[PATCH] notifiers: fix blocking_notifier_call_chain() scalability

while lock-profiling the -rt kernel i noticed weird contention during
mmap-intense workloads, and the tracer showed the following gem, in one
of our MM hotpaths:

 threaded-2771  1....   65us : sys_munmap (sysenter_do_call)
 threaded-2771  1....   66us : profile_munmap (sys_munmap)
 threaded-2771  1....   66us : blocking_notifier_call_chain (profile_munmap)
 threaded-2771  1....   66us : rt_down_read (blocking_notifier_call_chain)

ouch! a global rw-semaphore taken in one of the most performance-
sensitive codepaths of the kernel.  And i dont even have oprofile
enabled! All distro kernels have CONFIG_PROFILING enabled, so this
scalability problem affects the majority of Linux users.

The fix is to enhance blocking_notifier_call_chain() to only take the
lock if there appears to be work on the call-chain.

With this patch applied i get nicely saturated system, and much higher
munmap performance, on SMP systems.

And as a bonus this also fixes a similar scalability bottleneck in the
thread-exit codepath: profile_task_exit() ...

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Acked-by: Nick Piggin <nickpiggin@yahoo.com.au>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 11 insertions, 4 deletions

diff --git a/kernel/sys.c b/kernel/sys.c
index c7675c1bfdf2..6e2101dec0fc 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -323,11 +323,18 @@ EXPORT_SYMBOL_GPL(blocking_notifier_chain_unregister);
 int blocking_notifier_call_chain(struct blocking_notifier_head *nh,
                 unsigned long val, void *v)
 {
-        int ret;
+        int ret = NOTIFY_DONE;
 
-        down_read(&nh->rwsem);
-        ret = notifier_call_chain(&nh->head, val, v);
-        up_read(&nh->rwsem);
+        /*
+         * We check the head outside the lock, but if this access is
+         * racy then it does not matter what the result of the test
+         * is, we re-check the list after having taken the lock anyway:
+         */
+        if (rcu_dereference(nh->head)) {
+                down_read(&nh->rwsem);
+                ret = notifier_call_chain(&nh->head, val, v);
+                up_read(&nh->rwsem);
+        }
         return ret;
 }