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

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Message-ID: <20250813161708.GA115258@cmpxchg.org>
Date: Wed, 13 Aug 2025 12:17:08 -0400
From: Johannes Weiner <hannes@...xchg.org>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Michal Hocko <mhocko@...nel.org>,
	Roman Gushchin <roman.gushchin@...ux.dev>,
	Shakeel Butt <shakeel.butt@...ux.dev>,
	Muchun Song <muchun.song@...ux.dev>, cgroups@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] memcg: Optimize exit to user space

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>

Nice!

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