linux-kernel - Re: [PATCH v3 2/2] cpufreq: add virtual-cpufreq driver

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Date:   Fri, 4 Aug 2023 15:24:20 -0700
From:   Saravana Kannan <saravanak@...gle.com>
To:     Viresh Kumar <viresh.kumar@...aro.org>
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 Wed, Aug 2, 2023 at 10:52 PM Viresh Kumar <viresh.kumar@...aro.org> wrote:
>
> 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.

Will do.

Thanks,
Saravana