cfq-iosched: development update

- Implement logic for detecting cooperating processes, so we
  choose the best available queue whenever possible.

- Improve residual slice time accounting.

- Remove dead code: we no longer see async requests coming in on
  sync queues. That part was removed a long time ago. That means
  that we can also remove the difference between cfq_cfqq_sync()
  and cfq_cfqq_class_sync(), they are now indentical. And we can
  kill the on_dispatch array, just make it a counter.

- Allow a process to go into the current list, if it hasn't been
  serviced in this scheduler tick yet.

Possible future improvements including caching the cfqq lookup
in cfq_close_cooperator(), so we don't have to look it up twice.
cfq_get_best_queue() should just use that last decision instead
of doing it again.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

1 files changed, 261 insertions, 120 deletions

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index bfb396774cbb..28236f2cd908 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -56,13 +56,7 @@ static struct completion *ioc_gone;
 #define ASYNC                   (0)
 #define SYNC                    (1)
 
-#define cfq_cfqq_dispatched(cfqq)       \
-        ((cfqq)->on_dispatch[ASYNC] + (cfqq)->on_dispatch[SYNC])
-
-#define cfq_cfqq_class_sync(cfqq)       ((cfqq)->key != CFQ_KEY_ASYNC)
-
-#define cfq_cfqq_sync(cfqq)             \
-        (cfq_cfqq_class_sync(cfqq) || (cfqq)->on_dispatch[SYNC])
+#define cfq_cfqq_sync(cfqq)     ((cfqq)->key != CFQ_KEY_ASYNC)
 
 #define sample_valid(samples)   ((samples) > 80)
 
@@ -79,6 +73,7 @@ struct cfq_data {
         struct list_head busy_rr;
         struct list_head cur_rr;
         struct list_head idle_rr;
+        unsigned long cur_rr_tick;
         unsigned int busy_queues;
 
         /*
@@ -98,11 +93,12 @@ struct cfq_data {
         struct cfq_queue *active_queue;
         struct cfq_io_context *active_cic;
         int cur_prio, cur_end_prio;
+        unsigned long prio_time;
         unsigned int dispatch_slice;
 
         struct timer_list idle_class_timer;
 
-        sector_t last_sector;
+        sector_t last_position;
         unsigned long last_end_request;
 
         /*
@@ -117,6 +113,9 @@ struct cfq_data {
         unsigned int cfq_slice_idle;
 
         struct list_head cic_list;
+
+        sector_t new_seek_mean;
+        u64 new_seek_total;
 };
 
 /*
@@ -133,6 +132,8 @@ struct cfq_queue {
         unsigned int key;
         /* member of the rr/busy/cur/idle cfqd list */
         struct list_head cfq_list;
+        /* in what tick we were last serviced */
+        unsigned long rr_tick;
         /* sorted list of pending requests */
         struct rb_root sort_list;
         /* if fifo isn't expired, next request to serve */
@@ -148,10 +149,11 @@ struct cfq_queue {
 
         unsigned long slice_end;
         unsigned long service_last;
+        unsigned long slice_start;
         long slice_resid;
 
-        /* number of requests that are on the dispatch list */
-        int on_dispatch[2];
+        /* number of requests that are on the dispatch list or inside driver */
+        int dispatched;
 
         /* io prio of this group */
         unsigned short ioprio, org_ioprio;
@@ -159,6 +161,8 @@ struct cfq_queue {
 
         /* various state flags, see below */
         unsigned int flags;
+
+        sector_t last_request_pos;
 };
 
 enum cfqq_state_flags {
@@ -259,6 +263,8 @@ cfq_set_prio_slice(struct cfq_data *cfqd, struct cfq_queue *cfqq)
          * easily introduce oscillations.
          */
         cfqq->slice_resid = 0;
+
+        cfqq->slice_start = jiffies;
 }
 
 /*
@@ -307,7 +313,7 @@ cfq_choose_req(struct cfq_data *cfqd, struct request *rq1, struct request *rq2)
         s1 = rq1->sector;
         s2 = rq2->sector;
 
-        last = cfqd->last_sector;
+        last = cfqd->last_position;
 
         /*
          * by definition, 1KiB is 2 sectors
@@ -398,39 +404,42 @@ cfq_find_next_rq(struct cfq_data *cfqd, struct cfq_queue *cfqq,
         return cfq_choose_req(cfqd, next, prev);
 }
 
