linux-kernel - Re: [PATCH] profiling: remove prof_cpu

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Message-ID: <CAHk-=wh_mq+fvV2+V6FPYNF26DW0J_3zVd1i=ukf2oRe-B6O6A@mail.gmail.com>
Date: Sat, 27 Jul 2024 20:28:33 -0700
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc: Thomas Gleixner <tglx@...utronix.de>, Anna-Maria Gleixner <anna-maria@...utronix.de>, 
	Andrew Morton <akpm@...ux-foundation.org>, Arnd Bergmann <arnd@...db.de>, 
	Mel Gorman <mgorman@...hsingularity.net>, Michal Hocko <mhocko@...e.com>, 
	Peter Zijlstra <peterz@...radead.org>, Vlastimil Babka <vbabka@...e.cz>, Ingo Molnar <mingo@...nel.org>, 
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] profiling: remove prof_cpu_mask

On Sat, 27 Jul 2024 at 16:48, Tetsuo Handa
<penguin-kernel@...ove.sakura.ne.jp> wrote:
>
> What about emitting some kernel messages for investigating whether there
> are users who need this code, and wait for two years for whether someone
> says "I need this code" ?

Heh. We've tried that. Nobody reads the kernel messages (or the docs -
people have tried documenting "this will go away in a year" in our
kernel documentation instead).

People notice and let you know when the feature is gone, and generally
not one second before that.

Actually, what *has* worked is WARN_ONCE() kind of big really scary
messages, and that will actually make some people notice just because
they are *so* big that people see them almost by accident if they ever
happen to look at any logs.

But then that causes other problems (ie the denial-of-service on
platforms that have panic-on-warn set).

So while we have done that too, it's only workable for some "let's see
if anybody hits this during the release cycle", because you can't
release with the warning in place.

Generally the best option is probably just to remove it, see if
anybody notices, and add it back if it turns out to have a real and
valid use.

              Linus