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Subject: Re: [PATCH] x86,switch_mm: skip atomic operations for init_mm
From: Rik van Riel <riel@surriel.com>
To: Song Liu <songliubraving@fb.com>, Andy Lutomirski <luto@kernel.org>
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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.
>=20
> Yes, we are running netperf/netserver with 300 threads. We don't see
> this much overhead in with real workload.=20

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 :)

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