Marc Zyngier [off-list ref] writes:

On Thu, 26 Sep 2024 00:24:14 +0100,
Ankur Arora [off-list ref] wrote:

quoted

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