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

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

Marc Zyngier <maz@kernel.org> writes:

> On Wed, 16 Oct 2024 22:55:09 +0100,
> Ankur Arora <ankur.a.arora@oracle.com> wrote:
>>
>>
>> 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.
>
> I'd be interested to find out whether there is still some benefit in
> this series once you disable the WFx trapping heuristics.

The benefit of polling in idle is more than just avoiding the cost of
trapping and re-entering. The other benefit is that remote wakeups
can now be done just by setting need-resched, instead of sending an
IPI, and incurring the cost of handling the interrupt on the receiver
side.

But let me get you some numbers with that.

--
ankur