[PATCH] cpuset semaphore depth check optimize

Optimize the deadlock avoidance check on the global cpuset
semaphore cpuset_sem.  Instead of adding a depth counter to the
task struct of each task, rather just two words are enough, one
to store the depth and the other the current cpuset_sem holder.

Thanks to Nikita Danilov for the idea.
Signed-off-by: NPaul Jackson <pj@sgi.com>

[ We may want to change this further, but at least it's now
  a totally internal decision to the cpusets code ]
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>

include/linux/sched.h

@@ -785,7 +785,6 @@ struct task_struct {
 	short il_next;
 #endif
 #ifdef CONFIG_CPUSETS
-	short cpuset_sem_nest_depth;
 	struct cpuset *cpuset;
 	nodemask_t mems_allowed;
 	int cpuset_mems_generation;

kernel/cpuset.c

@@ -180,6 +180,8 @@ static struct super_block *cpuset_sb = NULL;
  */
 
 static DECLARE_MUTEX(cpuset_sem);
+static struct task_struct *cpuset_sem_owner;
+static int cpuset_sem_depth;
 
 /*
  * The global cpuset semaphore cpuset_sem can be needed by the
@@ -200,16 +202,19 @@ static DECLARE_MUTEX(cpuset_sem);
 
 static inline void cpuset_down(struct semaphore *psem)
 {
-	if (current->cpuset_sem_nest_depth == 0)
+	if (cpuset_sem_owner != current) {
 		down(psem);
-	current->cpuset_sem_nest_depth++;
+		cpuset_sem_owner = current;
+	}
+	cpuset_sem_depth++;
 }
 
 static inline void cpuset_up(struct semaphore *psem)
 {
-	current->cpuset_sem_nest_depth--;
-	if (current->cpuset_sem_nest_depth == 0)
+	if (--cpuset_sem_depth == 0) {
+		cpuset_sem_owner = NULL;
 		up(psem);
+	}
 }
 
 /*