cfq-iosched: schedule dispatch for noidle queue

A queue is idle at cfq_dispatch_requests(), but it gets noidle later. Unless
other task explictly does unplug or all requests are drained, we will not
deliever requests to the disk even cfq_arm_slice_timer doesn't make the
queue idle. For example, cfq_should_idle() returns true because of
service_tree->count == 1, and then other queues are added. Note, I didn't
see obvious performance impacts so far with the patch, but just thought
this could be a problem.

Signed-off-by: Shaohua Li <shaohua.li@intel.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index b8174bb4a6a16702bb88cdbd313947aca55fe2a2..986865e3fbc55b7d499669e82573cd16ed8f2ae3 100644 (file)
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -3255,6 +3255,10 @@ cfq_should_preempt(struct cfq_data *cfqd, struct cfq_queue *new_cfqq,
        if (cfq_class_rt(new_cfqq) && !cfq_class_rt(cfqq))
                return true;
 
+       /* An idle queue should not be idle now for some reason */
+       if (RB_EMPTY_ROOT(&cfqq->sort_list) && !cfq_should_idle(cfqd, cfqq))
+               return true;
+
        if (!cfqd->active_cic || !cfq_cfqq_wait_request(cfqq))
                return false;
 
@@ -3508,8 +3512,25 @@ static void cfq_completed_request(struct request_queue *q, struct request *rq)
                }
        }
 
-       if (!cfqd->rq_in_driver)
+       if (!cfqd->rq_in_driver) {
+               cfq_schedule_dispatch(cfqd);
+               return;
+       }
+       /*
+        * A queue is idle at cfq_dispatch_requests(), but it gets noidle
+        * later. We schedule a dispatch if the queue has no requests,
+        * otherwise the disk is actually in idle till all requests
+        * are finished even cfq_arm_slice_timer doesn't make the queue idle
+        * */
+       cfqq = cfqd->active_queue;
+       if (!cfqq)
+               return;
+
+       if (RB_EMPTY_ROOT(&cfqq->sort_list) && !cfq_should_idle(cfqd, cfqq) &&
+           (!cfqd->cfq_group_idle || cfqq->cfqg->nr_cfqq > 1)) {
+               cfq_del_timer(cfqd, cfqq);
                cfq_schedule_dispatch(cfqd);
+       }
 }
 
 /*