-static void cfq_resort_rr_list(struct cfq_queue *cfqq, int preempted)
+/*
+ * This function finds out where to insert a BE queue in the service hierarchy
+ */
+static void cfq_resort_be_queue(struct cfq_data *cfqd, struct cfq_queue *cfqq,
+                                int preempted)
 {
-        struct cfq_data *cfqd = cfqq->cfqd;
         struct list_head *list, *n;
         struct cfq_queue *__cfqq;
+        int add_tail = 0;
 
         /*
-         * Resorting requires the cfqq to be on the RR list already.
+         * if cfqq has requests in flight, don't allow it to be
+         * found in cfq_set_active_queue before it has finished them.
+         * this is done to increase fairness between a process that
+         * has lots of io pending vs one that only generates one
+         * sporadically or synchronously
          */
-        if (!cfq_cfqq_on_rr(cfqq))
-                return;
-
-        list_del(&cfqq->cfq_list);
-
-        if (cfq_class_rt(cfqq))
+        if (cfqq->dispatched)
+                list = &cfqd->busy_rr;
+        else if (cfqq->ioprio == (cfqd->cur_prio + 1) &&
+                 cfq_cfqq_sync(cfqq) &&
+                 (time_before(cfqd->prio_time, cfqq->service_last) ||
+                  cfq_cfqq_queue_new(cfqq) || preempted)) {
                 list = &cfqd->cur_rr;
-        else if (cfq_class_idle(cfqq))
-                list = &cfqd->idle_rr;
-        else {
+                add_tail = 1;
+        } else
+                list = &cfqd->rr_list[cfqq->ioprio];
+
+        if (!cfq_cfqq_sync(cfqq) || add_tail) {
                 /*
-                 * if cfqq has requests in flight, don't allow it to be
-                 * found in cfq_set_active_queue before it has finished them.
-                 * this is done to increase fairness between a process that
-                 * has lots of io pending vs one that only generates one
-                 * sporadically or synchronously
+                 * async queue always goes to the end. this wont be overly
+                 * unfair to writes, as the sort of the sync queue wont be
+                 * allowed to pass the async queue again.
                  */
-                if (cfq_cfqq_dispatched(cfqq))
-                        list = &cfqd->busy_rr;
-                else
-                        list = &cfqd->rr_list[cfqq->ioprio];
-        }
-
-        if (preempted || cfq_cfqq_queue_new(cfqq)) {
+                list_add_tail(&cfqq->cfq_list, list);
+        } else if (preempted || cfq_cfqq_queue_new(cfqq)) {
                 /*
                  * If this queue was preempted or is new (never been serviced),
                  * let it be added first for fairness but beind other new
@@ -444,14 +453,7 @@ static void cfq_resort_rr_list(struct cfq_queue *cfqq, int preempted)
 
                         n = n->next;
                 }
-                list_add_tail(&cfqq->cfq_list, n);
-        } else if (!cfq_cfqq_class_sync(cfqq)) {
-                /*
-                 * async queue always goes to the end. this wont be overly
-                 * unfair to writes, as the sort of the sync queue wont be
-                 * allowed to pass the async queue again.
-                 */
-                list_add_tail(&cfqq->cfq_list, list);
+                list_add(&cfqq->cfq_list, n);
         } else {
                 /*
                  * sort by last service, but don't cross a new or async
@@ -461,17 +463,54 @@ static void cfq_resort_rr_list(struct cfq_queue *cfqq, int preempted)
                  */
                 n = list;
                 while ((n = n->prev) != list) {
-                        struct cfq_queue *__cfqq = list_entry_cfqq(n);
+                        struct cfq_queue *__c = list_entry_cfqq(n);
 
-                        if (!cfq_cfqq_class_sync(cfqq) || !__cfqq->service_last)
+                        if (!cfq_cfqq_sync(__c) || !__c->service_last)
                                 break;
-                        if (time_before(__cfqq->service_last, cfqq->service_last))
+                        if (time_before(__c->service_last, cfqq->service_last))
                                 break;
                 }
                 list_add(&cfqq->cfq_list, n);
         }
 }
 
