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

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Date:   Thu, 16 Nov 2017 01:24:16 +0100
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     WANG Chao <chao.wang@...oud.cn>
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 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,
Rafael