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.

diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
index 8583b13..eb068d0 100644 (file)
--- 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)