+static void cfq_resort_rr_list(struct cfq_queue *cfqq, int preempted)
+{
+        struct cfq_data *cfqd = cfqq->cfqd;
+        struct list_head *n;
+
+        /*
+         * Resorting requires the cfqq to be on the RR list already.
+         */
+        if (!cfq_cfqq_on_rr(cfqq))
+                return;
+
+        list_del(&cfqq->cfq_list);
+
+        if (cfq_class_rt(cfqq)) {
+                /*
+                 * At to the front of the current list, but behind other
+                 * RT queues.
+                 */
+                n = &cfqd->cur_rr;
+                while (n->next != &cfqd->cur_rr)
+                        if (!cfq_class_rt(cfqq))
+                                break;
+
+                list_add(&cfqq->cfq_list, n);
+        } else if (cfq_class_idle(cfqq)) {
+                /*
+                 * IDLE goes to the tail of the idle list
+                 */
+                list_add_tail(&cfqq->cfq_list, &cfqd->idle_rr);
+        } else {
+                /*
+                 * So we get here, ergo the queue is a regular best-effort queue
+                 */
+                cfq_resort_be_queue(cfqd, cfqq, preempted);
+        }
+}
+
 /*
  * add to busy list of queues for service, trying to be fair in ordering
  * the pending list according to last request service
@@ -579,6 +618,8 @@ static void cfq_activate_request(request_queue_t *q, struct request *rq)
          */
         if (!cfqd->hw_tag && cfqd->rq_in_driver > 4)
                 cfqd->hw_tag = 1;
+
+        cfqd->last_position = rq->hard_sector + rq->hard_nr_sectors;
 }
 
 static void cfq_deactivate_request(request_queue_t *q, struct request *rq)
@@ -684,6 +725,7 @@ __cfq_set_active_queue(struct cfq_data *cfqd, struct cfq_queue *cfqq)
                 cfq_clear_cfqq_must_alloc_slice(cfqq);
                 cfq_clear_cfqq_fifo_expire(cfqq);
                 cfq_mark_cfqq_slice_new(cfqq);
+                cfqq->rr_tick = cfqd->cur_rr_tick;
         }
 
         cfqd->active_queue = cfqq;
@@ -786,10 +828,46 @@ static int cfq_get_next_prio_level(struct cfq_data *cfqd)
                 cfqd->cur_end_prio = 0;
         }
 
+        cfqd->cur_rr_tick++;
+        cfqd->prio_time = jiffies;
         return prio;
 }
 
-static struct cfq_queue *cfq_set_active_queue(struct cfq_data *cfqd)
+static inline sector_t cfq_dist_from_last(struct cfq_data *cfqd,
+                                          struct request *rq)
+{
+        if (rq->sector >= cfqd->last_position)
+                return rq->sector - cfqd->last_position;
+        else
+                return cfqd->last_position - rq->sector;
+}
+
+static struct cfq_queue *cfq_get_best_queue(struct cfq_data *cfqd)
+{
+        struct cfq_queue *cfqq = NULL, *__cfqq;
+        sector_t best = -1, dist;
+
+        list_for_each_entry(__cfqq, &cfqd->cur_rr, cfq_list) {
+                if (!__cfqq->next_rq || !cfq_cfqq_sync(__cfqq))
+                        continue;
+
+                dist = cfq_dist_from_last(cfqd, __cfqq->next_rq);
+                if (dist < best) {
+                        best = dist;
+                        cfqq = __cfqq;
+                }
+        }
+
+        /*
+         * Only async queue(s) available, grab first entry
+         */
+        if (!cfqq)
+                cfqq = list_entry_cfqq(cfqd->cur_rr.next);
+
+        return cfqq;
+}
+
+static struct cfq_queue *cfq_get_next_queue(struct cfq_data *cfqd)
 {
         struct cfq_queue *cfqq = NULL;
 
@@ -799,7 +877,7 @@ static struct cfq_queue *cfq_set_active_queue(struct cfq_data *cfqd)
                  * empty, get next prio level and grab first entry then if any
                  * are spliced
                  */
-                cfqq = list_entry_cfqq(cfqd->cur_rr.next);
+                cfqq = cfq_get_best_queue(cfqd);
         } else if (!list_empty(&cfqd->busy_rr)) {
                 /*
                  * If no new queues are available, check if the busy list has
@@ -820,49 +898,128 @@ static struct cfq_queue *cfq_set_active_queue(struct cfq_data *cfqd)
                         mod_timer(&cfqd->idle_class_timer, end);
         }
 
+        return cfqq;
+}
+
+static struct cfq_queue *cfq_set_active_queue(struct cfq_data *cfqd)
+{
+        struct cfq_queue *cfqq;
+
+        do {
+                long prio;
+
+                cfqq = cfq_get_next_queue(cfqd);
+                if (!cfqq)
+                        break;
+
+                prio = cfq_prio_to_slice(cfqd, cfqq);
+                if (cfqq->slice_resid > -prio)
+                        break;
+
+                cfqq->slice_resid += prio;
+                list_del_init(&cfqq->cfq_list);
+                list_add_tail(&cfqq->cfq_list, &cfqd->rr_list[cfqq->ioprio]);
+                cfqq = NULL;
+        } while (1);
+
         __cfq_set_active_queue(cfqd, cfqq);
         return cfqq;
 }
 
