimize-exit-to-user-space-fix
+++ a/mm/memcontrol.c
@@ -2213,9 +2213,6 @@ void __mem_cgroup_handle_over_high(gfp_t
 	struct mem_cgroup *memcg;
 	bool in_retry = false;
 
-	if (likely(!nr_pages))
-		return;
-
 	memcg = get_mem_cgroup_from_mm(current->mm);
 	current->memcg_nr_pages_over_high = 0;
 
_