Btrfs: reduce calls to wake_up on uncontended locks
The btrfs locks were unconditionally calling wake_up as the
locks were released. This lead to extra thrashing on the waitqueue,
especially for locks that were dominated by readers.
Signed-off-by: Chris Mason <chris.mason@fusionio.com>
diff --git a/fs/btrfs/locking.c b/fs/btrfs/locking.c
index 272f911..a44eff074 100644
--- a/fs/btrfs/locking.c
+++ b/fs/btrfs/locking.c
@@ -78,13 +78,15 @@
write_lock(&eb->lock);
WARN_ON(atomic_read(&eb->spinning_writers));
atomic_inc(&eb->spinning_writers);
- if (atomic_dec_and_test(&eb->blocking_writers))
+ if (atomic_dec_and_test(&eb->blocking_writers) &&
+ waitqueue_active(&eb->write_lock_wq))
wake_up(&eb->write_lock_wq);
} else if (rw == BTRFS_READ_LOCK_BLOCKING) {
BUG_ON(atomic_read(&eb->blocking_readers) == 0);
read_lock(&eb->lock);
atomic_inc(&eb->spinning_readers);
- if (atomic_dec_and_test(&eb->blocking_readers))
+ if (atomic_dec_and_test(&eb->blocking_readers) &&
+ waitqueue_active(&eb->read_lock_wq))
wake_up(&eb->read_lock_wq);
}
return;
@@ -199,7 +201,8 @@
}
btrfs_assert_tree_read_locked(eb);
WARN_ON(atomic_read(&eb->blocking_readers) == 0);
- if (atomic_dec_and_test(&eb->blocking_readers))
+ if (atomic_dec_and_test(&eb->blocking_readers) &&
+ waitqueue_active(&eb->read_lock_wq))
wake_up(&eb->read_lock_wq);
atomic_dec(&eb->read_locks);
}
@@ -247,8 +250,9 @@
if (blockers) {
WARN_ON(atomic_read(&eb->spinning_writers));
atomic_dec(&eb->blocking_writers);
- smp_wmb();
- wake_up(&eb->write_lock_wq);
+ smp_mb();
+ if (waitqueue_active(&eb->write_lock_wq))
+ wake_up(&eb->write_lock_wq);
} else {
WARN_ON(atomic_read(&eb->spinning_writers) != 1);
atomic_dec(&eb->spinning_writers);