-#define CIC_SEEKY(cic) ((cic)->seek_mean > (128 * 1024))
+static inline int cfq_rq_close(struct cfq_data *cfqd, struct request *rq)
+{
+        struct cfq_io_context *cic = cfqd->active_cic;
+
+        if (!sample_valid(cic->seek_samples))
+                return 0;
+
+        return cfq_dist_from_last(cfqd, rq) <= cic->seek_mean;
+}
+
+static struct cfq_queue *__cfq_close_cooperator(struct cfq_data *cfqd,
+                                                struct cfq_queue *cur_cfqq,
+                                                struct list_head *list)
+{
+        struct cfq_queue *cfqq;
+
+        list_for_each_entry(cfqq, list, cfq_list) {
+                if (cfqq == cur_cfqq || !cfq_cfqq_sync(cfqq))
+                        continue;
+
+                BUG_ON(!cfqq->next_rq);
+
+                if (cfq_rq_close(cfqd, cfqq->next_rq))
+                        return cfqq;
+        }
+
+        return NULL;
+}
+
+static int cfq_close_cooperator(struct cfq_data *cfqd,
+                                struct cfq_queue *cur_cfqq)
+{
+        struct cfq_queue *cfqq;
+
+        if (!cfqd->busy_queues)
+                return 0;
+
+        /*
+         * check cur_rr and same-prio rr_list for candidates
+         */
+        cfqq = __cfq_close_cooperator(cfqd, cur_cfqq, &cfqd->cur_rr);
+        if (cfqq)
+                return 1;
+
+        cfqq = __cfq_close_cooperator(cfqd, cur_cfqq, &cfqd->rr_list[cur_cfqq->ioprio]);
+        if (cfqq && (cfqq->rr_tick == cfqd->cur_rr_tick))
+                cfqq = NULL;
+
+        return cfqq != NULL;
+}
+
+#define CIC_SEEKY(cic) ((cic)->seek_mean > (8 * 1024))
 
-static int cfq_arm_slice_timer(struct cfq_data *cfqd)
+static void cfq_arm_slice_timer(struct cfq_data *cfqd)
 {
         struct cfq_queue *cfqq = cfqd->active_queue;
         struct cfq_io_context *cic;
         unsigned long sl;
 
         WARN_ON(!RB_EMPTY_ROOT(&cfqq->sort_list));
+        WARN_ON(cfq_cfqq_slice_new(cfqq));
 
         /*
          * idle is disabled, either manually or by past process history
          */
-        if (!cfqd->cfq_slice_idle)
-                return 0;
-        if (!cfq_cfqq_idle_window(cfqq))
-                return 0;
+        if (!cfqd->cfq_slice_idle || !cfq_cfqq_idle_window(cfqq))
+                return;
+
         /*
          * task has exited, don't wait
          */
         cic = cfqd->active_cic;
         if (!cic || !cic->ioc->task)
-                return 0;
+                return;
+
+        /*
+         * See if this prio level has a good candidate
+         */
+        if (cfq_close_cooperator(cfqd, cfqq))
+                return;
 
         cfq_mark_cfqq_must_dispatch(cfqq);
         cfq_mark_cfqq_wait_request(cfqq);
 
-        sl = min(cfqq->slice_end - 1, (unsigned long) cfqd->cfq_slice_idle);
-
         /*
          * we don't want to idle for seeks, but we do want to allow
          * fair distribution of slice time for a process doing back-to-back
          * seeks. so allow a little bit of time for him to submit a new rq
          */
+        sl = cfqd->cfq_slice_idle;
         if (sample_valid(cic->seek_samples) && CIC_SEEKY(cic))
                 sl = min(sl, msecs_to_jiffies(2));
 
         mod_timer(&cfqd->idle_slice_timer, jiffies + sl);
-        return 1;
 }
 
 static void cfq_dispatch_insert(request_queue_t *q, struct request *rq)
@@ -870,7 +1027,7 @@ static void cfq_dispatch_insert(request_queue_t *q, struct request *rq)
         struct cfq_queue *cfqq = RQ_CFQQ(rq);
 
         cfq_remove_request(rq);
-        cfqq->on_dispatch[rq_is_sync(rq)]++;
+        cfqq->dispatched++;
         elv_dispatch_sort(q, rq);
 }
 
