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

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Date:   Fri, 17 Nov 2017 12:27:07 +0800
From:   WANG Chao <chao.wang@...oud.cn>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        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 11/16/17 at 02:54P, Rafael J. Wysocki wrote:
> On Thursday, November 16, 2017 10:50:36 AM CET WANG Chao wrote:
> > On 11/16/17 at 01:24P, Rafael J. Wysocki wrote:
> > > On Wednesday, November 15, 2017 10:33:47 AM CET WANG Chao wrote:
> > > > On 11/15/17 at 02:13P, 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.
> > > > 
> > > > Hi, Rafael
> > > > 
> > > > I tested your patch. It's much faster.
> > > > 
> > > > But from what I got, calling aperfmperf_snapshot_khz() asynchronously
> > > > with 10ms sleep takes much longer than calling aperfmperf_snapshot_khz()
> > > > synchronously.
> > > > 
> > > > Here's my result on 64 CPUs:
> > > > 
> > > >  - async aperfmperf_snapshot_khz() w/ 10ms sleep:
> > > > 
> > > > # time cat /proc/cpuinfo > /dev/null
> > > > real    0m0.014s
> > > > user    0m0.000s
> > > > sys     0m0.002s
> > > > 
> > > >  - sync aperfmperf_snapshot_khz() w/o any sleep:
> > > > 
> > > > # time cat /proc/cpuinfo > /dev/null
> > > > real    0m0.002s
> > > > user    0m0.000s
> > > > sys     0m0.002s
> > > 
> > > Sure, but the delay is there, because without it the computed frequency
> > > may be way off for at least one of the CPUs.
> > 
> > Thanks, I understand now. In this case, The 10ms delay turns out to be
> > the interval of measuring aperf and mperf and computing their deltas.
> > 
> > Last question though, is 10ms best practice or can we make it shorter,
> > say 5ms?
> 
> Experimentally, I found 5 ms to be slightly too short.  It all depends on
> how accurate the numbers are expected to be, however, so there is some room
> for adjustments.
> 
> Regardless, I'd prefer to start with 10 ms as that is what has been used in
> intel_pstate for quite a long time at least and adjust later if need be.

Sure. Thanks for your explanation.

This patch looks good to me.

Reviewed-by: WANG Chao <chao.wang@...oud.cn>