Re: [PATCH v8 00/11] Enable haltpoll on arm64

[Ankur Arora <ankur.a.arora@oracle.com>]

Marc Zyngier <maz@kernel.org> writes:

> On Thu, 26 Sep 2024 00:24:14 +0100,
> Ankur Arora <ankur.a.arora@oracle.com> 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