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Date:   Tue, 1 Aug 2023 09:49:30 +0000
From:   Quentin Perret <qperret@google.com>
To:     David Dai <davidai@google.com>
Cc:     "Rafael J. Wysocki" <rafael@kernel.org>,
        Viresh Kumar <viresh.kumar@linaro.org>,
        Rob Herring <robh+dt@kernel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>,
        Conor Dooley <conor+dt@kernel.org>,
        Sudeep Holla <sudeep.holla@arm.com>,
        Saravana Kannan <saravanak@google.com>,
        Masami Hiramatsu <mhiramat@google.com>,
        Will Deacon <will@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Marc Zyngier <maz@kernel.org>,
        Oliver Upton <oliver.upton@linux.dev>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Pavan Kondeti <quic_pkondeti@quicinc.com>,
        Gupta Pankaj <pankaj.gupta@amd.com>,
        Mel Gorman <mgorman@suse.de>, kernel-team@android.com,
        linux-pm@vger.kernel.org, devicetree@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3 2/2] cpufreq: add virtual-cpufreq driver
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 <ZMjUMk5xXzahXjno@google.com>
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On Tuesday 01 Aug 2023 at 09:45:22 (+0000), Quentin Perret wrote:
> Hi David,
> 
> On Monday 31 Jul 2023 at 10:46:09 (-0700), David Dai wrote:
> > +static unsigned int virt_cpufreq_set_perf(struct cpufreq_policy *policy)
> > +{
> > +	struct virt_cpufreq_drv_data *data = policy->driver_data;
> > +	/*
> > +	 * Use cached frequency to avoid rounding to freq table entries
> > +	 * and undo 25% frequency boost applied by schedutil.
> > +	 */
> 
> The VMM would be a better place for this scaling I think, the driver
> can't/shouldn't make assumptions about the governor it is running with
> given that this is a guest userspace decision essentially.
> 
> IIRC the fast_switch() path is only used by schedutil, so one could
> probably make a case to scale things there, but it'd be inconsistent
> with the "slow" switch case, and would create a fragile dependency, so
> it's probably not worth pursuing.

Alternatively we could make the schedutil margin configurable via the
cmdline or something along those lines, so we can set it to 0 in the
guest and avoid the issue entirely.

Some partners have been asking for this IIRC , so I suspect there would
be interest from other parties.

Thanks,
Quentin