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h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=aNSs16S/O+GCzgCR/lVHsGqvnwDXgr7mkEgWPZzafz9LroGq4Ybv5WtgZnEnPvvAN dxhZAZ4I55h/Ib8/IK/qHWr6vJL4iOCi2MD95zT5CvXkJkE+SNvYVqarZz5AsGSFzv 6ueF83Kdm+VdpP+y1r0Mxid/L/h+/69IbhaaXgWc= Date: Wed, 13 Aug 2025 15:40:38 -0700 From: Andrew Morton To: Shakeel Butt Cc: Thomas Gleixner , linux-mm@kvack.org, LKML , Peter Zijlstra , Johannes Weiner , Michal Hocko , Roman Gushchin , Muchun Song , cgroups@vger.kernel.org Subject: Re: [PATCH] memcg: Optimize exit to user space Message-Id: <20250813154038.dee67ecd858b7223d6b5cc6c@linux-foundation.org> In-Reply-To: References: <87tt2b6zgs.ffs@tglx> X-Mailer: Sylpheed 3.8.0beta1 (GTK+ 2.24.33; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 894D280004 X-Stat-Signature: cg3hbsfnxuyo7y5jrgp7wjaqmsyn5pe8 X-Rspam-User: X-HE-Tag: 1755124840-966112 X-HE-Meta: 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 m6FEDfG1 1kKoV0ZUNJfgpwer9LqZOjPkbPxuP8RDC4BndSKvW7Tdn+BrZ5aUV844JTgTHAJ9BrRV+3eq6oonsxU8isLHg+OhB4+/VcTWMRUn0uarE0w7MTkRkyTOgJY8caPrQmAY8nKjzLwXS1Kx1zs+JyNIy3mzxI0hDss07h3PAO6LmZWwsD3hJmcf5sX3f8m1otaNRxjYvXZWAQwNGH1nQQIru/T1cWr0tqjRswDiRnpI50PeA3PN7lPnE7LoV86YUwUidJyMaIeyD4jH/JNvpisAQEs1216BulDwH7KTGZnj3SxtkVuZ2UBV5RhHsv8zRY2GC7djBsTPftRcZsjm9jh3sLkKi1umH7NLcyIXy+hXgnqkvE5n9mkO2pKcgRWflanHTgU8qkv9OJu5mDJg= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Wed, 13 Aug 2025 10:19:03 -0700 Shakeel Butt 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 > > Cc: Johannes Weiner > > Cc: Michal Hocko > > Cc: Roman Gushchin > > Cc: Shakeel Butt > > Cc: Muchun Song > > Cc: Andrew Morton > > Acked-by: Shakeel Butt > > 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; _