[PATCH] cpuset: skip rcu check if task is in root cpuset

For systems that aren't using cpusets, but have them CONFIG_CPUSET enabled in
their kernel (eventually this may be most distribution kernels), this patch
removes even the minimal rcu_read_lock() from the memory page allocation path.

Actually, it removes that rcu call for any task that is in the root cpuset
(top_cpuset), which on systems not actively using cpusets, is all tasks.

We don't need the rcu check for tasks in the top_cpuset, because the
top_cpuset is statically allocated, so at no risk of being freed out from
underneath us.
Signed-off-by: NPaul Jackson <pj@sgi.com>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>

kernel/cpuset.c

@@ -647,10 +647,15 @@ void cpuset_update_task_memory_state()
 	struct task_struct *tsk = current;
 	struct cpuset *cs;
 
+	if (tsk->cpuset == &top_cpuset) {
+		/* Don't need rcu for top_cpuset.  It's never freed. */
+		my_cpusets_mem_gen = top_cpuset.mems_generation;
+	} else {
 		rcu_read_lock();
 		cs = rcu_dereference(tsk->cpuset);
 		my_cpusets_mem_gen = cs->mems_generation;
 		rcu_read_unlock();
+	}
 
 	if (my_cpusets_mem_gen != tsk->cpuset_mems_generation) {
 		down(&callback_sem);