xen/blkback: Move the plugging/unplugging to a higher level.
We used to the plug/unplug on the submit_bio. But that means
if within a stream of WRITE, WRITE, WRITE,...,WRITE we have
one READ, it could stall the pipeline (as the 'submio_bio'
could trigger the unplug_fnc to be called and stall/sync
when doing the READ). Instead we want to move the unplugging
when the whole (or as a much as possible) ring buffer has been
processed. This also eliminates us doing plug/unplug for
each request.
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 c4bc85e..ed85ba94 100644
--- a/drivers/block/xen-blkback/blkback.c
+++ b/drivers/block/xen-blkback/blkback.c
@@ -276,6 +276,8 @@
printk(KERN_DEBUG "%s: started\n", current->comm);
while (!kthread_should_stop()) {
+ struct blk_plug plug;
+
if (try_to_freeze())
continue;
if (unlikely(vbd->size != vbd_sz(vbd)))
@@ -292,9 +294,13 @@
blkif->waiting_reqs = 0;
smp_mb(); /* clear flag *before* checking for work */
+ blk_start_plug(&plug);
+
if (do_block_io_op(blkif))
blkif->waiting_reqs = 1;
+ blk_finish_plug(&plug);
+
if (log_stats && time_after(jiffies, blkif->st_print))
print_stats(blkif);
}
@@ -547,7 +553,6 @@
struct bio *biolist[BLKIF_MAX_SEGMENTS_PER_REQUEST];
int i, nbio = 0;
int operation;
- struct blk_plug plug;
switch (req->operation) {
case BLKIF_OP_READ:
@@ -660,15 +665,9 @@
*/
atomic_set(&pending_req->pendcnt, nbio);
- /* Get a reference count for the disk queue and start sending I/O */
- blk_start_plug(&plug);
-
for (i = 0; i < nbio; i++)
submit_bio(operation, biolist[i]);
- blk_finish_plug(&plug);
- /* Let the I/Os go.. */
-
if (operation == READ)
blkif->st_rd_sect += preq.nr_sects;
else if (operation == WRITE || operation == WRITE_BARRIER)