On Wed, 16 Oct 2024 22:55:09 +0100, Ankur Arora <ankur.a.arora@xxxxxxxxxx> wrote: > > > Marc Zyngier <maz@xxxxxxxxxx> writes: > > > On Thu, 26 Sep 2024 00:24:14 +0100, > > Ankur Arora <ankur.a.arora@xxxxxxxxxx> wrote: > >> > >> This patchset enables the cpuidle-haltpoll driver and its namesake > >> governor on arm64. This is specifically interesting for KVM guests by > >> reducing IPC latencies. > >> > >> Comparing idle switching latencies on an arm64 KVM guest with > >> perf bench sched pipe: > >> > >> usecs/op %stdev > >> > >> no haltpoll (baseline) 13.48 +- 5.19% > >> with haltpoll 6.84 +- 22.07% > >> > >> > >> No change in performance for a similar test on x86: > >> > >> usecs/op %stdev > >> > >> haltpoll w/ cpu_relax() (baseline) 4.75 +- 1.76% > >> haltpoll w/ smp_cond_load_relaxed() 4.78 +- 2.31% > >> > >> Both sets of tests were on otherwise idle systems with guest VCPUs > >> pinned to specific PCPUs. One reason for the higher stdev on arm64 > >> is that trapping of the WFE instruction by the host KVM is contingent > >> on the number of tasks on the runqueue. > > > > Sorry to state the obvious, but if that's the variable trapping of > > WFI/WFE is the cause of your trouble, why don't you simply turn it off > > (see 0b5afe05377d for the details)? Given that you pin your vcpus to > > physical CPUs, there is no need for any trapping. > > Good point. Thanks. That should help reduce the guessing games around > the variance in these tests. I'd be interested to find out whether there is still some benefit in this series once you disable the WFx trapping heuristics. Thanks, M. -- Without deviation from the norm, progress is not possible.