@@ -891,13 +1048,13 @@ static inline struct request *cfq_check_fifo(struct cfq_queue *cfqq)
         if (list_empty(&cfqq->fifo))
                 return NULL;
 
-        fifo = cfq_cfqq_class_sync(cfqq);
+        fifo = cfq_cfqq_sync(cfqq);
         rq = rq_entry_fifo(cfqq->fifo.next);
 
-        if (time_after(jiffies, rq->start_time + cfqd->cfq_fifo_expire[fifo]))
-                return rq;
+        if (time_before(jiffies, rq->start_time + cfqd->cfq_fifo_expire[fifo]))
+                return NULL;
 
-        return NULL;
+        return rq;
 }
 
 static inline int
@@ -922,23 +1079,26 @@ static struct cfq_queue *cfq_select_queue(struct cfq_data *cfqd)
                 goto new_queue;
 
         /*
-         * slice has expired
+         * The active queue has run out of time, expire it and select new.
          */
-        if (!cfq_cfqq_must_dispatch(cfqq) && cfq_slice_used(cfqq))
+        if (cfq_slice_used(cfqq))
                 goto expire;
 
         /*
-         * if queue has requests, dispatch one. if not, check if
-         * enough slice is left to wait for one
+         * The active queue has requests and isn't expired, allow it to
+         * dispatch.
          */
         if (!RB_EMPTY_ROOT(&cfqq->sort_list))
                 goto keep_queue;
-        else if (cfq_cfqq_slice_new(cfqq) || cfq_cfqq_dispatched(cfqq)) {
+
+        /*
+         * No requests pending. If the active queue still has requests in
+         * flight or is idling for a new request, allow either of these
+         * conditions to happen (or time out) before selecting a new queue.
+         */
+        if (cfqq->dispatched || timer_pending(&cfqd->idle_slice_timer)) {
                 cfqq = NULL;
                 goto keep_queue;
-        } else if (cfq_cfqq_class_sync(cfqq)) {
-                if (cfq_arm_slice_timer(cfqd))
-                        return NULL;
         }
 
 expire:
@@ -1039,7 +1199,7 @@ static int
 cfq_dispatch_requests(request_queue_t *q, int force)
 {
         struct cfq_data *cfqd = q->elevator->elevator_data;
-        struct cfq_queue *cfqq, *prev_cfqq;
+        struct cfq_queue *cfqq;
         int dispatched;
 
         if (!cfqd->busy_queues)
@@ -1049,23 +1209,19 @@ cfq_dispatch_requests(request_queue_t *q, int force)
                 return cfq_forced_dispatch(cfqd);
 
         dispatched = 0;
-        prev_cfqq = NULL;
         while ((cfqq = cfq_select_queue(cfqd)) != NULL) {
                 int max_dispatch;
 
                 if (cfqd->busy_queues > 1) {
                         /*
-                         * Don't repeat dispatch from the previous queue.
-                         */
-                        if (prev_cfqq == cfqq)
-                                break;
-
-                        /*
                          * So we have dispatched before in this round, if the
                          * next queue has idling enabled (must be sync), don't
-                         * allow it service until the previous have continued.
+                         * allow it service until the previous have completed.
                          */
-                        if (cfqd->rq_in_driver && cfq_cfqq_idle_window(cfqq))
+                        if (cfqd->rq_in_driver && cfq_cfqq_idle_window(cfqq) &&
+                            dispatched)
+                                break;
+                        if (cfqq->dispatched >= cfqd->cfq_quantum)
                                 break;
                 }
 
@@ -1078,7 +1234,6 @@ cfq_dispatch_requests(request_queue_t *q, int force)
                         max_dispatch = 1;
 
                 dispatched += __cfq_dispatch_requests(cfqd, cfqq, max_dispatch);
-                prev_cfqq = cfqq;
         }
 
         return dispatched;
@@ -1520,7 +1675,8 @@ cfq_update_io_thinktime(struct cfq_data *cfqd, struct cfq_io_context *cic)
 }
 
 static void
-cfq_update_io_seektime(struct cfq_io_context *cic, struct request *rq)
+cfq_update_io_seektime(struct cfq_data *cfqd, struct cfq_io_context *cic,
+                       struct request *rq)
 {
         sector_t sdist;
         u64 total;
@@ -1530,6 +1686,11 @@ cfq_update_io_seektime(struct cfq_io_context *cic, struct request *rq)
         else
                 sdist = cic->last_request_pos - rq->sector;
 
