cfq-iosched: don't delay queue kick for a merged request

"Zhang, Yanmin" <yanmin_zhang@linux.intel.com> reports that commit
b029195d introduced a regression
of about 50% with sequential threaded read workloads. The test
case is:

tiotest -k0 -k1 -k3 -f 80 -t 32

which starts 32 threads each reading a 80MB file. Twiddle the kick
queue logic so that we do start IO immediately, if it appears to be
a fully merged request. We can't really detect that, so just check
if the request is bigger than a page or not. The assumption is that
since single bio issues will first queue a single request with just
one page attached and then later do merges on that, if we already
have more than a page worth of data in the request, then the request
is most likely good to go.

Verified that this doesn't cause a regression with the test case that
commit b029195d was fixing. It does not,
we still see maximum sized requests for the queue-then-merge cases.
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

block/cfq-iosched.c

@@ -1903,10 +1903,17 @@ cfq_rq_enqueued(struct cfq_data *cfqd, struct cfq_queue *cfqq,
 		 * Remember that we saw a request from this process, but
 		 * don't start queuing just yet. Otherwise we risk seeing lots
 		 * of tiny requests, because we disrupt the normal plugging
-		 * and merging.
+		 * and merging. If the request is already larger than a single
+		 * page, let it rip immediately. For that case we assume that
+		 * merging is already done.
 		 */
-		if (cfq_cfqq_wait_request(cfqq))
+		if (cfq_cfqq_wait_request(cfqq)) {
+			if (blk_rq_bytes(rq) > PAGE_CACHE_SIZE) {
+				del_timer(&cfqd->idle_slice_timer);
+				blk_start_queueing(cfqd->queue);
+			}
 			cfq_mark_cfqq_must_dispatch(cfqq);
+		}
 	} else if (cfq_should_preempt(cfqd, cfqq, rq)) {
 		/*
 		 * not the active queue - expire current slice if it is