[PATCH] allow the IO scheduler to pass an allocation hint to

From: Nick Piggin <piggin@cyberone.com.au>


This patch implements a hint so that AS can tell the request allocator to
allocate a request even if there are none left (the accounting is quite
flexible and easily handles overallocations).

elv_may_queue semantics have changed from "the elevator does _not_ want
another request allocated" to "the elevator _insists_ that another request is
allocated".  I couldn't see any harm ;)

Now in practice, AS will only allow _1_ request over the limit, because as
soon as the request is sent to AS, it stops anticipating.

drivers/block/as-iosched.c

@@ -1641,6 +1641,20 @@ static int as_set_request(request_queue_t *q, struct request *rq, int gfp_mask)
 	return 1;
 }
 
+static int as_may_queue(request_queue_t *q, int rw)
+{
+	struct as_data *ad = q->elevator.elevator_data;
+	struct as_io_context *aic;
+	if (ad->antic_status == ANTIC_WAIT_REQ ||
+			ad->antic_status == ANTIC_WAIT_NEXT) {
+		aic = get_as_io_context();
+		if (ad->as_io_context == aic)
+			return 1;
+	}
+
+	return 0;
+}
+
 static void as_exit(request_queue_t *q, elevator_t *e)
 {
 	struct as_data *ad = e->elevator_data;
@@ -1879,6 +1893,7 @@ elevator_t iosched_as = {
 	.elevator_latter_req_fn =	as_latter_request,
 	.elevator_set_req_fn =		as_set_request,
 	.elevator_put_req_fn =		as_put_request,
+	.elevator_may_queue_fn =	as_may_queue,
 	.elevator_init_fn =		as_init,
 	.elevator_exit_fn =		as_exit,
 

drivers/block/elevator.c

@@ -368,7 +368,7 @@ int elv_may_queue(request_queue_t *q, int rw)
 	if (e->elevator_may_queue_fn)
 		return e->elevator_may_queue_fn(q, rw);
 
-	return 1;
+	return 0;
 }
 
 void elv_completed_request(request_queue_t *q, struct request *rq)

drivers/block/ll_rw_blk.c

@@ -1315,7 +1315,7 @@ static struct request *get_request(request_queue_t *q, int rw, int gfp_mask)
 	struct request_list *rl = &q->rq;
 
 	spin_lock_irq(q->queue_lock);
-	if (rl->count[rw] >= q->nr_requests || !elv_may_queue(q, rw)) {
+	if (rl->count[rw] >= q->nr_requests && !elv_may_queue(q, rw)) {
 		spin_unlock_irq(q->queue_lock);
 		goto out;
 	}