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

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Date:	Tue, 31 Mar 2015 04:04:35 +0200
From:	Mike Galbraith <umgwanakikbuti@...il.com>
To:	Don Zickus <dzickus@...hat.com>
Cc:	cmetcalf@...hip.com, Andrew Morton <akpm@...ux-foundation.org>,
	Andrew Jones <drjones@...hat.com>,
	chai wen <chaiw.fnst@...fujitsu.com>,
	Ingo Molnar <mingo@...nel.org>,
	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>,
	Frederic Weisbecker <fweisbec@...il.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 Mon, 2015-03-30 at 15:12 -0400, Don Zickus wrote:
> On Mon, Mar 30, 2015 at 02:51:05PM -0400, cmetcalf@...hip.com wrote:
> > From: Chris Metcalf <cmetcalf@...hip.com>
> > 
> > Running watchdog can be a helpful debugging feature on regular
> > cores, but it's incompatible with nohz_full, since it forces
> > regular scheduling events.  Accordingly, just exit out immediately
> > from any nohz_full core.
> > 
> > An alternate approach would be to add a flags field or function to
> > smp_hotplug_thread to control on which cores the percpu threads
> > are created, but it wasn't clear that much mechanism was useful.
> 
> Hi Chris,
> 
> It seems like the correct solution would be to hook into the idle_loop
> somehow.  If the cpu is idle, then it seems unlikely that a lockup could
> occur.
> 
> My fear with this apporach is a lockup would occur on the nohz cpu and it
> would go undetected because that cpu is disabled.  Further no printk is
> thrown out to even indicate a cpu is disabled making it more difficult to
> debug.

Hm, I don't see why this is needed, for debugging/testing you turn it
on, when you set up for critical operation, you turn it off.

A bigger deal is the clocksource watchdog methinks.  Measurement
inspired me to make it dead yesterday.

	-Mike

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