linux-kernel - Re: [PATCH-next v3] mm/memcg: Properly handle memcg_stock access for PREEMPT

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Date:   Fri, 17 Dec 2021 20:48:47 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Waiman Long <longman@...hat.com>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
        linux-mm@...ck.org, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH-next v3] mm/memcg: Properly handle memcg_stock access for
 PREEMPT_RT

On 2021-12-17 13:46:53 [-0500], Waiman Long wrote:
> > annotation. Looking at the history, I'm also impressed by that fact that
> > disabling/ enabling interrupts is *so* expensive that all this is
> > actually worth it.
> 
> For !RT with voluntary or no preemption, preempt_disable() is just a
> compiler barrier. So it is definitely cheaper than disabling interrupt. The
> performance benefit is less with preemptible but !RT kernel. Microbenchmark
> testing shows a performance improvement of a few percents depending on the
> exact benchmark.

Thanks for confirming. I got the feeling that this optimisation is for
!CONFIG_PREEMPTION. So I instead of depending on CONFIG_PREEMPT_RT I'm
leaning towards CONFIG_PREEMPT instead.

> Cheers,
> Longman

Sebastian