Re: [PATCH] x86,switch_mm: skip atomic operations for init_mm

[Rik van Riel <riel@surriel.com>]

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On Wed, 2018-06-06 at 11:17 -0700, Andy Lutomirski wrote:
> On Sat, Jun 2, 2018 at 6:38 PM Rik van Riel <riel@surriel.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.

Oh, I still have it saved, but the cpumask per
NUMA node looks like it could have a big impact,
with less guesswork or side effects.

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