linux-kernel - Re: [PATCH] x86,switch_mm: skip atomic operations for init

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Message-ID: <CALCETrUVFdrtorJBVg1fizUx+c23MVVqNnzGs3oWpN8LHyEvMA@mail.gmail.com>
Date:   Fri, 1 Jun 2018 08:11:06 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     riel@...riel.com
Cc:     LKML <linux-kernel@...r.kernel.org>, songliubraving@...com,
        kernel-team <kernel-team@...com>, Ingo Molnar <mingo@...hat.com>,
        Andrew Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>, X86 ML <x86@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Mike Galbraith <efault@....de>
Subject: Re: [PATCH] x86,switch_mm: skip atomic operations for init_mm

On Fri, Jun 1, 2018 at 5:28 AM Rik van Riel <riel@...riel.com> wrote:
>
> Song noticed switch_mm_irqs_off taking a lot of CPU time in recent
> kernels,using 2.4% of a 48 CPU system during a netperf to localhost run.
> Digging into the profile, we noticed that cpumask_clear_cpu and
> cpumask_set_cpu together take about half of the CPU time taken by
> switch_mm_irqs_off.
>
> However, the CPUs running netperf end up switching back and forth
> between netperf and the idle task, which does not require changes
> to the mm_cpumask. Furthermore, the init_mm cpumask ends up being
> the most heavily contended one in the system.`
>
> Skipping cpumask_clear_cpu and cpumask_set_cpu for init_mm
> (mostly the idle task) reduced CPU use of switch_mm_irqs_off
> from 2.4% of the CPU to 1.9% of the CPU, with the following
> netperf commandline:

I'm conceptually fine with this change.  Does mm_cpumask(&init_mm) end
up in a deterministic state?

Mike, depending on exactly what's going on with your benchmark, this
might help recover a bit of your performance, too.

--Andy