cfq-iosched: Allow RT requests to pre-empt ongoing BE timeslice

This patch adds the ability to pre-empt an ongoing BE timeslice when a RT
request is waiting for the current timeslice to complete. This reduces the
wait time to disk for RT requests from an upper bound of 4 (current value
of cfq_quantum) to 1 disk request.

Applied Jens' suggeested changes to avoid the rb lookup and use !cfq_class_rt()
and retested.

Latency(secs) for the RT task when doing sequential reads from 10G file.
                       | only RT | RT + BE | RT + BE + this patch
small (512 byte) reads | 143     | 163     | 145
large (1Mb) reads      | 142     | 158     | 146

Signed-off-by: Divyesh Shah <dpshah@google.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

1 files changed, 38 insertions, 1 deletions

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index e8525fa72823..664ebfd092ec 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -84,6 +84,11 @@ struct cfq_data {
          */
         struct cfq_rb_root service_tree;
         unsigned int busy_queues;
+        /*
+         * Used to track any pending rt requests so we can pre-empt current
+         * non-RT cfqq in service when this value is non-zero.
+         */
+        unsigned int busy_rt_queues;
 
         int rq_in_driver;
         int sync_flight;
@@ -562,6 +567,8 @@ static void cfq_add_cfqq_rr(struct cfq_data *cfqd, struct cfq_queue *cfqq)
         BUG_ON(cfq_cfqq_on_rr(cfqq));
         cfq_mark_cfqq_on_rr(cfqq);
         cfqd->busy_queues++;
+        if (cfq_class_rt(cfqq))
+                cfqd->busy_rt_queues++;
 
         cfq_resort_rr_list(cfqd, cfqq);
 }
@@ -581,6 +588,8 @@ static void cfq_del_cfqq_rr(struct cfq_data *cfqd, struct cfq_queue *cfqq)
 
         BUG_ON(!cfqd->busy_queues);
         cfqd->busy_queues--;
+        if (cfq_class_rt(cfqq))
+                cfqd->busy_rt_queues--;
 }
 
 /*
@@ -1005,6 +1014,20 @@ static struct cfq_queue *cfq_select_queue(struct cfq_data *cfqd)
                 goto expire;
 
         /*
+         * If we have a RT cfqq waiting, then we pre-empt the current non-rt
+         * cfqq.
+         */
+        if (!cfq_class_rt(cfqq) && cfqd->busy_rt_queues) {
+                /*
+                 * We simulate this as cfqq timed out so that it gets to bank
+                 * the remaining of its time slice.
+                 */
+                cfq_log_cfqq(cfqd, cfqq, "preempt");
+                cfq_slice_expired(cfqd, 1);
+                goto new_queue;
+        }
+
+        /*
          * The active queue has requests and isn't expired, allow it to
          * dispatch.
          */
@@ -1067,6 +1090,13 @@ __cfq_dispatch_requests(struct cfq_data *cfqd, struct cfq_queue *cfqq,
                 if (RB_EMPTY_ROOT(&cfqq->sort_list))
                         break;
 
+                /*
+                 * If there is a non-empty RT cfqq waiting for current
+                 * cfqq's timeslice to complete, pre-empt this cfqq
+                 */
+                if (!cfq_class_rt(cfqq) && cfqd->busy_rt_queues)
+                        break;
+
         } while (dispatched < max_dispatch);
 
         /*
@@ -1801,6 +1831,12 @@ cfq_should_preempt(struct cfq_data *cfqd, struct cfq_queue *new_cfqq,
         if (rq_is_meta(rq) && !cfqq->meta_pending)
                 return 1;
 
+        /*
+         * Allow an RT request to pre-empt an ongoing non-RT cfqq timeslice.
+         */
+        if (cfq_class_rt(new_cfqq) && !cfq_class_rt(cfqq))
+                return 1;
+
         if (!cfqd->active_cic || !cfq_cfqq_wait_request(cfqq))
                 return 0;
 
@@ -1870,7 +1906,8 @@ cfq_rq_enqueued(struct cfq_data *cfqd, struct cfq_queue *cfqq,
                 /*
                  * not the active queue - expire current slice if it is
                  * idle and has expired it's mean thinktime or this new queue
-                 * has some old slice time left and is of higher priority
+                 * has some old slice time left and is of higher priority or
+                 * this new queue is RT and the current one is BE
                  */
                 cfq_preempt_queue(cfqd, cfqq);
                 cfq_mark_cfqq_must_dispatch(cfqq);