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

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Message-ID: <CALCETrU6bqVyBzeMu9RSoXq1Fm6rdHA6rDskqVqo5+W9snkz=A@mail.gmail.com>
Date:   Wed, 6 Jun 2018 11:17:46 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Rik van Riel <riel@...riel.com>
Cc:     songliubraving@...com, Andrew Lutomirski <luto@...nel.org>,
        Mike Galbraith <efault@....de>,
        LKML <linux-kernel@...r.kernel.org>,
        kernel-team <Kernel-team@...com>, Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>, X86 ML <x86@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] x86,switch_mm: skip atomic operations for init_mm

On Sat, Jun 2, 2018 at 6:38 PM Rik van Riel <riel@...riel.com> wrote:
>
> On Sun, 2018-06-03 at 00:51 +0000, Song Liu wrote:
>
> > > Just to check: in the workload where you're seeing this problem,
> > > are
> > > you using an mm with many threads?  I would imagine that, if you
> > > only
> > > have one or two threads, the bit operations aren't so bad.
> >
> > Yes, we are running netperf/netserver with 300 threads. We don't see
> > this much overhead in with real workload.
>
> We may not, but there are some crazy workloads out
> there in the world. Think of some Java programs with
> thousands of threads, causing a million context
> switches a second on a large system.
>
> I like Andy's idea of having one cache line with
> a cpumask per node. That seems like it will have
> fewer downsides for tasks with fewer threads running
> on giant systems.
>
> I'll throw out the code I was working on, and look
> into implementing that :)
>

I'm not sure you should throw your patch out.  It's a decent idea, too.