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Message-ID: <20230803055157.6buxd5lcxjo4wrbk@vireshk-i7>
Date: Thu, 3 Aug 2023 11:21:57 +0530
From: Viresh Kumar <viresh.kumar@...aro.org>
To: Saravana Kannan <saravanak@...gle.com>
Cc: David Dai <davidai@...gle.com>,
"Rafael J. Wysocki" <rafael@...nel.org>,
Rob Herring <robh+dt@...nel.org>,
Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
Conor Dooley <conor+dt@...nel.org>,
Sudeep Holla <sudeep.holla@....com>,
Quentin Perret <qperret@...gle.com>,
Masami Hiramatsu <mhiramat@...gle.com>,
Will Deacon <will@...nel.org>,
Peter Zijlstra <peterz@...radead.org>,
Vincent Guittot <vincent.guittot@...aro.org>,
Marc Zyngier <maz@...nel.org>,
Oliver Upton <oliver.upton@...ux.dev>,
Dietmar Eggemann <dietmar.eggemann@....com>,
Pavan Kondeti <quic_pkondeti@...cinc.com>,
Gupta Pankaj <pankaj.gupta@....com>,
Mel Gorman <mgorman@...e.de>, kernel-team@...roid.com,
linux-pm@...r.kernel.org, devicetree@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 2/2] cpufreq: add virtual-cpufreq driver
On 02-08-23, 15:16, Saravana Kannan wrote:
> This is mainly an optimization to reduce the latency of the "frequency
> change" which has a huge impact on the performance (as can be seen
> from the numbers in the cover letter).
>
> Setting this flag means that the vCPU thread triggering the MMIO
> handling (on the host side) is the thread on which the host needs to
> apply any uclamp settings. So this avoids the VMM having to look up
> the right vCPU thread that corresponds to this CPU, and any
> permissions issues wrt setting another threads uclamp, etc. This
> becomes even more important if/when BPF support is added for handling
> simple MMIO read/writes. Will Deacon has been working on the eBPF
> part[1] and IIUC, not setting this flag adds a lot of extra overhead
> on the BPF side.
>
> So, yeah, this flag is very helpful wrt reducing latency/simplifying
> host side implementation and that's why we want it here.
>
> [1] - https://kvm-forum.qemu.org/2023/talk/AZKC77/
Would be good to have a (big) comment in the code explaining that as
it isn't obvious. Thanks.
--
viresh
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