On Fri, 2018-06-01 at 08:11 -0700, Andy Lutomirski wrote: > On Fri, Jun 1, 2018 at 5:28 AM Rik van Riel <r...@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. -- All Rights Reversed.
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