Messages in this thread | | | Subject | Re: [PATCH] x86,switch_mm: skip atomic operations for init_mm | From | Mike Galbraith <> | Date | Fri, 01 Jun 2018 20:48:02 +0200 |
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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@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.
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
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