sched, cpuacct: refactoring cpuusage_read / cpuusage_write

Impact: micro-optimize the code on 64-bit architectures

In the thread regarding to 'export percpu cpuacct cgroup stats'
http://lkml.org/lkml/2008/12/7/13

akpm pointed out that current cpuacct code is inefficient.  This patch
refactoring the following:

* make cpu_rq locking only on 32-bit
* change iterator to each_present_cpu instead of each_possible_cpu to
  make it hotplug friendly.

It's a bit of code churn, but I was rewarded with 160 byte code size saving
on x86-64 arch and zero code size change on i386.

Signed-off-by: Ken Chen <kenchen@google.com>
Cc: Paul Menage <menage@google.com>
Cc: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/sched.c

@@ -9275,6 +9275,41 @@ cpuacct_destroy(struct cgroup_subsys *ss, struct cgroup *cgrp)
 	kfree(ca);
 }
 
+static u64 cpuacct_cpuusage_read(struct cpuacct *ca, int cpu)
+{
+	u64 *cpuusage = percpu_ptr(ca->cpuusage, cpu);
+	u64 data;
+
+#ifndef CONFIG_64BIT
+	/*
+	 * Take rq->lock to make 64-bit read safe on 32-bit platforms.
+	 */
+	spin_lock_irq(&cpu_rq(cpu)->lock);
+	data = *cpuusage;
+	spin_unlock_irq(&cpu_rq(cpu)->lock);
+#else
+	data = *cpuusage;
+#endif
+
+	return data;
+}
+
+static void cpuacct_cpuusage_write(struct cpuacct *ca, int cpu, u64 val)
+{
+	u64 *cpuusage = percpu_ptr(ca->cpuusage, cpu);
+
+#ifndef CONFIG_64BIT
+	/*
+	 * Take rq->lock to make 64-bit write safe on 32-bit platforms.
+	 */
+	spin_lock_irq(&cpu_rq(cpu)->lock);
+	*cpuusage = val;
+	spin_unlock_irq(&cpu_rq(cpu)->lock);
+#else
+	*cpuusage = val;
+#endif
+}
+
 /* return total cpu usage (in nanoseconds) of a group */
 static u64 cpuusage_read(struct cgroup *cgrp, struct cftype *cft)
 {
@@ -9282,17 +9317,8 @@ static u64 cpuusage_read(struct cgroup *cgrp, struct cftype *cft)
 	u64 totalcpuusage = 0;
 	int i;
 
-	for_each_possible_cpu(i) {
-		u64 *cpuusage = percpu_ptr(ca->cpuusage, i);
-
-		/*
-		 * Take rq->lock to make 64-bit addition safe on 32-bit
-		 * platforms.
-		 */
-		spin_lock_irq(&cpu_rq(i)->lock);
-		totalcpuusage += *cpuusage;
-		spin_unlock_irq(&cpu_rq(i)->lock);
-	}
+	for_each_present_cpu(i)
+		totalcpuusage += cpuacct_cpuusage_read(ca, i);
 
 	return totalcpuusage;
 }
@@ -9309,13 +9335,9 @@ static int cpuusage_write(struct cgroup *cgrp, struct cftype *cftype,
 		goto out;
 	}
 
-	for_each_possible_cpu(i) {
-		u64 *cpuusage = percpu_ptr(ca->cpuusage, i);
+	for_each_present_cpu(i)
+		cpuacct_cpuusage_write(ca, i, 0);
 
-		spin_lock_irq(&cpu_rq(i)->lock);
-		*cpuusage = 0;
-		spin_unlock_irq(&cpu_rq(i)->lock);
-	}
 out:
 	return err;
 }