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

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Message-ID: <CA+55aFzHqhxzv1p7fqdXPugMUVRyM44pL4KO3f_T6yFJUACXvw@mail.gmail.com>
Date:   Wed, 15 Nov 2017 09:27:07 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Thomas Gleixner <tglx@...utronix.de>,
        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, Nov 14, 2017 at 11:43 PM, Ingo Molnar <mingo@...nel.org> wrote:
>
> I also think that /proc/cpuinfo is a pretty bad interface for many uses - I
> personally only very rarely need the cpuinfo of _all_ CPUs.
>
> We we should eventually have /proc/cpu/N/info or so, so that 99% of the times
> cpuinfo is needed to report bugs we can do:
>
>         cat /proc/cpu/0/info

I don't disagree in theory, and I used to enthusiastically support the
"let's make one value per file" model.

But in practice it's a pain, and I no longer really think it's viable
except for debugging.

As an example, I can do

  cat /proc/cpuinfo | grep MHz

and get a nice overview of what's going on. If I were a system stat
program, I'd do that too.

In contrast, tell me where in /sys something useful is?

Honestly, the /sys directory structure _looks_ much more organized,
and it's lovely for some things, but it's a confusing jungle, and
finding things there is a complete and utter pain.

Yes, you can get the same thing:

     cat /sys/devices/system/cpu/cpu*/cpufreq/scaling_cur_freq

but here's the deal: time that on your 120-core machine some day.

In other words, /proc/cpuinfo is not perfect. But the "let's split
things out" really doesn't work either. It's worse. It _sounds_ good
in theory, but it really really sucks.

I'm convinced that /sys is wonderful for management purposes. But no,
this whole "we should have individual files" is often a huge huge
mistake.

In fact, I think the current patch for CPU MHz shows exactly why
/proc/cpuinfo is actually hugely superior to the crazy "one file per
cpu" model: we could gather the statistics in parallel, all together,
and make it be reasonable.

Again, try that "cat" example again, and time it.

(And yes, I also know that "cpu*/cpufreq" is a symlink, but the direct
names are illogical, and no faster for me. Try it if you like:

  time cat /sys/devices/system/cpu/cpufreq/policy*/scaling_cur_freq > /dev/null

and then look at /proc/cpuinfo again, and realize how lovely that
human-readable file actually is).

                      Linus