linux-kernel - Re: [PATCH] x86 / CPU: Always show current CPU frequency in /proc/cpuinfo

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Date:   Wed, 15 Nov 2017 09:47:57 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        WANG Chao <chao.wang@...oud.cn>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Vikas Shivappa <vikas.shivappa@...ux.intel.com>,
        Kate Stewart <kstewart@...uxfoundation.org>,
        Len Brown <len.brown@...el.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Philippe Ombredanne <pombredanne@...b.com>,
        Mathias Krause <minipli@...glemail.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>
Subject: Re: [PATCH] x86 / CPU: Always show current CPU frequency in
 /proc/cpuinfo

On Wed, 15 Nov 2017, Rafael J. Wysocki wrote:

> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> 
> After commit 890da9cf0983 (Revert "x86: do not use cpufreq_quick_get()
> for /proc/cpuinfo "cpu MHz"") the "cpu MHz" number in /proc/cpuinfo
> on x86 can be either the nominal CPU frequency (which is constant)
> or the frequency most recently requested by a scaling governor in
> cpufreq, depending on the cpufreq configuration.  That is somewhat
> inconsistent and is different from what it was before 4.13, so in
> order to restore the previous behavior, make it report the current
> CPU frequency like the scaling_cur_freq sysfs file in cpufreq.
> 
> To that end, modify the /proc/cpuinfo implementation on x86 to use
> aperfmperf_snapshot_khz() to snapshot the APERF and MPERF feedback
> registers, if available, and use their values to compute the CPU
> frequency to be reported as "cpu MHz".
> 
> However, do that carefully enough to avoid accumulating delays that
> lead to unacceptable access times for /proc/cpuinfo on systems with
> many CPUs.  Run aperfmperf_snapshot_khz() once on all CPUs
> asynchronously at the /proc/cpuinfo open time, add a single delay
> upfront (if necessary) at that point and simply compute the current
> frequency while running show_cpuinfo() for each individual CPU.
> 
> Also, to avoid slowing down /proc/cpuinfo accesses too much, reduce
> the default delay between consecutive APERF and MPERF reads to 10 ms,
> which should be sufficient to get large enough numbers for the
> frequency computation in all cases.
> 
> Fixes: 890da9cf0983 (Revert "x86: do not use cpufreq_quick_get() for /proc/cpuinfo "cpu MHz"")
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>

Acked-by: Thomas Gleixner <tglx@...utronix.de>
Tested-by: Thomas Gleixner <tglx@...utronix.de>