[patch 7/9] blk: efficiency improvements

Nick Piggin
2005-04-12T13:02:29Z

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7/9

--
SUSE Labs, Novell Inc.

[-- Attachment #2: blk-efficient.patch --]
[-- Type: text/plain, Size: 3583 bytes --]

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>

Index: linux-2.6/drivers/block/ll_rw_blk.c
===================================================================
--- linux-2.6.orig/drivers/block/ll_rw_blk.c 2005-04-12 22:26:14.000000000 +1000
+++ linux-2.6/drivers/block/ll_rw_blk.c 2005-04-12 22:26:14.000000000 +1000
@@ -1952,10 +1952,11 @@ out:
*/
static struct request *get_request_wait(request_queue_t *q, int rw)
{
- DEFINE_WAIT(wait);
struct request *rq;

- do {
+ rq = get_request(q, rw, GFP_NOIO);
+ while (!rq) {
+ DEFINE_WAIT(wait);
struct request_list *rl = &q->rq;

prepare_to_wait_exclusive(&rl->wait[rw], &wait,
@@ -1980,7 +1981,7 @@ static struct request *get_request_wait(
put_io_context(ioc);
}
finish_wait(&rl->wait[rw], &wait);
- } while (!rq);
+ }

return rq;
}
@@ -2557,7 +2558,7 @@ 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;
sector_t sector;

@@ -2582,14 +2583,9 @@ static int __make_request(request_queue_
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);
@@ -2632,40 +2628,23 @@ 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);
+ /*
+ * 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, GFP_ATOMIC)) == NULL) {
- /*
- * READA bit set
- */
- err = -EWOULDBLOCK;
- if (bio_rw_ahead(bio))
- goto end_io;
-
- freereq = get_request_wait(q, rw);
- }
- goto again;
- }

req->flags |= REQ_CMD;

@@ -2693,10 +2672,11 @@ 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 (bio_sync(bio))
__generic_unplug_device(q);