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

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Message-ID: <1427741465-15747-1-git-send-email-cmetcalf@ezchip.com>
Date:	Mon, 30 Mar 2015 14:51:05 -0400
From:	<cmetcalf@...hip.com>
To:	Andrew Morton <akpm@...ux-foundation.org>,
	Don Zickus <dzickus@...hat.com>,
	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>
CC:	Chris Metcalf <cmetcalf@...hip.com>
Subject: [PATCH] watchdog: nohz: don't run watchdog on nohz_full cores

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.

Signed-off-by: Chris Metcalf <cmetcalf@...hip.com>
---
 kernel/watchdog.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index 3174bf8e3538..8a46d9d8a66f 100644
--- a/kernel/watchdog.c
+++ b/kernel/watchdog.c
@@ -19,6 +19,7 @@
 #include <linux/sysctl.h>
 #include <linux/smpboot.h>
 #include <linux/sched/rt.h>
+#include <linux/tick.h>
 
 #include <asm/irq_regs.h>
 #include <linux/kvm_para.h>
@@ -431,6 +432,10 @@ static void watchdog_enable(unsigned int cpu)
 	hrtimer_init(hrtimer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
 	hrtimer->function = watchdog_timer_fn;
 
+	/* nohz_full cpus do not do watchdog checking. */
+	if (tick_nohz_full_cpu(cpu))
+		do_exit(0);
+
 	/* Enable the perf event */
 	watchdog_nmi_enable(cpu);
 
-- 
2.1.2

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