linux-kernel - [PATCH 5/7] cpuacct: Remove redundant NULL checks in cpuacct

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Date:	Thu, 28 Mar 2013 12:12:59 +0800
From:	Li Zefan <lizefan@...wei.com>
To:	Ingo Molnar <mingo@...nel.org>
CC:	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: [PATCH 5/7] cpuacct: Remove redundant NULL checks in cpuacct_charge()

This is micro optimization for the hot path.

- We don't need to check if @ca is NULL in parent_ca().
- We don't need to check if @ca is NULL in the beginning of the for loop.

Signed-off-by: Li Zefan <lizefan@...wei.com>
---
 kernel/sched/cpuacct.c | 6 +++++-
 kernel/sched/cpuacct.h | 2 +-
 2 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/cpuacct.c b/kernel/sched/cpuacct.c
index 72bd971..b2aaaba 100644
--- a/kernel/sched/cpuacct.c
+++ b/kernel/sched/cpuacct.c
@@ -210,9 +210,13 @@ void cpuacct_charge(struct task_struct *tsk, u64 cputime)
 
 	ca = task_ca(tsk);
 
-	for (; ca; ca = parent_ca(ca)) {
+	while (true) {
 		u64 *cpuusage = per_cpu_ptr(ca->cpuusage, cpu);
 		*cpuusage += cputime;
+
+		ca = parent_ca(ca);
+		if (!ca)
+			break;
 	}
 
 	rcu_read_unlock();
diff --git a/kernel/sched/cpuacct.h b/kernel/sched/cpuacct.h
index bd0409b..45c1682 100644
--- a/kernel/sched/cpuacct.h
+++ b/kernel/sched/cpuacct.h
@@ -36,7 +36,7 @@ static inline struct cpuacct *task_ca(struct task_struct *tsk)
 
 static inline struct cpuacct *parent_ca(struct cpuacct *ca)
 {
-	if (!ca || !ca->css.cgroup->parent)
+	if (!ca->css.cgroup->parent)
 		return NULL;
 	return cgroup_ca(ca->css.cgroup->parent);
 }
-- 
1.8.0.2
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