linux-kernel - Re: [PATCH v2 6/7] arch_topology,arm,arm64: define arch_scale_freq

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Date:   Thu, 30 Jul 2020 12:44:48 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Ionela Voinescu <ionela.voinescu@....com>
Cc:     rjw@...ysocki.net, viresh.kumar@...aro.org,
        dietmar.eggemann@....com, sudeep.holla@....com, will@...nel.org,
        linux@...linux.org.uk, mingo@...hat.com, peterz@...radead.org,
        linux-pm@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org,
        Valentin Schneider <valentin.schneider@....com>
Subject: Re: [PATCH v2 6/7] arch_topology,arm,arm64: define
 arch_scale_freq_invariant()

On Wed, Jul 22, 2020 at 10:37:31AM +0100, Ionela Voinescu wrote:
> From: Valentin Schneider <valentin.schneider@....com>
> 
> arch_scale_freq_invariant() is used by schedutil to determine whether
> the scheduler's load-tracking signals are frequency invariant. Its
> definition is overridable, though by default it is hardcoded to 'true'
> if arch_scale_freq_capacity() is defined ('false' otherwise).
> 
> This behaviour is not overridden on arm, arm64 and other users of the
> generic arch topology driver, which is somewhat precarious:
> arch_scale_freq_capacity() will always be defined, yet not all cpufreq
> drivers are guaranteed to drive the frequency invariance scale factor
> setting. In other words, the load-tracking signals may very well *not*
> be frequency invariant.
> 
> Now that cpufreq can be queried on whether the current driver is driving
> the Frequency Invariance (FI) scale setting, the current situation can
> be improved. This combines the query of whether cpufreq supports the
> setting of the frequency scale factor, with whether all online CPUs are
> counter-based FI enabled.
> 
> While cpufreq FI enablement applies at system level, for all CPUs,
> counter-based FI support could also be used for only a subset of CPUs to
> set the invariance scale factor. Therefore, if cpufreq-based FI support
> is present, we consider the system to be invariant. If missing, we
> require all online CPUs to be counter-based FI enabled in order for the
> full system to be considered invariant.
> 
> If the system ends up not being invariant, a new condition is needed in
> the counter initialization code that disables all scale factor setting
> based on counters.
> 
> Precedence of counters over cpufreq use is not important here. The
> invariant status is only given to the system if all CPUs have at least
> one method of setting the frequency scale factor.
> 
> Signed-off-by: Valentin Schneider <valentin.schneider@....com>
> Signed-off-by: Ionela Voinescu <ionela.voinescu@....com>
> Cc: Catalin Marinas <catalin.marinas@....com>
> Cc: Will Deacon <will@...nel.org>
> Cc: Sudeep Holla <sudeep.holla@....com>

Acked-by: Catalin Marinas <catalin.marinas@....com>