From mboxrd@z Thu Jan  1 00:00:00 1970
From: Sebastian Andrzej Siewior <bigeasy-hfZtesqFncYOwBW4kG4KsQ@public.gmane.org>
Subject: Re: [RFC PATCH 1/3] mm/memcg: Protect per-CPU counter by disabling
 preemption on PREEMPT_RT
Date: Thu, 13 Jan 2022 14:08:10 +0100
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To: Michal =?utf-8?Q?Koutn=C3=BD?= <mkoutny-IBi9RG/b67k@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 2022-01-05 15:16:53 [+0100], Michal Koutn=C3=BD wrote:
> On Wed, Dec 22, 2021 at 12:41:09PM +0100, Sebastian Andrzej Siewior <bige=
asy-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()).
>=20
> The resulting construct in uncharge_batch() raises eybrows. If you can de=
couple
> 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)?

That would mean that mem_cgroup_event_ratelimit() needs a
local_irq_save(). If that is okay then sure I can move it that way.

> (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).)

I added a preempt-disable() section restricted to RT to
mem_cgroup_event_ratelimit(). I had to exclude memcg_check_events() from
the block because of the spinlock_t locks involved down the road
(eventfd_signal() for instance).

I remember Michal (Hocko) suggested excluding/ rejecting soft limit but
I didn't know where exactly and its implications. In this block here I
just followed the replacement of irq-off with preempt-off for RT.

> Thanks,
> Michal

Sebastian