sched/cpuacct: Remove redundant NULL checks in cpuacct_acount_field()

This is a micro optimazation for a hot path.

- We don't need to check if @ca returned from task_ca() is NULL.
- We don't need to check if @ca returned from parent_ca() is NULL.
Signed-off-by: Li Zefan <lizefan@huawei.com>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/515536B7.6060602@huawei.comSigned-off-by: Ingo Molnar <mingo@kernel.org>

kernel/sched/cpuacct.c

@@ -237,10 +237,10 @@ void cpuacct_account_field(struct task_struct *p, int index, u64 val)
 
 	rcu_read_lock();
 	ca = task_ca(p);
-	while (ca && (ca != &root_cpuacct)) {
+	while (ca != &root_cpuacct) {
 		kcpustat = this_cpu_ptr(ca->cpustat);
 		kcpustat->cpustat[index] += val;
-		ca = parent_ca(ca);
+		ca = __parent_ca(ca);
 	}
 	rcu_read_unlock();
 }

kernel/sched/cpuacct.h

@@ -34,6 +34,11 @@ static inline struct cpuacct *task_ca(struct task_struct *tsk)
 			    struct cpuacct, css);
 }
 
+static inline struct cpuacct *__parent_ca(struct cpuacct *ca)
+{
+	return cgroup_ca(ca->css.cgroup->parent);
+}
+
 static inline struct cpuacct *parent_ca(struct cpuacct *ca)
 {
 	if (!ca->css.cgroup->parent)