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

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Message-ID: <86sesv3zvf.wl-maz@kernel.org>
Date: Thu, 17 Oct 2024 09:19:48 +0100
From: Marc Zyngier <maz@...nel.org>
To: Ankur Arora <ankur.a.arora@...cle.com>
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Subject: Re: [PATCH v8 00/11] Enable haltpoll on arm64

On Wed, 16 Oct 2024 22:55:09 +0100,
Ankur Arora <ankur.a.arora@...cle.com> wrote:
> 
> 
> Marc Zyngier <maz@...nel.org> writes:
> 
> > On Thu, 26 Sep 2024 00:24:14 +0100,
> > Ankur Arora <ankur.a.arora@...cle.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.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.