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Date: Wed, 29 Apr 2015 15:26:29 -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>,
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        Fabian Frederick <fabf@skynet.be>, Aaron Tomlin <atomlin@redhat.com>,
        Ben Zhang <benzh@chromium.org>, "Christoph Lameter" <cl@linux.com>,
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        "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.

Could you please expand on the patch motivation?  "would prevent the
desired tickless operation on those cores" is quite vague.

Exactly what userspace-visible problem is the current implementation
causing and how does the patchset improve things?

If any of this is quantifiable (wakeups/sec, joules/hour etc) then some
attempt to perform those measurements would also be useful.