Re: [PATCH v3 1/5] mm/memcg: Revert ("mm/memcg: optimize user context object stock access")

[Shakeel Butt <shakeelb@google.com>]

On Thu, Feb 17, 2022 at 1:48 AM Sebastian Andrzej Siewior
<bigeasy@linutronix.de> wrote:
>
> From: Michal Hocko <mhocko@suse.com>
>
> The optimisation is based on a micro benchmark where local_irq_save() is
> more expensive than a preempt_disable(). There is no evidence that it is
> visible in a real-world workload and there are CPUs where the opposite is
> true (local_irq_save() is cheaper than preempt_disable()).
>
> Based on micro benchmarks, the optimisation makes sense on PREEMPT_NONE
> where preempt_disable() is optimized away. There is no improvement with
> PREEMPT_DYNAMIC since the preemption counter is always available.
>
> The optimization makes also the PREEMPT_RT integration more complicated
> since most of the assumption are not true on PREEMPT_RT.
>
> Revert the optimisation since it complicates the PREEMPT_RT integration
> and the improvement is hardly visible.
>
> [ bigeasy: Patch body around Michal's diff ]
>
> Link: https://lore.kernel.org/all/YgOGkXXCrD%2F1k+p4@dhcp22.suse.cz
> Link: https://lkml.kernel.org/r/YdX+INO9gQje6d0S@linutronix.de
> Signed-off-by: Michal Hocko <mhocko@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>