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

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

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On Fri, 2018-06-01 at 08:11 -0700, Andy Lutomirski wrote:
> On Fri, Jun 1, 2018 at 5:28 AM Rik van Riel <riel@surriel.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?

Given that we do not touch mm_cpumask(&init_mm)
any more, and that bitmask never appears to be
used for things like tlb shootdowns (kernel TLB
shootdowns simply go to everybody), I suspect
it ends up in whatever state it is initialized
to on startup.

I had not looked into this much, because it does
not appear to be used for anything.

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

It will be interesting to know how this change
impacts others.

-- 
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