linux-kernel - Re: [PATCH v9 2/3] watchdog: add watchdog

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Message-Id: <20150429152629.74212a1808833d979c7b797d@linux-foundation.org>
Date:	Wed, 29 Apr 2015 15:26:29 -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 v9 2/3] watchdog: add watchdog_cpumask sysctl to assist
 nohz

On Fri, 17 Apr 2015 14:37:17 -0400 Chris Metcalf <cmetcalf@...hip.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.



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