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

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Message-ID: <20200130153325.GA5208@arm.com>
Date:   Thu, 30 Jan 2020 15:33:25 +0000
From:   Ionela Voinescu <ionela.voinescu@....com>
To:     Peter Zijlstra <peterz@...radead.org>
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 Wednesday 29 Jan 2020 at 20:37:41 (+0100), Peter Zijlstra wrote:
> 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

Thanks, Peter, I'll drop this one next time around.

Ionela.