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Date: Wed, 29 Apr 2015 15:26:44 -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>,
        chai wen <chaiw.fnst@cn.fujitsu.com>,
        Ulrich Obergfell <uobergfe@redhat.com>,
        Fabian Frederick <fabf@skynet.be>, Aaron Tomlin <atomlin@redhat.com>,
        Ben Zhang <benzh@chromium.org>, "Christoph Lameter" <cl@linux.com>,
        Gilad Ben-Yossef <gilad@benyossef.com>,
        "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 v9 2/3] watchdog: add watchdog_cpumask sysctl to assist
 nohz
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On Fri, 17 Apr 2015 14:37:17 -0400 Chris Metcalf <cmetcalf@ezchip.com> wrote:

> Change the default behavior of watchdog so it only runs on the
> housekeeping cores when nohz_full is enabled at build and boot time.
> Allow modifying the set of cores the watchdog is currently running
> on with a new kernel.watchdog_cpumask sysctl.
> 
> If we allowed the watchdog to run on nohz_full cores, the timer
> interrupts and scheduler work would prevent the desired tickless
> operation on those cores.  But if we disable the watchdog globally,
> then the housekeeping cores can't benefit from the watchdog
> functionality.  So we allow disabling it only on some cores.
> See Documentation/lockup-watchdogs.txt for more information.
> 
> ...
>  
> +watchdog_cpumask:
> +
> +This value can be used to control on which cpus the watchdog may run.
> +The default cpumask is all possible cores, but if NO_HZ_FULL is
> +enabled in the kernel config, and cores are specified with the
> +nohz_full= boot argument, those cores are excluded by default.
> +Offline cores can be included in this mask, and if the core is later
> +brought online, the watchdog will be started based on the mask value.
> +
> +Typically this value would only be touched in the nohz_full case
> +to re-enable cores that by default were not running the watchdog,
> +if a kernel lockup was suspected on those cores.

Now the reader is wondering "how the heck do I specify a cpumask".  Is
it hex encoded?  decimal?  binary string?  Which bit corresponds to
which CPU?

A little example would help things along.