Revert "xen/blkback: Move the plugging/unplugging to a higher level."

This reverts commit 97961ef46b9b5a6a7c918a38b898a7b3e49869f4 b/c
we lose about 15% performance if we do the unplugging and the
end of the reading the ring buffer.

1 files changed, 7 insertions, 6 deletions

diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
index 8583b130499a..eb068d0b47ea 100644
--- a/drivers/block/xen-blkback/blkback.c
+++ b/drivers/block/xen-blkback/blkback.c
@@ -276,8 +276,6 @@ int xen_blkif_schedule(void *arg)
                 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)))
@@ -294,13 +292,9 @@ int xen_blkif_schedule(void *arg)
                 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);
         }
@@ -553,6 +547,7 @@ static void dispatch_rw_block_io(struct blkif_st *blkif,
         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:
@@ -665,9 +660,15 @@ static void dispatch_rw_block_io(struct blkif_st *blkif,
          */
         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)