linux-kernel - [PATCH 2/2] nohz: don't fetch per

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Date:	Fri, 25 Apr 2014 20:57:30 +0200
From:	Denys Vlasenko <dvlasenk@...hat.com>
To:	linux-kernel@...r.kernel.org
Cc:	Denys Vlasenko <dvlasenk@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>,
	Fernando Luis Vazquez Cao <fernando_b1@....ntt.co.jp>,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Oleg Nesterov <oleg@...hat.com>
Subject: [PATCH 2/2] nohz: don't fetch per_cpu variable before we need it

In get_cpu_{idle,iowait}_time_us, we were fetching
per_cpu(tick_cpu_sched, cpu) upfront. But them we do
        if (!tick_nohz_active)
                return -1;
which may make that fetch pointless.
Moving it further down, closer to the usage point.

Signed-off-by: Denys Vlasenko <dvlasenk@...hat.com>
Cc: Frederic Weisbecker <fweisbec@...il.com>
Cc: Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>
Cc: Fernando Luis Vazquez Cao <fernando_b1@....ntt.co.jp>
Cc: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Arjan van de Ven <arjan@...ux.intel.com>
Cc: Oleg Nesterov <oleg@...hat.com>
---
 kernel/time/tick-sched.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index d78c942..4ffccd0 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -476,7 +476,7 @@ void tick_nohz_iowait_to_idle(int cpu)
  */
 u64 get_cpu_idle_time_us(int cpu, u64 *last_update_time)
 {
-	struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);
+	struct tick_sched *ts;
 	ktime_t now, idle;
 	unsigned int seq;
 
@@ -487,6 +487,8 @@ u64 get_cpu_idle_time_us(int cpu, u64 *last_update_time)
 	if (last_update_time)
 		*last_update_time = ktime_to_us(now);
 
+	ts = &per_cpu(tick_cpu_sched, cpu);
+
 	do {
 		ktime_t start, delta, iowait_exit;
 
@@ -531,7 +533,7 @@ EXPORT_SYMBOL_GPL(get_cpu_idle_time_us);
  */
 u64 get_cpu_iowait_time_us(int cpu, u64 *last_update_time)
 {
-	struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);
+	struct tick_sched *ts;
 	ktime_t now, iowait;
 	unsigned int seq;
 
@@ -542,6 +544,8 @@ u64 get_cpu_iowait_time_us(int cpu, u64 *last_update_time)
 	if (last_update_time)
 		*last_update_time = ktime_to_us(now);
 
+	ts = &per_cpu(tick_cpu_sched, cpu);
+
 	do {
 		ktime_t delta, end;
 
-- 
1.8.1.4

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