From: Ingo Molnar Date: Tue, 23 Jan 2007 09:45:50 +0000 (+0100) Subject: [PATCH] notifiers: fix blocking_notifier_call_chain() scalability X-Git-Tag: v2.6.20-rc6~23 X-Git-Url: http://pilppa.com/gitweb/?a=commitdiff_plain;h=1b5180b65122666a36a1a232b7b9b38b21a9dcdd;p=linux-2.6-omap-h63xx.git [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 Acked-by: Peter Zijlstra Acked-by: Nick Piggin Signed-off-by: Linus Torvalds --- diff --git a/kernel/sys.c b/kernel/sys.c index c7675c1bfdf..6e2101dec0f 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; }