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. -- ankur