In blkback driver, after I/O requests are submitted to Dom-0 block I/O subsystem, blkback goes to 'sleep' effectively without letting blkfront know about it (req_event isn't set appropriately). Hence blkfront doesn't notify blkback when it submits a new I/O thus delaying the 'dispatch' of the new I/O to Dom-0 block I/O subsystem. The new I/O is dispatched as soon as one of the previous I/Os completes.
As a result of this issue, the block I/O latency performance is degraded for some workloads on Xen guests using blkfront-blkback stack.
Signed-off-by: Pradeep Vincent <pradeepv@xxxxxxxxxx>
diff --git a/drivers/xen/blkback/blkback.c b/drivers/xen/blkback/blkback.c
@@ -383,6 +383,12 @@ static int do_block_io_op(blkif_t *blkif)
+ /* If blkback might go to sleep (i.e. more_to_do == 0) then we better
+ let blkfront know about it (by setting req_event appropriately) so that
+ blkfront will bother to wake us up (via interrupt) when it submits a
+ new I/O */
+ if (!more_to_do)
+ RING_FINAL_CHECK_FOR_REQUESTS(&blk_rings->common, more_to_do);