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