xen/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.
Signed-off-by: Daniel Stodden <daniel.stodden@citrix.com>
[v1: Rebased and fixed compile problems]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
index 5cf2993..f0537f3 100644
--- a/drivers/block/xen-blkback/blkback.c
+++ b/drivers/block/xen-blkback/blkback.c
@@ -458,7 +458,8 @@
* (which has the sectors we want, number of them, grant references, etc),
* and transmute it to the block API to hand it over to the proper block disk.
*/
-static int do_block_io_op(struct xen_blkif *blkif)
+static int
+__do_block_io_op(struct xen_blkif *blkif)
{
union blkif_back_rings *blk_rings = &blkif->blk_rings;
struct blkif_request req;
@@ -515,6 +516,23 @@
return more_to_do;
}
+static int
+do_block_io_op(struct xen_blkif *blkif)
+{
+ union blkif_back_rings *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;
+}
+
/*
* Transmutation of the 'struct blkif_request' to a proper 'struct bio'
* and call the 'submit_bio' to pass it to the underlying storage.
@@ -700,7 +718,6 @@
struct blkif_response resp;
unsigned long flags;
union blkif_back_rings *blk_rings = &blkif->blk_rings;
- int more_to_do = 0;
int notify;
resp.id = id;
@@ -727,22 +744,7 @@
}
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);
}
