linux-kernel - [PATCH] timers: Avoid finding cpu for migration of running timers in __mod

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Date:	Fri, 28 Sep 2012 12:32:19 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	tglx@...utronix.de, linux-kernel@...r.kernel.org
Cc:	paul.mckenney@...aro.org, peterz@...radead.org,
	robin.randhawa@....com, Steve.Bannister@....com,
	Arvind.Chauhan@....com, amit.kucheria@...aro.org,
	vincent.guittot@...aro.org, linaro-dev@...ts.linaro.org,
	patches@...aro.org, Viresh Kumar <viresh.kumar@...aro.org>
Subject: [PATCH] timers: Avoid finding cpu for migration of running timers in __mod_timer()

Currently we don't support migration of currently running timers. Current code
flow in __mod_timer() is:
- Find a cpu where we should migrate the timer
- Check if timer is currently running
- If yes, don't migrate.

In this process, the first step is a unnecessary activiy, if the timer is
currently running. This patch tries to avoid it in such cases.

Signed-off-by: Viresh Kumar <viresh.kumar@...aro.org>
---
 kernel/timer.c | 26 +++++++++++++-------------
 1 file changed, 13 insertions(+), 13 deletions(-)

diff --git a/kernel/timer.c b/kernel/timer.c
index 8c5e7b9..6e5bf98 100644
--- a/kernel/timer.c
+++ b/kernel/timer.c
@@ -755,23 +755,23 @@ __mod_timer(struct timer_list *timer, unsigned long expires,
 
 	debug_activate(timer, expires);
 
-	cpu = smp_processor_id();
+	/*
+	 * Should we try to migrate timer?
+	 * However we can't change timer's base while it is running, otherwise
+	 * del_timer_sync() can't detect that the timer's handler yet has not
+	 * finished. This also guarantees that the timer is serialized wrt
+	 * itself.
+	 */
+	if (likely(base->running_timer != timer)) {
+		cpu = smp_processor_id();
 
 #if defined(CONFIG_NO_HZ) && defined(CONFIG_SMP)
-	if (!pinned && get_sysctl_timer_migration() && idle_cpu(cpu))
-		cpu = get_nohz_timer_target();
+		if (!pinned && get_sysctl_timer_migration() && idle_cpu(cpu))
+			cpu = get_nohz_timer_target();
 #endif
-	new_base = per_cpu(tvec_bases, cpu);
+		new_base = per_cpu(tvec_bases, cpu);
 
-	if (base != new_base) {
-		/*
-		 * We are trying to schedule the timer on the local CPU.
-		 * However we can't change timer's base while it is running,
-		 * otherwise del_timer_sync() can't detect that the timer's
-		 * handler yet has not finished. This also guarantees that
-		 * the timer is serialized wrt itself.
-		 */
-		if (likely(base->running_timer != timer)) {
+		if (base != new_base) {
 			/* See the comment in lock_timer_base() */
 			timer_set_base(timer, NULL);
 			spin_unlock(&base->lock);
-- 
1.7.12.rc2.18.g61b472e


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