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

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Message-ID: <552BE540.3040108@ezchip.com>
Date:	Mon, 13 Apr 2015 11:48:16 -0400
From:	Chris Metcalf <cmetcalf@...hip.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
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 04/10/2015 05:11 PM, Andrew Morton wrote:
> 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?

Thanks for the feedback.  It's easy to assume everyone knows
everything about what's being done in the kernel :-)

I'll add some more descriptive language around what the point
of nohz_full is, and why the watchdog interferes with it, in v8.

>
> 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?
>

-- 
Chris Metcalf, EZChip Semiconductor
http://www.ezchip.com

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