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

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Date:   Tue, 1 Aug 2023 09:45:22 +0000
From:   Quentin Perret <qperret@...gle.com>
To:     David Dai <davidai@...gle.com>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Viresh Kumar <viresh.kumar@...aro.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>,
        Saravana Kannan <saravanak@...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

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.

> +	u32 freq = mult_frac(policy->cached_target_freq, 80, 100);
> +
> +	data->ops->set_freq(policy, freq);
> +	return 0;
> +}

Thanks,
Quentin