[PATCH] cpuset: use rcu directly optimization

Optimize the cpuset impact on page allocation, the most performance critical
cpuset hook in the kernel.

On each page allocation, the cpuset hook needs to check for a possible change
in the current tasks cpuset.  It can now handle the common case, of no change,
without taking any spinlock or semaphore, thanks to RCU.

Convert a spinlock on the current task to an rcu_read_lock(), saving
approximately a memory barrier and an atomic op, depending on architecture.

This is done by adding rcu_assign_pointer() and synchronize_rcu() calls to the
write side of the task->cpuset pointer, in cpuset.c:attach_task(), to delay
freeing up a detached cpuset until after any critical sections referencing
that pointer.

Thanks to Andi Kleen, Nick Piggin and Eric Dumazet for ideas.
Signed-off-by: Paul Jackson <pj@sgi.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

kernel/cpuset.c

@@ -39,6 +39,7 @@
 #include <linux/namei.h>
 #include <linux/pagemap.h>
 #include <linux/proc_fs.h>
+#include <linux/rcupdate.h>
 #include <linux/sched.h>
 #include <linux/seq_file.h>
 #include <linux/slab.h>
@@ -248,6 +249,11 @@ static struct super_block *cpuset_sb;
  * a tasks cpuset pointer we use task_lock(), which acts on a spinlock
  * (task->alloc_lock) already in the task_struct routinely used for
  * such matters.
+ *
+ * P.S.  One more locking exception.  RCU is used to guard the
+ * update of a tasks cpuset pointer by attach_task() and the
+ * access of task->cpuset->mems_generation via that pointer in
+ * the routine cpuset_update_task_memory_state().
  */
 
 static DECLARE_MUTEX(manage_sem);
@@ -610,12 +616,24 @@ static void guarantee_online_mems(const struct cpuset *cs, nodemask_t *pmask)
  * cpuset pointer.  This routine also might acquire callback_sem and
  * current->mm->mmap_sem during call.
  *
- * The task_lock() is required to dereference current->cpuset safely.
- * Without it, we could pick up the pointer value of current->cpuset
- * in one instruction, and then attach_task could give us a different
- * cpuset, and then the cpuset we had could be removed and freed,
- * and then on our next instruction, we could dereference a no longer
- * valid cpuset pointer to get its mems_generation field.
+ * Reading current->cpuset->mems_generation doesn't need task_lock
+ * to guard the current->cpuset derefence, because it is guarded
+ * from concurrent freeing of current->cpuset by attach_task(),
+ * using RCU.
+ *
+ * The rcu_dereference() is technically probably not needed,
+ * as I don't actually mind if I see a new cpuset pointer but
+ * an old value of mems_generation.  However this really only
+ * matters on alpha systems using cpusets heavily.  If I dropped
+ * that rcu_dereference(), it would save them a memory barrier.
+ * For all other arch's, rcu_dereference is a no-op anyway, and for
+ * alpha systems not using cpusets, another planned optimization,
+ * avoiding the rcu critical section for tasks in the root cpuset
+ * which is statically allocated, so can't vanish, will make this
+ * irrelevant.  Better to use RCU as intended, than to engage in
+ * some cute trick to save a memory barrier that is impossible to
+ * test, for alpha systems using cpusets heavily, which might not
+ * even exist.
  *
  * This routine is needed to update the per-task mems_allowed data,
  * within the tasks context, when it is trying to allocate memory
@@ -627,11 +645,12 @@ void cpuset_update_task_memory_state()
 {
 	int my_cpusets_mem_gen;
 	struct task_struct *tsk = current;
-	struct cpuset *cs = tsk->cpuset;
+	struct cpuset *cs;
 
-	task_lock(tsk);
+	rcu_read_lock();
+	cs = rcu_dereference(tsk->cpuset);
 	my_cpusets_mem_gen = cs->mems_generation;
-	task_unlock(tsk);
+	rcu_read_unlock();
 
 	if (my_cpusets_mem_gen != tsk->cpuset_mems_generation) {
 		down(&callback_sem);
@@ -1131,7 +1150,7 @@ static int attach_task(struct cpuset *cs, char *pidbuf, char **ppathbuf)
 		return -ESRCH;
 	}
 	atomic_inc(&cs->count);
-	tsk->cpuset = cs;
+	rcu_assign_pointer(tsk->cpuset, cs);
 	task_unlock(tsk);
 
 	guarantee_online_cpus(cs, &cpus);
@@ -1151,6 +1170,7 @@ static int attach_task(struct cpuset *cs, char *pidbuf, char **ppathbuf)
 	if (is_memory_migrate(cs))
 		do_migrate_pages(tsk->mm, &from, &to, MPOL_MF_MOVE_ALL);
 	put_task_struct(tsk);
+	synchronize_rcu();
 	if (atomic_dec_and_test(&oldcs->count))
 		check_for_release(oldcs, ppathbuf);
 	return 0;