[PATCH] remove contention on profile_lock

profile_hook unconditionally takes a read lock on profile_lock if kernel
profiling is enabled.  The lock protects the profile_hook notifier chain
from being written while it's being called.  The routine profile_hook is
called in a very hot path though: every timer tick on every CPU.  As you
can imagine, on a large system, this makes the cacheline containing
profile_lock pretty hot.  Since oprofile was the only user of the
profile_hook, I removed the notifier chain altogether in favor of a simple
function pointer with the help of John Levon.  This removes all of the
contention in the hot path since the variable is very seldom written and
simplifies things a little to boot.

Acked-by: John Levon <levon@movementarian.org>
Signed-off-by: Jesse Barnes <jbarnes@sgi.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 17 insertions, 28 deletions

diff --git a/kernel/profile.c b/kernel/profile.c
index e7ff9b32d822..ff62fa98328a 100644
--- a/kernel/profile.c
+++ b/kernel/profile.c
@@ -34,6 +34,9 @@ struct profile_hit {
 #define NR_PROFILE_HIT		(PAGE_SIZE/sizeof(struct profile_hit))
 #define NR_PROFILE_GRP		(NR_PROFILE_HIT/PROFILE_GRPSZ)
 
+/* Oprofile timer tick hook */
+int (*timer_hook)(struct pt_regs *);
+
 static atomic_t *prof_buffer;
 static unsigned long prof_len, prof_shift;
 static int prof_on;
@@ -168,38 +171,24 @@ int profile_event_unregister(enum profile_type type, struct notifier_block * n)
 	return err;
 }
 
-static struct notifier_block * profile_listeners;
-static rwlock_t profile_lock = RW_LOCK_UNLOCKED;
- 
-int register_profile_notifier(struct notifier_block * nb)
-{
-	int err;
-	write_lock_irq(&profile_lock);
-	err = notifier_chain_register(&profile_listeners, nb);
-	write_unlock_irq(&profile_lock);
-	return err;
-}
-
-
-int unregister_profile_notifier(struct notifier_block * nb)
+int register_timer_hook(int (*hook)(struct pt_regs *))
 {
-	int err;
-	write_lock_irq(&profile_lock);
-	err = notifier_chain_unregister(&profile_listeners, nb);
-	write_unlock_irq(&profile_lock);
-	return err;
+	if (timer_hook)
+		return -EBUSY;
+	timer_hook = hook;
+	return 0;
 }
 
-
-void profile_hook(struct pt_regs * regs)
+void unregister_timer_hook(int (*hook)(struct pt_regs *))
 {
-	read_lock(&profile_lock);
-	notifier_call_chain(&profile_listeners, 0, regs);
-	read_unlock(&profile_lock);
+	WARN_ON(hook != timer_hook);
+	timer_hook = NULL;
+	/* make sure all CPUs see the NULL hook */
+	synchronize_kernel();
 }
 
-EXPORT_SYMBOL_GPL(register_profile_notifier);
-EXPORT_SYMBOL_GPL(unregister_profile_notifier);
+EXPORT_SYMBOL_GPL(register_timer_hook);
+EXPORT_SYMBOL_GPL(unregister_timer_hook);
 EXPORT_SYMBOL_GPL(task_handoff_register);
 EXPORT_SYMBOL_GPL(task_handoff_unregister);
 
@@ -394,8 +383,8 @@ void profile_hit(int type, void *__pc)
 
 void profile_tick(int type, struct pt_regs *regs)
 {
-	if (type == CPU_PROFILING)
-		profile_hook(regs);
+	if (type == CPU_PROFILING && timer_hook)
+		timer_hook(regs);
 	if (!user_mode(regs) && cpu_isset(smp_processor_id(), prof_cpu_mask))
 		profile_hit(type, (void *)profile_pc(regs));
 }