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

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Message-ID: <8634kx5yqd.wl-maz@kernel.org>
Date: Tue, 15 Oct 2024 13:36:58 +0100
From: Marc Zyngier <maz@...nel.org>
To: Ankur Arora <ankur.a.arora@...cle.com>
Cc: linux-pm@...r.kernel.org,
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Subject: Re: [PATCH v8 00/11] Enable haltpoll on arm64

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.

	M.

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