[PATCH] cpuacct: VIRT_CPU_ACCOUNTING don't prevent percpu cputime count

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

I'm not cpuacct expert. please give me comment.

====================
Subject: [PATCH] cpuacct: VIRT_CPU_ACCOUNTING don't prevent percpu cputime caching

impact: little performance improvement

cpuacct_update_stats() is called at every tick updating. and it use percpu_counter
for avoiding performance degression.

Unfortunately, it doesn't works on VIRT_CPU_ACCOUNTING=y environment properly.
if VIRT_CPU_ACCOUNTING=y, every tick update much than 1000 cputime.
Thus every percpu_counter_add() makes spinlock grabbing and update non-percpu-variable.

This patch change the batch rule. now, every cpu can store "percpu_counter_bach x jiffies"
cputime in percpu cache.
it mean this patch don't have behavior change if VIRT_CPU_ACCOUNTING=n, but
works well on VIRT_CPU_ACCOUNTING=y too.


Cc: Bharata B Rao <bharata@linux.vnet.ibm.com>
Cc: Balaji Rao <balajirrao@gmail.com>
Cc: Dhaval Giani <dhaval@linux.vnet.ibm.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Balbir Singh <balbir@linux.vnet.ibm.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
---
 kernel/sched.c |    6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

Index: b/kernel/sched.c
===================================================================
--- a/kernel/sched.c	2009-04-28 14:18:36.000000000 +0900
+++ b/kernel/sched.c	2009-04-28 15:18:07.000000000 +0900
@@ -10117,6 +10117,7 @@ struct cpuacct {
 };
 
 struct cgroup_subsys cpuacct_subsys;
+static s32 cpuacct_batch;
 
 /* return cpu accounting group corresponding to this container */
 static inline struct cpuacct *cgroup_ca(struct cgroup *cgrp)
@@ -10146,6 +10147,9 @@ static struct cgroup_subsys_state *cpuac
 	if (!ca->cpuusage)
 		goto out_free_ca;
 
+	if (!cpuacct_batch)
+		cpuacct_batch = jiffies_to_cputime(percpu_counter_batch);
+
 	for (i = 0; i < CPUACCT_STAT_NSTATS; i++)
 		if (percpu_counter_init(&ca->cpustat[i], 0))
 			goto out_free_counters;
@@ -10342,7 +10346,7 @@ static void cpuacct_update_stats(struct 
 	ca = task_ca(tsk);
 
 	do {
-		percpu_counter_add(&ca->cpustat[idx], val);
+		__percpu_counter_add(&ca->cpustat[idx], val, cpuacct_batch);
 		ca = ca->parent;
 	} while (ca);
 	rcu_read_unlock();