linux-kernel - Re: [PATCH] x86: Calculate MHz using APERF/MPERF for cpuinfo and scaling_cur

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Message-ID: <20160401081639.GD3448@twins.programming.kicks-ass.net>
Date:	Fri, 1 Apr 2016 10:16:39 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Len Brown <lenb@...nel.org>
Cc:	x86@...nel.org, linux-pm@...r.kernel.org,
	linux-kernel@...r.kernel.org, Len Brown <len.brown@...el.com>
Subject: Re: [PATCH] x86: Calculate MHz using APERF/MPERF for cpuinfo and
 scaling_cur_freq

On Fri, Apr 01, 2016 at 12:37:00AM -0400, Len Brown wrote:
> From: Len Brown <len.brown@...el.com>
> 
> For x86 processors with APERF/MPERF and TSC,
> return meaningful and consistent MHz in
> /proc/cpuinfo and
> /sys/devices/system/cpu/cpu*/cpufreq/scaling_cur_freq
> 
> MHz is computed like so:
> 
> MHz = base_MHz * delta_APERF / delta_MPERF
> 
> MHz is the average frequency of the busy processor
> over a measurement interval.  The interval is
> defined to be the time between successive reads
> of the frequency on that processor, whether from
> /proc/cpuinfo or from sysfs cpufreq/scaling_cur_freq.
> As with previous methods of calculating MHz,
> idle time is excluded.

Is this really a semantic you want to pin down?

Since we're looking at doing something like:

lkml.kernel.org/r/20160303162829.GB6375@...ns.programming.kicks-ass.net

We could also just return cpu_khz * whatever fraction we store there,
knowing it is something recent.