Subject: [patch] notifiers: fix blocking_notifier_call_chain() scalability
From: Ingo Molnar <[email protected]>
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]>
---
kernel/sys.c | 15 +++++++++++----
1 file changed, 11 insertions(+), 4 deletions(-)
Index: linux/kernel/sys.c
===================================================================
--- linux.orig/kernel/sys.c
+++ linux/kernel/sys.c
@@ -325,11 +325,18 @@ EXPORT_SYMBOL_GPL(blocking_notifier_chai
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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