bio: skip atomic inc/dec of ->bi_remaining for non-chains

Struct bio has an atomic ref count for chained bio's, and we use this
to know when to end IO on the bio. However, most bio's are not chained,
so we don't need to always introduce this atomic operation as part of
ending IO.

Add a helper to elevate the bi_remaining count, and flag the bio as
now actually needing the decrement at end_io time. Rename the field
to __bi_remaining to catch any current users of this doing the
incrementing manually.

For high IOPS workloads, this reduces the overhead of bio_endio()
substantially.
Tested-by: Robert Elliott <elliott@hp.com>
Acked-by: Kent Overstreet <kent.overstreet@gmail.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@fb.com>

block/bio.c

@@ -270,7 +270,7 @@ void bio_init(struct bio *bio)
 {
 	memset(bio, 0, sizeof(*bio));
 	bio->bi_flags = 1 << BIO_UPTODATE;
-	atomic_set(&bio->bi_remaining, 1);
+	atomic_set(&bio->__bi_remaining, 1);
 	atomic_set(&bio->bi_cnt, 1);
 }
 EXPORT_SYMBOL(bio_init);
@@ -292,8 +292,8 @@ void bio_reset(struct bio *bio)
 	__bio_free(bio);
 
 	memset(bio, 0, BIO_RESET_BYTES);
-	bio->bi_flags = flags|(1 << BIO_UPTODATE);
-	atomic_set(&bio->bi_remaining, 1);
+	bio->bi_flags = flags | (1 << BIO_UPTODATE);
+	atomic_set(&bio->__bi_remaining, 1);
 }
 EXPORT_SYMBOL(bio_reset);
 
@@ -320,7 +320,7 @@ void bio_chain(struct bio *bio, struct bio *parent)
 
 	bio->bi_private = parent;
 	bio->bi_end_io	= bio_chain_endio;
-	atomic_inc(&parent->bi_remaining);
+	bio_inc_remaining(parent);
 }
 EXPORT_SYMBOL(bio_chain);
 
@@ -1741,6 +1741,23 @@ void bio_flush_dcache_pages(struct bio *bi)
 EXPORT_SYMBOL(bio_flush_dcache_pages);
 #endif
 
+static inline bool bio_remaining_done(struct bio *bio)
+{
+	/*
+	 * If we're not chaining, then ->__bi_remaining is always 1 and
+	 * we always end io on the first invocation.
+	 */
+	if (!bio_flagged(bio, BIO_CHAIN))
+		return true;
+
+	BUG_ON(atomic_read(&bio->__bi_remaining) <= 0);
+
+	if (atomic_dec_and_test(&bio->__bi_remaining))
+		return true;
+
+	return false;
+}
+
 /**
  * bio_endio - end I/O on a bio
  * @bio:	bio
@@ -1758,15 +1775,13 @@ EXPORT_SYMBOL(bio_flush_dcache_pages);
 void bio_endio(struct bio *bio, int error)
 {
 	while (bio) {
-		BUG_ON(atomic_read(&bio->bi_remaining) <= 0);
-
 		if (error)
 			clear_bit(BIO_UPTODATE, &bio->bi_flags);
 		else if (!test_bit(BIO_UPTODATE, &bio->bi_flags))
 			error = -EIO;
 
-		if (!atomic_dec_and_test(&bio->bi_remaining))
-			return;
+		if (unlikely(!bio_remaining_done(bio)))
+			break;
 
 		/*
 		 * Need to have a real endio function for chained bios,
@@ -1799,7 +1814,12 @@ EXPORT_SYMBOL(bio_endio);
  **/
 void bio_endio_nodec(struct bio *bio, int error)
 {
-	atomic_inc(&bio->bi_remaining);
+	/*
+	 * If it's not flagged as a chain, we are not going to dec the count
+	 */
+	if (bio_flagged(bio, BIO_CHAIN))
+		bio_inc_remaining(bio);
+
 	bio_endio(bio, error);
 }
 EXPORT_SYMBOL(bio_endio_nodec);

drivers/md/dm-cache-target.c

@@ -91,7 +91,7 @@ static void dm_unhook_bio(struct dm_hook_info *h, struct bio *bio)
 	 * Must bump bi_remaining to allow bio to complete with
 	 * restored bi_end_io.
 	 */
-	atomic_inc(&bio->bi_remaining);
+	bio_inc_remaining(bio);
 }
 
 /*----------------------------------------------------------------*/

drivers/md/dm-raid1.c

@@ -1254,7 +1254,7 @@ static int mirror_end_io(struct dm_target *ti, struct bio *bio, int error)
 			dm_bio_restore(bd, bio);
 			bio_record->details.bi_bdev = NULL;
 
-			atomic_inc(&bio->bi_remaining);
+			bio_inc_remaining(bio);
 
 			queue_bio(ms, bio, rw);
 			return DM_ENDIO_INCOMPLETE;

drivers/md/dm-snap.c

@@ -1478,7 +1478,7 @@ static void pending_complete(struct dm_snap_pending_exception *pe, int success)
 	if (full_bio) {
 		full_bio->bi_end_io = pe->full_bio_end_io;
 		full_bio->bi_private = pe->full_bio_private;
-		atomic_inc(&full_bio->bi_remaining);
+		bio_inc_remaining(full_bio);
 	}
 	increment_pending_exceptions_done_count();
 

drivers/md/dm-thin.c

@@ -795,7 +795,7 @@ static void process_prepared_mapping_fail(struct dm_thin_new_mapping *m)
 {
 	if (m->bio) {
 		m->bio->bi_end_io = m->saved_bi_end_io;
-		atomic_inc(&m->bio->bi_remaining);
+		bio_inc_remaining(m->bio);
 	}
 	cell_error(m->tc->pool, m->cell);
 	list_del(&m->list);
@@ -812,7 +812,7 @@ static void process_prepared_mapping(struct dm_thin_new_mapping *m)
 	bio = m->bio;
 	if (bio) {
 		bio->bi_end_io = m->saved_bi_end_io;
-		atomic_inc(&bio->bi_remaining);
+		bio_inc_remaining(bio);
 	}
 
 	if (m->err) {

include/linux/bio.h

@@ -644,6 +644,17 @@ static inline struct bio *bio_list_get(struct bio_list *bl)
 	return bio;
 }
 
+/*
+ * Increment chain count for the bio. Make sure the CHAIN flag update
+ * is visible before the raised count.
+ */
+static inline void bio_inc_remaining(struct bio *bio)
+{
+	bio->bi_flags |= (1 << BIO_CHAIN);
+	smp_mb__before_atomic();
+	atomic_inc(&bio->__bi_remaining);
+}
+
 /*
  * bio_set is used to allow other portions of the IO system to
  * allocate their own private memory pools for bio and iovec structures.

include/linux/blk_types.h

@@ -65,7 +65,7 @@ struct bio {
 	unsigned int		bi_seg_front_size;
 	unsigned int		bi_seg_back_size;
 
-	atomic_t		bi_remaining;
+	atomic_t		__bi_remaining;
 
 	bio_end_io_t		*bi_end_io;
 
@@ -122,6 +122,7 @@ struct bio {
 #define BIO_NULL_MAPPED 8	/* contains invalid user pages */
 #define BIO_QUIET	9	/* Make BIO Quiet */
 #define BIO_SNAP_STABLE	10	/* bio data must be snapshotted during write */
+#define BIO_CHAIN	11	/* chained bio, ->bi_remaining in effect */
 
 /*
  * Flags starting here get preserved by bio_reset() - this includes