sched: report CPU usage in CFS cgroup directories

Adds a cpu.usage file to the CFS cgroup that reports CPU usage in
milliseconds for that cgroup's tasks

[ mingo@elte.hu: style cleanups. ]
Signed-off-by: Paul Menage <menage@google.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/sched.c

@@ -7211,15 +7211,43 @@ static u64 cpu_shares_read_uint(struct cgroup *cgrp, struct cftype *cft)
 	return (u64) tg->shares;
 }
 
-static struct cftype cpu_shares = {
+static u64 cpu_usage_read(struct cgroup *cgrp, struct cftype *cft)
+{
+	struct task_group *tg = cgroup_tg(cgrp);
+	unsigned long flags;
+	u64 res = 0;
+	int i;
+
+	for_each_possible_cpu(i) {
+		/*
+		 * Lock to prevent races with updating 64-bit counters
+		 * on 32-bit arches.
+		 */
+		spin_lock_irqsave(&cpu_rq(i)->lock, flags);
+		res += tg->se[i]->sum_exec_runtime;
+		spin_unlock_irqrestore(&cpu_rq(i)->lock, flags);
+	}
+	/* Convert from ns to ms */
+	do_div(res, 1000000);
+
+	return res;
+}
+
+static struct cftype cpu_files[] = {
+	{
 		.name = "shares",
 		.read_uint = cpu_shares_read_uint,
 		.write_uint = cpu_shares_write_uint,
+	},
+	{
+		.name = "usage",
+		.read_uint = cpu_usage_read,
+	},
 };
 
 static int cpu_cgroup_populate(struct cgroup_subsys *ss, struct cgroup *cont)
 {
-	return cgroup_add_file(cont, ss, &cpu_shares);
+	return cgroup_add_files(cont, ss, cpu_files, ARRAY_SIZE(cpu_files));
 }
 
 struct cgroup_subsys cpu_cgroup_subsys = {