linux-kernel - [PATCH 1/2] nohz: Disable LOCKUP_DETECTOR when NO_HZ

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Date:	Tue, 14 May 2013 18:02:51 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: [PATCH 1/2] nohz: Disable LOCKUP_DETECTOR when NO_HZ_FULL is enabled

From: Steven Rostedt <rostedt@...dmis.org>

Trying to test the nohz_full code, I was not able to get it to work.
Finally I enabled the tick_stop tracepoint and it showed:

 tick_stop:            success=no msg=perf events running

I talked to Frederic Weisbecker about this and he informed me that
perf is used by the lockup detector. I checked the code, and sure
enough it is.

As perf is always running when LOCKUP_DETECTOR is enabled, which
will always disable nohz_full from working, instead of confusing
users, disable LOCKUP_DETECTOR when NO_HZ_FULL is enabled.

When perf is changed such that it does not prevent nohz_full from
working, then we can and should remove this constraint.

Signed-off-by: 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: H. Peter Anvin <hpa@...or.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
---
 lib/Kconfig.debug |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)

diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 566cf2b..1364d09 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -174,6 +174,8 @@ config DEBUG_SHIRQ
 config LOCKUP_DETECTOR
 	bool "Detect Hard and Soft Lockups"
 	depends on DEBUG_KERNEL && !S390
+	# Lockup detector currently prevents NO_HZ_FULL from working
+	depends on !NO_HZ_FULL
 	help
 	  Say Y here to enable the kernel to act as a watchdog to detect
 	  hard and soft lockups.
-- 
1.7.5.4

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