blkback-final-check-fix.diff

# HG changeset patch
# Parent 9ac29cb432cd98e27ac54482f0c963c3dd4aefdb
blkback: Don't let in-flight requests defer pending ones.

Running RING_FINAL_CHECK_FOR_REQUESTS from make_response is a bad
idea. It means that in-flight I/O is essentially blocking continued
batches. This essentially kills throughput on frontends which unplug
(or even just notify) early and rightfully assume addtional requests
will be picked up on time, not synchronously. 

Saw cache writeback going up from 45MB/s top 65MB/s for xen-blkfront
on ISCSI.

Signed-off-by: Daniel Stodden <daniel.stodden@citrix.com>

diff -r 9ac29cb432cd drivers/xen/blkback/blkback.c
--- a/drivers/xen/blkback/blkback.c	Fri May 27 15:27:45 2011 -0700
+++ b/drivers/xen/blkback/blkback.c	Fri May 27 15:28:52 2011 -0700
@@ -552,7 +552,7 @@
  * DOWNWARD CALLS -- These interface with the block-device layer proper.
  */
 
-static int do_block_io_op(blkif_t *blkif)
+static int __do_block_io_op(blkif_t *blkif)
 {
 	blkif_back_rings_t *blk_rings = &blkif->blk_rings;
 	blkif_request_t req;
@@ -630,6 +630,22 @@
 	return more_to_do;
 }
 
+static int do_block_io_op(blkif_t *blkif)
+{
+	blkif_back_rings_t *blk_rings = &blkif->blk_rings;
+	int more_to_do;
+
+	do {
+		more_to_do = __do_block_io_op(blkif);
+		if (more_to_do)
+			break;
+
+		RING_FINAL_CHECK_FOR_REQUESTS(&blk_rings->common, more_to_do);
+	} while (more_to_do);
+
+	return more_to_do;
+}
+
 static void dispatch_rw_block_io(blkif_t *blkif,
 				 blkif_request_t *req,
 				 pending_req_t *pending_req)
@@ -819,7 +837,6 @@
 	blkif_response_t  resp;
 	unsigned long     flags;
 	blkif_back_rings_t *blk_rings = &blkif->blk_rings;
-	int more_to_do = 0;
 	int notify;
 
 	resp.id        = id;
@@ -846,22 +863,7 @@
 	}
 	blk_rings->common.rsp_prod_pvt++;
 	RING_PUSH_RESPONSES_AND_CHECK_NOTIFY(&blk_rings->common, notify);
-	if (blk_rings->common.rsp_prod_pvt == blk_rings->common.req_cons) {
-		/*
-		 * Tail check for pending requests. Allows frontend to avoid
-		 * notifications if requests are already in flight (lower
-		 * overheads and promotes batching).
-		 */
-		RING_FINAL_CHECK_FOR_REQUESTS(&blk_rings->common, more_to_do);
-
-	} else if (RING_HAS_UNCONSUMED_REQUESTS(&blk_rings->common)) {
-		more_to_do = 1;
-	}
-
 	spin_unlock_irqrestore(&blkif->blk_ring_lock, flags);
-
-	if (more_to_do)
-		blkif_notify_work(blkif);
 	if (notify)
 		notify_remote_via_irq(blkif->irq);
 }