linux-kernel - [PATCH 28/41] nohz: Don't restart the tick before scheduling to idle

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Date:	Tue,  1 May 2012 01:55:02 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	LKML <linux-kernel@...r.kernel.org>,
	linaro-sched-sig@...ts.linaro.org
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Alessio Igor Bogani <abogani@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Avi Kivity <avi@...hat.com>,
	Chris Metcalf <cmetcalf@...era.com>,
	Christoph Lameter <cl@...ux.com>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Geoff Levand <geoff@...radead.org>,
	Gilad Ben Yossef <gilad@...yossef.com>,
	Hakan Akkan <hakanakkan@...il.com>,
	Ingo Molnar <mingo@...nel.org>, Kevin Hilman <khilman@...com>,
	Max Krasnyansky <maxk@...lcomm.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Stephen Hemminger <shemminger@...tta.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Sven-Thorsten Dietrich <thebigcorporation@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: [PATCH 28/41] nohz: Don't restart the tick before scheduling to idle

If we were running adaptive tickless but then we schedule out and
enter the idle task, we don't need to restart the tick because
tick_nohz_idle_enter() is going to be called right away.

The only thing we need to do is to save the jiffies such that
when we later restart the tick we can account the CPU time spent
while idle was tickless.

Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
Cc: Alessio Igor Bogani <abogani@...nel.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Avi Kivity <avi@...hat.com>
Cc: Chris Metcalf <cmetcalf@...era.com>
Cc: Christoph Lameter <cl@...ux.com>
Cc: Daniel Lezcano <daniel.lezcano@...aro.org>
Cc: Geoff Levand <geoff@...radead.org>
Cc: Gilad Ben Yossef <gilad@...yossef.com>
Cc: Hakan Akkan <hakanakkan@...il.com>
Cc: Ingo Molnar <mingo@...nel.org>
Cc: Kevin Hilman <khilman@...com>
Cc: Max Krasnyansky <maxk@...lcomm.com>
Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Stephen Hemminger <shemminger@...tta.com>
Cc: Steven Rostedt <rostedt@...dmis.org>
Cc: Sven-Thorsten Dietrich <thebigcorporation@...il.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
---
 kernel/time/tick-sched.c |   18 +++++++++++-------
 1 files changed, 11 insertions(+), 7 deletions(-)

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 5933506..8217409 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -1029,14 +1029,18 @@ void tick_nohz_pre_schedule(void)
 void tick_nohz_post_schedule(void)
 {
 	struct tick_sched *ts = &__get_cpu_var(tick_cpu_sched);
+	unsigned long flags;
 
-	/*
-	 * No need to disable irqs here. The worst that can happen
-	 * is an irq that comes and restart the tick before us.
-	 * tick_nohz_restart_sched_tick() is irq safe.
-	 */
-	if (ts->tick_stopped)
-		tick_nohz_restart_sched_tick();
+	local_irq_save(flags);
+	if (ts->tick_stopped) {
+		if (is_idle_task(current)) {
+			ts->saved_jiffies = jiffies;
+			ts->saved_jiffies_whence = JIFFIES_SAVED_IDLE;
+		} else {
+			tick_nohz_restart_sched_tick();
+		}
+	}
+	local_irq_restore(flags);
 }
 
 void tick_nohz_flush_current_times(bool restart_tick)
-- 
1.7.5.4

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