linux-kernel - [PATCH 2/6] nohz: Warn if the machine can not perform nohz

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Date:	Wed, 12 Jun 2013 16:02:34 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	LKML <linux-kernel@...r.kernel.org>
Cc:	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>,
	"H. Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Borislav Petkov <bp@...en8.de>,
	Li Zhong <zhong@...ux.vnet.ibm.com>,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: [PATCH 2/6] nohz: Warn if the machine can not perform nohz_full

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

If the user configures NO_HZ_FULL and defines nohz_full=XXX on the
kernel command line, or enables NO_HZ_FULL_ALL, but nohz fails
due to the machine having a unstable clock, warn about it.

We do not want users thinking that they are getting the benefit
of nohz when their machine can not support it.

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>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Li Zhong <zhong@...ux.vnet.ibm.com>
Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
---
 kernel/time/tick-sched.c |    5 +++++
 1 files changed, 5 insertions(+), 0 deletions(-)

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 0cf1c14..dbb8f76 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -178,6 +178,11 @@ static bool can_stop_full_tick(void)
 	 */
 	if (!sched_clock_stable) {
 		trace_tick_stop(0, "unstable sched clock\n");
+		/*
+		 * Don't allow the user to think they can get
+		 * full NO_HZ with this machine.
+		 */
+		WARN_ONCE(1, "NO_HZ FULL will not work with unstable sched clock");
 		return false;
 	}
 #endif
-- 
1.7.5.4

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