linux-kernel - Re: [PATCH v2 5/6] TEMP: sched: add interface for counter-based frequency invariance

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Message-ID: <20200129193741.GU14914@hirez.programming.kicks-ass.net>
Date:   Wed, 29 Jan 2020 20:37:41 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Ionela Voinescu <ionela.voinescu@....com>
Cc:     catalin.marinas@....com, will@...nel.org, mark.rutland@....com,
        maz@...nel.org, suzuki.poulose@....com, sudeep.holla@....com,
        dietmar.eggemann@....com, mingo@...hat.com, ggherdovich@...e.cz,
        vincent.guittot@...aro.org, linux-arm-kernel@...ts.infradead.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        Juri Lelli <juri.lelli@...hat.com>
Subject: Re: [PATCH v2 5/6] TEMP: sched: add interface for counter-based
 frequency invariance

On Wed, Dec 18, 2019 at 06:26:06PM +0000, Ionela Voinescu wrote:
> To be noted that this patch is a temporary one. It introduces the
> interface added by the patches at [1] to allow update of the frequency
> invariance scale factor based on counters. If [1] is merged there is
> not need for this patch.
> 
> For platforms that support counters (x86 - APERF/MPERF, arm64 - AMU
> counters) the frequency invariance correction factor can be obtained
> using a core counter and a fixed counter to get information on the
> performance (frequency based only) obtained in a period of time. This
> will more accurately reflect the actual current frequency of the CPU,
> compared with the alternative implementation that reflects the request
> of a performance level from the OS through the cpufreq framework
> (arch_set_freq_scale).
> 
> Therefore, introduce an interface - arch_scale_freq_tick, to be
> implemented by each architecture and called for each CPU on the tick
> to update the scale factor based on the delta in the counter values,
> if counter support is present on the CPU.
> 
> Either because reading counters is expensive or because reading
> counters from remote CPUs is not possible or is expensive, only
> update the counter based frequency scale factor on the tick for
> now. A tick based update will definitely be necessary either due to
> it being the only point of update for certain architectures or in
> order to cache the counter values for a particular CPU, if a
> further update from that CPU is not possible.
> 
> [1]
> https://lore.kernel.org/lkml/20191113124654.18122-1-ggherdovich@suse.cz/

FWIW, those patches just landed in tip/sched/core