Btrfs: Add ordered async work queues

Btrfs uses kernel threads to create async work queues for cpu intensive
operations such as checksumming and decompression.  These work well,
but they make it difficult to keep IO order intact.

A single writepages call from pdflush or fsync will turn into a number
of bios, and each bio is checksummed in parallel.  Once the checksum is
computed, the bio is sent down to the disk, and since we don't control
the order in which the parallel operations happen, they might go down to
the disk in almost any order.

The code deals with this somewhat by having deep work queues for a single
kernel thread, making it very likely that a single thread will process all
the bios for a single inode.

This patch introduces an explicitly ordered work queue.  As work structs
are placed into the queue they are put onto the tail of a list.  They have
three callbacks:

->func (cpu intensive processing here)
->ordered_func (order sensitive processing here)
->ordered_free (free the work struct, all processing is done)

The work struct has three callbacks.  The func callback does the cpu intensive
work, and when it completes the work struct is marked as done.

Every time a work struct completes, the list is checked to see if the head
is marked as done.  If so the ordered_func callback is used to do the
order sensitive processing and the ordered_free callback is used to do
any cleanup.  Then we loop back and check the head of the list again.

This patch also changes the checksumming code to use the ordered workqueues.
One a 4 drive array, it increases streaming writes from 280MB/s to 350MB/s.

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

1 files changed, 17 insertions, 2 deletions

diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 806708dd7e38..3df0ffad976e 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -881,7 +881,7 @@ int btrfs_merge_bio_hook(struct page *page, unsigned long offset,
  * At IO completion time the cums attached on the ordered extent record
  * are inserted into the btree
  */
-int __btrfs_submit_bio_hook(struct inode *inode, int rw, struct bio *bio,
+int __btrfs_submit_bio_start(struct inode *inode, int rw, struct bio *bio,
                           int mirror_num, unsigned long bio_flags)
 {
         struct btrfs_root *root = BTRFS_I(inode)->root;
@@ -889,7 +889,21 @@ int __btrfs_submit_bio_hook(struct inode *inode, int rw, struct bio *bio,
 
         ret = btrfs_csum_one_bio(root, inode, bio);
         BUG_ON(ret);
+        return 0;
+}
 
+/*
+ * in order to insert checksums into the metadata in large chunks,
+ * we wait until bio submission time.   All the pages in the bio are
+ * checksummed and sums are attached onto the ordered extent record.
+ *
+ * At IO completion time the cums attached on the ordered extent record
+ * are inserted into the btree
+ */
+int __btrfs_submit_bio_done(struct inode *inode, int rw, struct bio *bio,
+                          int mirror_num, unsigned long bio_flags)
+{
+        struct btrfs_root *root = BTRFS_I(inode)->root;
         return btrfs_map_bio(root, rw, bio, mirror_num, 1);
 }
 
@@ -922,7 +936,8 @@ int btrfs_submit_bio_hook(struct inode *inode, int rw, struct bio *bio,
                 /* we're doing a write, do the async checksumming */
                 return btrfs_wq_submit_bio(BTRFS_I(inode)->root->fs_info,
                                    inode, rw, bio, mirror_num,
-                                   bio_flags, __btrfs_submit_bio_hook);
+                                   bio_flags, __btrfs_submit_bio_start,
+                                   __btrfs_submit_bio_done);
         }
 
 mapit: