linux-kernel - Re: [PATCH] watchdog: nohz: don't run watchdog on nohz

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Date:	Thu, 2 Apr 2015 18:48:48 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Chris Metcalf <cmetcalf@...hip.com>
Cc:	Don Zickus <dzickus@...hat.com>, Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.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>,
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] watchdog: nohz: don't run watchdog on nohz_full cores

On Thu, Apr 02, 2015 at 11:42:43AM -0400, Chris Metcalf wrote:
> OK, I will respin v2 of the patch as follows:
> 
> - Provide a watchdog_cpumask as suggested by Don.
> - On a non-NO_HZ_FULL build, it defaults to cpu_possible as normal
> - On a NO_HZ_FULL build, it defaults to the housekeeping cpus

Ah note that NO_HZ_FULL is only the capability. Nohz full is actually
only running if the nohz_full parameter is passed (or NO_HZ_FULL_ALL=y).

And now generalist distros enable NO_HZ_FULL so that anybody can use it.

So better check tick_nohz_full_enabled() instead of the CONFIG.

Thanks.


> - If the mask is modified, we disable and then re-enable the watchdog,
>   so that the watchdog init code can exit() the appropriate threads as
>   they start up
> 
> This should address the various concerns that have been raised.
> 
> -- 
> Chris Metcalf, EZChip Semiconductor
> http://www.ezchip.com
> 
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