Re: [PATCH v2 6/7] arch_topology,arm,arm64: define arch_scale_freq_invariant()

From: Catalin Marinas <catalin.marinas@arm.com>
To: Ionela Voinescu <ionela.voinescu@arm.com>
Cc: linux-pm@vger.kernel.org, peterz@infradead.org,
	viresh.kumar@linaro.org,
	Valentin Schneider <valentin.schneider@arm.com>,
	rjw@rjwysocki.net, linux@armlinux.org.uk,
	linux-kernel@vger.kernel.org, mingo@redhat.com,
	sudeep.holla@arm.com, will@kernel.org, dietmar.eggemann@arm.com,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v2 6/7] arch_topology,arm,arm64: define arch_scale_freq_invariant()
Date: Thu, 30 Jul 2020 12:44:48 +0100	[thread overview]
Message-ID: <20200730114447.GK25149@gaia> (raw)
In-Reply-To: <20200722093732.14297-7-ionela.voinescu@arm.com>

On Wed, Jul 22, 2020 at 10:37:31AM +0100, Ionela Voinescu wrote:
> From: Valentin Schneider <valentin.schneider@arm.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@arm.com>
> Signed-off-by: Ionela Voinescu <ionela.voinescu@arm.com>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: Sudeep Holla <sudeep.holla@arm.com>

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

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