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

[Mike Galbraith]

On Fri, 2018-06-01 at 14:22 -0400, Rik van Riel wrote:
> 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@xxxxxxxxxxx> 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.

previous pipe-test numbers
4.13.16         2.024978 usecs/loop -- avg 2.045250 977.9 KHz
4.14.47         2.234518 usecs/loop -- avg 2.227716 897.8 KHz
4.15.18         2.287815 usecs/loop -- avg 2.295858 871.1 KHz
4.16.13         2.286036 usecs/loop -- avg 2.279057 877.6 KHz
4.17.0.g88a8676 2.288231 usecs/loop -- avg 2.288917 873.8 KHz

new numbers
4.17.0.g0512e01 2.268629 usecs/loop -- avg 2.269493 881.3 KHz
4.17.0.g0512e01 2.035401 usecs/loop -- avg 2.038341 981.2 KHz +andy
4.17.0.g0512e01 2.238701 usecs/loop -- avg 2.231828 896.1 KHz -andy+rik

There might be something there with your change Rik, but it's small
enough to be wary of variance.  Andy's "invert the return of
tlb_defer_switch_to_init_mm()" is OTOH pretty clear.

	-Mike