Re: [PATCH v3 2/5] mm/memcg: Disable threshold event handlers on PREEMPT_RT

[Shakeel Butt <shakeelb@google.com>]

On Thu, Feb 17, 2022 at 1:48 AM Sebastian Andrzej Siewior
<bigeasy@linutronix.de> wrote:
>
> During the integration of PREEMPT_RT support, the code flow around
> memcg_check_events() resulted in `twisted code'. Moving the code around
> and avoiding then would then lead to an additional local-irq-save
> section within memcg_check_events(). While looking better, it adds a
> local-irq-save section to code flow which is usually within an
> local-irq-off block on non-PREEMPT_RT configurations.
>
> The threshold event handler is a deprecated memcg v1 feature. Instead of
> trying to get it to work under PREEMPT_RT just disable it. There should
> be no users on PREEMPT_RT. From that perspective it makes even less
> sense to get it to work under PREEMPT_RT while having zero users.
>
> Make memory.soft_limit_in_bytes and cgroup.event_control return
> -EOPNOTSUPP on PREEMPT_RT. Make an empty memcg_check_events() and
> memcg_write_event_control() which return only -EOPNOTSUPP on PREEMPT_RT.
> Document that the two knobs are disabled on PREEMPT_RT.
>
> Suggested-by: Michal Hocko <mhocko@kernel.org>
> Suggested-by: Michal Koutný <mkoutny@suse.com>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> Acked-by: Roman Gushchin <guro@fb.com>
> Acked-by: Johannes Weiner <hannes@cmpxchg.org>

Reviewed-by: Shakeel Butt <shakeelb@google.com>