Message-ID: <552BE540.3040108@ezchip.com>
Date: Mon, 13 Apr 2015 11:48:16 -0400
From: Chris Metcalf <cmetcalf@ezchip.com>
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To: Andrew Morton <akpm@linux-foundation.org>
CC: Frederic Weisbecker <fweisbec@gmail.com>, Don Zickus <dzickus@redhat.com>,
        Ingo Molnar <mingo@kernel.org>, Andrew Jones <drjones@redhat.com>,
        chai wen <chaiw.fnst@cn.fujitsu.com>,
        Ulrich Obergfell <uobergfe@redhat.com>,
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        Ben Zhang <benzh@chromium.org>, Christoph Lameter <cl@linux.com>,
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        Steven Rostedt <rostedt@goodmis.org>, <linux-kernel@vger.kernel.org>,
        Jonathan Corbet <corbet@lwn.net>, <linux-doc@vger.kernel.org>,
        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 04/10/2015 05:11 PM, Andrew Morton wrote:
> 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?

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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