From mboxrd@z Thu Jan  1 00:00:00 1970
From: Michal =?iso-8859-1?Q?Koutn=FD?= <mkoutny-IBi9RG/b67k@public.gmane.org>
Subject: Re: [RFC PATCH 1/3] mm/memcg: Protect per-CPU counter by disabling
 preemption on PREEMPT_RT
Date: Wed, 5 Jan 2022 15:16:53 +0100
Message-ID: <20220105141653.GA6464@blackbody.suse.cz>
References: <20211222114111.2206248-1-bigeasy@linutronix.de>
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To: Sebastian Andrzej Siewior <bigeasy-hfZtesqFncYOwBW4kG4KsQ@public.gmane.org>
Cc: cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org, Johannes Weiner <hannes-druUgvl0LCNAfugRpC6u6w@public.gmane.org>, Michal Hocko <mhocko-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>, Vladimir Davydov <vdavydov.dev-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>, Andrew Morton <akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>, Thomas Gleixner <tglx-hfZtesqFncYOwBW4kG4KsQ@public.gmane.org>, Waiman Long <longman-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>, Peter Zijlstra <peterz-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>

On Wed, Dec 22, 2021 at 12:41:09PM +0100, Sebastian Andrzej Siewior <bigeasy-hfZtesqFncYOwBW4kG4KsQ@public.gmane.org> wrote:
> The sections with disabled preemption must exclude
> memcg_check_events() so that spinlock_t locks can still be acquired
> (for instance in eventfd_signal()).

The resulting construct in uncharge_batch() raises eybrows. If you can decouple
per-cpu updates from memcg_check_events() on PREEMPT_RT, why not tackle
it the same way on !PREEMPT_RT too (and have just one variant of the
block)?

(Actually, it doesn't seem to me that memcg_check_events() can be
extracted like this from the preempt disabled block since
mem_cgroup_event_ratelimit() relies on similar RMW pattern.
Things would be simpler if PREEMPT_RT didn't allow the threshold event
handlers (akin to Michal Hocko's suggestion of rejecting soft limit).)

Thanks,
Michal