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| author | Ingo Molnar <mingo@elte.hu> | 2007-01-23 12:45:50 +0300 |
|---|---|---|
| committer | Linus Torvalds <torvalds@woody.linux-foundation.org> | 2007-01-23 22:08:03 +0300 |
| commit | 1b5180b65122666a36a1a232b7b9b38b21a9dcdd (patch) | |
| tree | a61b7cea4cd410755bd5f87ea886554ad1d05676 /kernel/sys.c | |
| parent | b53d0b919ebe227e2b8e49b6e0ddee506be83aa8 (diff) | |
| download | linux-1b5180b65122666a36a1a232b7b9b38b21a9dcdd.tar.xz | |
[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>
Diffstat (limited to 'kernel/sys.c')
| -rw-r--r-- | kernel/sys.c | 15 |
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; } |