Commit:     1b5180b65122666a36a1a232b7b9b38b21a9dcdd
Parent:     b53d0b919ebe227e2b8e49b6e0ddee506be83aa8
Author:     Ingo Molnar <[EMAIL PROTECTED]>
AuthorDate: Tue Jan 23 10:45:50 2007 +0100
Committer:  Linus Torvalds <[EMAIL PROTECTED]>
CommitDate: Tue Jan 23 11:08:03 2007 -0800

    [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 <[EMAIL PROTECTED]>
    Acked-by: Peter Zijlstra <[EMAIL PROTECTED]>
    Acked-by: Nick Piggin <[EMAIL PROTECTED]>
    Signed-off-by: Linus Torvalds <[EMAIL PROTECTED]>
 kernel/sys.c |   15 +++++++++++----
 1 files changed, 11 insertions(+), 4 deletions(-)

diff --git a/kernel/sys.c b/kernel/sys.c
index c7675c1..6e2101d 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;
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