[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: Paul Jackson <pj@sgi.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 9 insertions, 4 deletions

diff --git a/kernel/cpuset.c b/kernel/cpuset.c
index 681a5d58d40d..e04c2da9dadb 100644
--- a/kernel/cpuset.c
+++ b/kernel/cpuset.c
@@ -647,10 +647,15 @@ void cpuset_update_task_memory_state()
         struct task_struct *tsk = current;
         struct cpuset *cs;
 
-        rcu_read_lock();
-        cs = rcu_dereference(tsk->cpuset);
-        my_cpusets_mem_gen = cs->mems_generation;
-        rcu_read_unlock();
+        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);