[PATCH] blk: __make_request efficiency

In the case where the request is not able to be merged by the elevator, don't
retake the lock and retry the merge mechanism after allocating a new request.

Instead assume that the chance of a merge remains slim, and now that we've
done most of the work allocating a request we may as well just go with it.

Also be rid of the GFP_ATOMIC allocation: we've got working mempools for the
block layer now, so let's save atomic memory for things like networking.

Lastly, in get_request_wait, do an initial get_request call before going into
the waitqueue.  This is reported to help efficiency.

Signed-off-by: Nick Piggin <nickpiggin@yahoo.com.au>
Cc: Jens Axboe <axboe@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 21 insertions, 41 deletions

diff --git a/drivers/block/ll_rw_blk.c b/drivers/block/ll_rw_blk.c
index 6c98cf042714..67431f280154 100644
--- a/drivers/block/ll_rw_blk.c
+++ b/drivers/block/ll_rw_blk.c
@@ -1971,10 +1971,11 @@ out:
 static struct request *get_request_wait(request_queue_t *q, int rw,
                                         struct bio *bio)
 {
-        DEFINE_WAIT(wait);
         struct request *rq;
 
-        do {
+        rq = get_request(q, rw, bio, GFP_NOIO);
+        while (!rq) {
+                DEFINE_WAIT(wait);
                 struct request_list *rl = &q->rq;
 
                 prepare_to_wait_exclusive(&rl->wait[rw], &wait,
@@ -1999,7 +2000,7 @@ static struct request *get_request_wait(request_queue_t *q, int rw,
                         put_io_context(ioc);
                 }
                 finish_wait(&rl->wait[rw], &wait);
-        } while (!rq);
+        }
 
         return rq;
 }
@@ -2521,7 +2522,7 @@ EXPORT_SYMBOL(blk_attempt_remerge);
 
 static int __make_request(request_queue_t *q, struct bio *bio)
 {
-        struct request *req, *freereq = NULL;
+        struct request *req;
         int el_ret, rw, nr_sectors, cur_nr_sectors, barrier, err, sync;
         unsigned short prio;
         sector_t sector;
@@ -2549,14 +2550,9 @@ static int __make_request(request_queue_t *q, struct bio *bio)
                 goto end_io;
         }
 
-again:
         spin_lock_irq(q->queue_lock);
 
-        if (elv_queue_empty(q)) {
-                blk_plug_device(q);
-                goto get_rq;
-        }
-        if (barrier)
+        if (unlikely(barrier) || elv_queue_empty(q))
                 goto get_rq;
 
         el_ret = elv_merge(q, &req, bio);
@@ -2601,40 +2597,23 @@ again:
                                 elv_merged_request(q, req);
                         goto out;
 
-                /*
-                 * elevator says don't/can't merge. get new request
-                 */
-                case ELEVATOR_NO_MERGE:
-                        break;
-
+                /* ELV_NO_MERGE: elevator says don't/can't merge. */
                 default:
-                        printk("elevator returned crap (%d)\n", el_ret);
-                        BUG();
+                        ;
         }
 
+get_rq:
         /*
-         * Grab a free request from the freelist - if that is empty, check
-         * if we are doing read ahead and abort instead of blocking for
-         * a free slot.
+         * Grab a free request. This is might sleep but can not fail.
+         */
+        spin_unlock_irq(q->queue_lock);
+        req = get_request_wait(q, rw, bio);
+        /*
+         * After dropping the lock and possibly sleeping here, our request
+         * may now be mergeable after it had proven unmergeable (above).
+         * We don't worry about that case for efficiency. It won't happen
+         * often, and the elevators are able to handle it.
          */
-get_rq:
-        if (freereq) {
-                req = freereq;
-                freereq = NULL;
-        } else {
-                spin_unlock_irq(q->queue_lock);
-                if ((freereq = get_request(q, rw, bio, GFP_ATOMIC)) == NULL) {
-                        /*
-                         * READA bit set
-                         */
-                        err = -EWOULDBLOCK;
-                        if (bio_rw_ahead(bio))
-                                goto end_io;
-        
-                        freereq = get_request_wait(q, rw, bio);
-                }
-                goto again;
-        }
 
         req->flags |= REQ_CMD;
 
@@ -2663,10 +2642,11 @@ get_rq:
         req->rq_disk = bio->bi_bdev->bd_disk;
         req->start_time = jiffies;
 
+        spin_lock_irq(q->queue_lock);
+        if (elv_queue_empty(q))
+                blk_plug_device(q);
         add_request(q, req);
 out:
-        if (freereq)
-                __blk_put_request(q, freereq);
         if (sync)
                 __generic_unplug_device(q);