linux-kernel - [PATCH 3/8] watchdog: Boot-disable by default on full dynticks

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Message-Id: <1369065716-22801-4-git-send-email-fweisbec@gmail.com>
Date:	Mon, 20 May 2013 18:01:51 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	LKML <linux-kernel@...r.kernel.org>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Li Zhong <zhong@...ux.vnet.ibm.com>,
	Don Zickus <dzickus@...hat.com>
Subject: [PATCH 3/8] watchdog: Boot-disable by default on full dynticks

When the watchdog runs, it prevents the full dynticks
CPUs from stopping their tick because the hard lockup
detector uses perf events internally, which in turn
rely on the periodic tick.

Since this is a rather confusing behaviour that is not
easy to track down and identify for those who want to
test CONFIG_NO_HZ_FULL, let's default disable the
watchdog on boot time when full dynticks is enabled.

The user can still enable it later on runtime using
proc or sysctl.

Reported-by: Steven Rostedt <rostedt@...dmis.org>
Suggested-by: Peter Zijlstra <peterz@...radead.org>
Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
Cc: Steven Rostedt <rostedt@...dmis.org>
Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Cc: Ingo Molnar <mingo@...nel.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Li Zhong <zhong@...ux.vnet.ibm.com>
Cc: Don Zickus <dzickus@...hat.com>
---
 kernel/watchdog.c |    6 ++++++
 1 files changed, 6 insertions(+), 0 deletions(-)

diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index 05039e3..7e1a021 100644
--- a/kernel/watchdog.c
+++ b/kernel/watchdog.c
@@ -543,6 +543,12 @@ static struct smp_hotplug_thread watchdog_threads = {
 
 void __init lockup_detector_init(void)
 {
+#ifdef CONFIG_NO_HZ_FULL
+	watchdog_enabled = 0;
+	watchdog_disabled = 1;
+	pr_warning("Disabled lockup detectors by default because of full dynticks\n");
+	pr_warning("You can overwrite that with 'sysctl -w kernel.watchdog=1'\n");
+#endif
 	set_sample_period();
 	if (smpboot_register_percpu_thread(&watchdog_threads)) {
 		pr_err("Failed to create watchdog threads, disabled\n");
-- 
1.7.5.4

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