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Date: Fri, 10 Apr 2015 14:11:41 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Chris Metcalf <cmetcalf@ezchip.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>, Don Zickus <dzickus@redhat.com>,
        Ingo Molnar <mingo@kernel.org>, Andrew Jones <drjones@redhat.com>,
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        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH v7 1/3] smpboot: allow excluding cpus from the smpboot
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On Fri, 10 Apr 2015 16:48:18 -0400 Chris Metcalf <cmetcalf@ezchip.com> wrote:

> This change allows some cores to be excluded from running the
> smp_hotplug_thread tasks.  The motivating example for this is
> the watchdog threads, which by default we don't want to run
> on any enabled nohz_full cores.

Why not?

I can guess, but I'd rather not guess.  Please fully explain the
end-user value of this change.  Providing a benefit to users is the
whole point of the patchset, but the above assertion is the only
description we have.

This info should be in Documentation/lockup-watchdogs.txt and/or
Documentation/sysctl/kernel.txt as well as the changelogs, so users
have an answer to "why the heck should I enable this".

Please also describe the downside of the change.  I assume this is
"lockups will go undetected on some CPUs"?  Let's expand on this so we
can understand where the best tradeoff point lies.

If people are experiencing <whatever this problem is> then they can
disable the watchdog altogether.  What value is there in this partial
disabling?  Why is it worth doing this?

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