Move last looked up sem_undo struct to the head of the task's undo list.
Attempt to move common entries to the front of the list so search time is
reduced. This reduces lookup_undo on oprofile of problematic SAP workload
by 30% (see patch 4 for a description of SAP workload).

Signed-off-by: Nick Piggin <npiggin@suse.de>
---
 ipc/sem.c |   26 ++++++++++++++++++++------
 1 file changed, 20 insertions(+), 6 deletions(-)

Index: linux-2.6/ipc/sem.c
===================================================================
--- linux-2.6.orig/ipc/sem.c
+++ linux-2.6/ipc/sem.c
@@ -961,17 +961,31 @@ static inline int get_undo_list(struct s
 	return 0;
 }
 
-static struct sem_undo *lookup_undo(struct sem_undo_list *ulp, int semid)
+static struct sem_undo *__lookup_undo(struct sem_undo_list *ulp, int semid)
 {
-	struct sem_undo *walk;
+	struct sem_undo *un;
 
-	list_for_each_entry_rcu(walk, &ulp->list_proc, list_proc) {
-		if (walk->semid == semid)
-			return walk;
+	list_for_each_entry_rcu(un, &ulp->list_proc, list_proc) {
+		if (un->semid == semid)
+			return un;
 	}
 	return NULL;
 }
 
+static struct sem_undo *lookup_undo(struct sem_undo_list *ulp, int semid)
+{
+	struct sem_undo *un;
+
+  	assert_spin_locked(&ulp->lock);
+
+	un = __lookup_undo(ulp, semid);
+	if (un) {
+		list_del_rcu(&un->list_proc);
+		list_add_rcu(&un->list_proc, &ulp->list_proc);
+	}
+	return un;
+}
+
 /**
  * find_alloc_undo - Lookup (and if not present create) undo array
  * @ns: namespace
@@ -1307,7 +1321,7 @@ void exit_sem(struct task_struct *tsk)
 		if (IS_ERR(sma))
 			continue;
 
-		un = lookup_undo(ulp, semid);
+		un = __lookup_undo(ulp, semid);
 		if (un == NULL) {
 			/* exit_sem raced with IPC_RMID+semget() that created
 			 * exactly the same semid. Nothing to do.


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