linux-kernel - [RFC PATCH 02/15] nohz_task: Avoid nohz task cpu as non-idle timer target

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Date:	Mon, 20 Dec 2010 16:24:09 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	LKML <linux-kernel@...r.kernel.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Steven Rostedt <rostedt@...dmis.org>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Anton Blanchard <anton@....ibm.com>,
	Tim Pepper <lnxninja@...ux.vnet.ibm.com>
Subject: [RFC PATCH 02/15] nohz_task: Avoid nohz task cpu as non-idle timer target

Unbound timers are preferably targeted for non idle cpu. If
possible though, prioritize idle cpus over nohz task cpus,
because the main point of nohz task is to avoid unnecessary
timer interrupts.

Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Cc: Ingo Molnar <mingo@...e.hu>
Cc: Steven Rostedt <rostedt@...dmis.org>
Cc: Lai Jiangshan <laijs@...fujitsu.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Anton Blanchard <anton@....ibm.com>
Cc: Tim Pepper <lnxninja@...ux.vnet.ibm.com>
---
 kernel/sched.c |   17 +++++++++++++++--
 1 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/kernel/sched.c b/kernel/sched.c
index dc91a4d..2cd6823 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -1209,16 +1209,29 @@ static void resched_cpu(int cpu)
 int get_nohz_timer_target(void)
 {
 	int cpu = smp_processor_id();
+	int fallback = -1;
 	int i;
 	struct sched_domain *sd;
 
 	for_each_domain(cpu, sd) {
-		for_each_cpu(i, sched_domain_span(sd))
+		for_each_cpu(i, sched_domain_span(sd)) {
+			if (cpu_has_nohz_task(i))
+				continue;
+
 			if (!idle_cpu(i))
 				return i;
+
+			if (fallback == -1 || i == cpu)
+				fallback = i;
+		}
 	}
-	return cpu;
+
+	if (fallback == -1)
+		fallback = cpu;
+
+	return fallback;
 }
+
 /*
  * When add_timer_on() enqueues a timer into the timer wheel of an
  * idle CPU then this timer might expire before the next timer event
-- 
1.7.3.2

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