linux-kernel - Re: [PATCH v7 1/3] smpboot: allow excluding cpus from the smpboot threads

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Message-Id: <20150410141141.5fda4083dbaa874fd2690658@linux-foundation.org>
Date:	Fri, 10 Apr 2015 14:11:41 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Chris Metcalf <cmetcalf@...hip.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Don Zickus <dzickus@...hat.com>,
	Ingo Molnar <mingo@...nel.org>,
	Andrew Jones <drjones@...hat.com>,
	chai wen <chaiw.fnst@...fujitsu.com>,
	Ulrich Obergfell <uobergfe@...hat.com>,
	Fabian Frederick <fabf@...net.be>,
	Aaron Tomlin <atomlin@...hat.com>,
	Ben Zhang <benzh@...omium.org>,
	"Christoph Lameter" <cl@...ux.com>,
	Gilad Ben-Yossef <gilad@...yossef.com>,
	"Steven Rostedt" <rostedt@...dmis.org>,
	<linux-kernel@...r.kernel.org>, "Jonathan Corbet" <corbet@....net>,
	<linux-doc@...r.kernel.org>, Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v7 1/3] smpboot: allow excluding cpus from the smpboot
 threads

On Fri, 10 Apr 2015 16:48:18 -0400 Chris Metcalf <cmetcalf@...hip.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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