[PATCH] cfq-iosched: use last service point as the fairness criteria

Right now we use slice_start, which gives async queues an unfair
advantage. Chance that to service_last, and base the resorter
on that.

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

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index 975cdfc8d61406b19d05d6380caf3b81a3fd15c9..7a8ef0f09699f521613bacb0a9295a22e2e44359 100644 (file)
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -146,9 +146,9 @@ struct cfq_queue {
        /* fifo list of requests in sort_list */
        struct list_head fifo;
 
-       unsigned long slice_start;
        unsigned long slice_end;
        unsigned long slice_left;
+       unsigned long service_last;
 
        /* number of requests that are on the dispatch list */
        int on_dispatch[2];
@@ -355,7 +355,8 @@ cfq_find_next_rq(struct cfq_data *cfqd, struct cfq_queue *cfqq,
 static void cfq_resort_rr_list(struct cfq_queue *cfqq, int preempted)
 {
        struct cfq_data *cfqd = cfqq->cfqd;
-       struct list_head *list;
+       struct list_head *list, *n;
+       struct cfq_queue *__cfqq;
 
        /*
         * Resorting requires the cfqq to be on the RR list already.
@@ -383,15 +384,13 @@ static void cfq_resort_rr_list(struct cfq_queue *cfqq, int preempted)
                        list = &cfqd->rr_list[cfqq->ioprio];
        }
 
-       /*
-        * If this queue was preempted or is new (never been serviced), let
-        * it be added first for fairness but beind other new queues.
-        * Otherwise, just add to the back  of the list.
-        */
        if (preempted || cfq_cfqq_queue_new(cfqq)) {
-               struct list_head *n = list;
-               struct cfq_queue *__cfqq;
-
+               /*
+                * If this queue was preempted or is new (never been serviced),
+                * let it be added first for fairness but beind other new
+                * queues.
+                */
+               n = list;
                while (n->next != list) {
                        __cfqq = list_entry_cfqq(n->next);
                        if (!cfq_cfqq_queue_new(__cfqq))
@@ -399,11 +398,32 @@ 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);
+       } else {
+               /*
+                * sort by last service, but don't cross a new or async
+                * queue. we don't cross a new queue because it hasn't been
+                * service before, and we don't cross an async queue because
+                * it gets added to the end on expire.
+                */
+               n = list;
+               while ((n = n->prev) != list) {
+                       struct cfq_queue *__cfqq = list_entry_cfqq(n);
 
-               list = n;
+                       if (!cfq_cfqq_class_sync(cfqq) || !__cfqq->service_last)
+                               break;
+                       if (time_before(__cfqq->service_last, cfqq->service_last))
+                               break;
+               }
+               list_add(&cfqq->cfq_list, n);
        }
-
-       list_add_tail(&cfqq->cfq_list, list);
 }
 
 /*
@@ -608,7 +628,6 @@ __cfq_set_active_queue(struct cfq_data *cfqd, struct cfq_queue *cfqq)
                 */
                del_timer(&cfqd->idle_class_timer);
 
-               cfqq->slice_start = jiffies;
                cfqq->slice_end = 0;
                cfqq->slice_left = 0;
                cfq_clear_cfqq_must_alloc_slice(cfqq);
@@ -1688,6 +1707,7 @@ static void cfq_completed_request(request_queue_t *q, struct request *rq)
        WARN_ON(!cfqq->on_dispatch[sync]);
        cfqd->rq_in_driver--;
        cfqq->on_dispatch[sync]--;
+       cfqq->service_last = now;
 
        if (!cfq_class_idle(cfqq))
                cfqd->last_end_request = now;