Btrfs: Change btree locking to use explicit blocking points

Most of the btrfs metadata operations can be protected by a spinlock,
but some operations still need to schedule.

So far, btrfs has been using a mutex along with a trylock loop,
most of the time it is able to avoid going for the full mutex, so
the trylock loop is a big performance gain.

This commit is step one for getting rid of the blocking locks entirely.
btrfs_tree_lock takes a spinlock, and the code explicitly switches
to a blocking lock when it starts an operation that can schedule.

We'll be able get rid of the blocking locks in smaller pieces over time.
Tracing allows us to find the most common cause of blocking, so we
can start with the hot spots first.

The basic idea is:

btrfs_tree_lock() returns with the spin lock held

btrfs_set_lock_blocking() sets the EXTENT_BUFFER_BLOCKING bit in
the extent buffer flags, and then drops the spin lock.  The buffer is
still considered locked by all of the btrfs code.

If btrfs_tree_lock gets the spinlock but finds the blocking bit set, it drops
the spin lock and waits on a wait queue for the blocking bit to go away.

Much of the code that needs to set the blocking bit finishes without actually
blocking a good percentage of the time.  So, an adaptive spin is still
used against the blocking bit to avoid very high context switch rates.

btrfs_clear_lock_blocking() clears the blocking bit and returns
with the spinlock held again.

btrfs_tree_unlock() can be called on either blocking or spinning locks,
it does the right thing based on the blocking bit.

ctree.c has a helper function to set/clear all the locked buffers in a
path as blocking.

Signed-off-by: Chris Mason <chris.mason@oracle.com>

1 files changed, 8 insertions, 2 deletions

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 549271607c17..5aebddd71193 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -799,7 +799,7 @@ struct extent_buffer *read_tree_block(struct btrfs_root *root, u64 bytenr,
         ret = btree_read_extent_buffer_pages(root, buf, 0, parent_transid);
 
         if (ret == 0)
-                buf->flags |= EXTENT_UPTODATE;
+                set_bit(EXTENT_BUFFER_UPTODATE, &buf->bflags);
         else
                 WARN_ON(1);
         return buf;
@@ -813,6 +813,10 @@ int clean_tree_block(struct btrfs_trans_handle *trans, struct btrfs_root *root,
         if (btrfs_header_generation(buf) ==
             root->fs_info->running_transaction->transid) {
                 WARN_ON(!btrfs_tree_locked(buf));
+
+                /* ugh, clear_extent_buffer_dirty can be expensive */
+                btrfs_set_lock_blocking(buf);
+
                 clear_extent_buffer_dirty(&BTRFS_I(btree_inode)->io_tree,
                                           buf);
         }
@@ -2311,6 +2315,8 @@ void btrfs_mark_buffer_dirty(struct extent_buffer *buf)
         u64 transid = btrfs_header_generation(buf);
         struct inode *btree_inode = root->fs_info->btree_inode;
 
+        btrfs_set_lock_blocking(buf);
+
         WARN_ON(!btrfs_tree_locked(buf));
         if (transid != root->fs_info->generation) {
                 printk(KERN_CRIT "btrfs transid mismatch buffer %llu, "
@@ -2353,7 +2359,7 @@ int btrfs_read_buffer(struct extent_buffer *buf, u64 parent_transid)
         int ret;
         ret = btree_read_extent_buffer_pages(root, buf, 0, parent_transid);
         if (ret == 0)
-                buf->flags |= EXTENT_UPTODATE;
+                set_bit(EXTENT_BUFFER_UPTODATE, &buf->bflags);
         return ret;
 }