[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: Paul 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: Linus Torvalds <torvalds@osdl.org>

2 files changed, 9 insertions, 5 deletions

diff --git a/include/linux/sched.h b/include/linux/sched.h
index ed3bb19d1337..38c8654aaa96 100644
--- a/include/linux/sched.h
+++ b/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;
diff --git a/kernel/cpuset.c b/kernel/cpuset.c
index 407b5f0a8c8e..79866bc6b3a1 100644
--- a/kernel/cpuset.c
+++ b/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);
+        }
 }
 
 /*