linux-kernel - Re: [PATCH] x86: use cpufreq_quick_get() for /proc/cpuinfo "cpu MHz" again

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Date:   Wed, 15 Nov 2017 09:47:11 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        WANG Chao <chao.wang@...oud.cn>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        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: use cpufreq_quick_get() for /proc/cpuinfo "cpu MHz"
 again

On Tue, 14 Nov 2017, Linus Torvalds wrote:
> On Tue, Nov 14, 2017 at 3:53 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> > Current head + Raphaels patch:
> >
> > real    0m0.029s
> > user    0m0.000s
> > sys     0m0.010s
> >
> > So that patch is actually slower.
> 
> Oh it definitely is expected to be slower, because it does the IPI to
> all the cores and actually gets their frequency right.
> 
> It was the old one that we had to revert (because it did so
> sequentially) that was really bad, and took something like 2+ seconds
> on Ingo's 160-core thing, iirc.
> 

Tired brain did not connect it to the revert.

On that machine with ea0ee3398877: Revert "x86: CPU: Fix up "cpu MHz" in
/proc/cpuinfo" reverted it takes:

real	0m4.497s
user	0m0.012s
sys	0m0.000s

> It sounds like the current patch is slower, but likely acceptable
> considering that you get the right results now ..

Correct and the factor 10, i.e. 30ms vs. 3ms is not horrible, while the 4.5
seconds are.

Thanks,

	tglx