+        if (!cic->seek_samples) {
+                cfqd->new_seek_total = (7*cic->seek_total + (u64)256*sdist) / 8;
+                cfqd->new_seek_mean = cfqd->new_seek_total / 256;
+        }
+
         /*
          * Don't allow the seek distance to get too large from the
          * odd fragment, pagein, etc
@@ -1580,13 +1741,16 @@ static int
 cfq_should_preempt(struct cfq_data *cfqd, struct cfq_queue *new_cfqq,
                    struct request *rq)
 {
-        struct cfq_queue *cfqq = cfqd->active_queue;
-        sector_t dist;
+        struct cfq_queue *cfqq;
 
-        if (cfq_class_idle(new_cfqq))
+        cfqq = cfqd->active_queue;
+        if (!cfqq)
                 return 0;
 
-        if (!cfqq)
+        if (cfq_slice_used(cfqq))
+                return 1;
+
+        if (cfq_class_idle(new_cfqq))
                 return 0;
 
         if (cfq_class_idle(cfqq))
@@ -1613,12 +1777,7 @@ cfq_should_preempt(struct cfq_data *cfqd, struct cfq_queue *new_cfqq,
          * if this request is as-good as one we would expect from the
          * current cfqq, let it preempt
          */
-        if (rq->sector > cfqd->last_sector)
-                dist = rq->sector - cfqd->last_sector;
-        else
-                dist = cfqd->last_sector - rq->sector;
-
-        if (dist <= cfqd->active_cic->seek_mean)
+        if (cfq_rq_close(cfqd, rq))
                 return 1;
 
         return 0;
@@ -1656,28 +1815,12 @@ cfq_rq_enqueued(struct cfq_data *cfqd, struct cfq_queue *cfqq,
         if (rq_is_meta(rq))
                 cfqq->meta_pending++;
 
-        /*
-         * we never wait for an async request and we don't allow preemption
-         * of an async request. so just return early
-         */
-        if (!rq_is_sync(rq)) {
-                /*
-                 * sync process issued an async request, if it's waiting
-                 * then expire it and kick rq handling.
-                 */
-                if (cic == cfqd->active_cic &&
-                    del_timer(&cfqd->idle_slice_timer)) {
-                        cfq_slice_expired(cfqd, 0, 0);
-                        blk_start_queueing(cfqd->queue);
-                }
-                return;
-        }
-
         cfq_update_io_thinktime(cfqd, cic);
-        cfq_update_io_seektime(cic, rq);
+        cfq_update_io_seektime(cfqd, cic, rq);
         cfq_update_idle_window(cfqd, cfqq, cic);
 
         cic->last_request_pos = rq->sector + rq->nr_sectors;
+        cfqq->last_request_pos = cic->last_request_pos;
 
         if (cfqq == cfqd->active_queue) {
                 /*
@@ -1726,13 +1869,11 @@ static void cfq_completed_request(request_queue_t *q, struct request *rq)
         now = jiffies;
 
         WARN_ON(!cfqd->rq_in_driver);
-        WARN_ON(!cfqq->on_dispatch[sync]);
+        WARN_ON(!cfqq->dispatched);
         cfqd->rq_in_driver--;
-        cfqq->on_dispatch[sync]--;
+        cfqq->dispatched--;
         cfqq->service_last = now;
 
-        cfqd->last_sector = rq->hard_sector + rq->hard_nr_sectors;
-
         if (!cfq_class_idle(cfqq))
                 cfqd->last_end_request = now;
 
@@ -1752,11 +1893,12 @@ static void cfq_completed_request(request_queue_t *q, struct request *rq)
                 }
                 if (cfq_slice_used(cfqq))
                         cfq_slice_expired(cfqd, 0, 1);
-                else if (sync && RB_EMPTY_ROOT(&cfqq->sort_list)) {
-                        if (!cfq_arm_slice_timer(cfqd))
-                                cfq_schedule_dispatch(cfqd);
-                }
+                else if (sync && RB_EMPTY_ROOT(&cfqq->sort_list))
+                        cfq_arm_slice_timer(cfqd);
         }
+
+        if (!cfqd->rq_in_driver)
+                cfq_schedule_dispatch(cfqd);
 }
 
 /*
@@ -2101,7 +2243,6 @@ fail:
 /*
  * sysfs parts below -->
  */
-
 static ssize_t
 cfq_var_show(unsigned int var, char *page)
 {