linux-kernel - Re: [PATCH v3] watchdog: add watchdog

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Message-ID: <20150402183308.GB27490@worktop.programming.kicks-ass.net>
Date:	Thu, 2 Apr 2015 20:33:08 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Chris Metcalf <cmetcalf@...hip.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	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>,
	linux-kernel@...r.kernel.org, Jonathan Corbet <corbet@....net>,
	linux-doc@...r.kernel.org
Subject: Re: [PATCH v3] watchdog: add watchdog_cpumask sysctl to assist nohz

On Thu, Apr 02, 2015 at 02:16:09PM -0400, Chris Metcalf wrote:
> On 04/02/2015 02:06 PM, Peter Zijlstra wrote:
> >On Thu, Apr 02, 2015 at 01:39:28PM -0400, cmetcalf@...hip.com wrote:
> >>@@ -431,6 +434,10 @@ static void watchdog_enable(unsigned int cpu)
> >>  	hrtimer_init(hrtimer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
> >>  	hrtimer->function = watchdog_timer_fn;
> >>+	/* Exit if the cpu is not allowed for watchdog. */
> >>+	if (!cpumask_test_cpu(cpu, watchdog_mask))
> >>+		do_exit(0);
> >>+
> >Ick, that doesn't look right for smpboot threads.
> 
> I didn't see a better way to make this happen without adding
> a bunch of infrastructure to the smpboot thread mechanism
> to use a cpumask other than for_each_online_cpu().  The exit
> seems benign in my testing, but I agree it's not the cleanest
> way to express what we're trying to do here.
> 
> Perhaps something like an optional cpumask_t pointer in
> struct smp_hotplug_thread, which if present specifies the
> cpus to run on, and otherwise we stick with cpu_online_mask?

What's wrong with just leaving the thread be but making sure it'll never
actually do anything?
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