cfq-iosched: fix queue depth detection

CFQ's detection of queueing devices assumes a non-queuing device and detects
if the queue depth reaches a certain threshold.  Under some workloads (e.g.
synchronous reads), CFQ effectively forces a unit queue depth, thus defeating
the detection logic.  This leads to poor performance on queuing hardware,
since the idle window remains enabled.

This patch inverts the sense of the logic: assume a queuing-capable device,
and detect if the depth does not exceed the threshold.

Signed-off-by: Aaron Carroll <aaronc@gelato.unsw.edu.au>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

1 files changed, 38 insertions, 9 deletions

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index 5f6fd287c18..494b6fdcb18 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -39,6 +39,7 @@ static int cfq_slice_idle = HZ / 125;
 #define CFQ_MIN_TT              (2)
 
 #define CFQ_SLICE_SCALE         (5)
+#define CFQ_HW_QUEUE_MIN        (5)
 
 #define RQ_CIC(rq)              \
         ((struct cfq_io_context *) (rq)->elevator_private)
@@ -86,7 +87,14 @@ struct cfq_data {
 
         int rq_in_driver;
         int sync_flight;
+
+        /*
+         * queue-depth detection
+         */
+        int rq_queued;
         int hw_tag;
+        int hw_tag_samples;
+        int rq_in_driver_peak;
 
         /*
          * idle window management
@@ -654,15 +662,6 @@ static void cfq_activate_request(struct request_queue *q, struct request *rq)
         cfq_log_cfqq(cfqd, RQ_CFQQ(rq), "activate rq, drv=%d",
                                                 cfqd->rq_in_driver);
 
-        /*
-         * If the depth is larger 1, it really could be queueing. But lets
-         * make the mark a little higher - idling could still be good for
-         * low queueing, and a low queueing number could also just indicate
-         * a SCSI mid layer like behaviour where limit+1 is often seen.
-         */
-        if (!cfqd->hw_tag && cfqd->rq_in_driver > 4)
-                cfqd->hw_tag = 1;
-
         cfqd->last_position = rq->hard_sector + rq->hard_nr_sectors;
 }
 
@@ -686,6 +685,7 @@ static void cfq_remove_request(struct request *rq)
         list_del_init(&rq->queuelist);
         cfq_del_rq_rb(rq);
 
+        cfqq->cfqd->rq_queued--;
         if (rq_is_meta(rq)) {
                 WARN_ON(!cfqq->meta_pending);
                 cfqq->meta_pending--;
@@ -1833,6 +1833,7 @@ cfq_rq_enqueued(struct cfq_data *cfqd, struct cfq_queue *cfqq,
 {
         struct cfq_io_context *cic = RQ_CIC(rq);
 
+        cfqd->rq_queued++;
         if (rq_is_meta(rq))
                 cfqq->meta_pending++;
 
@@ -1880,6 +1881,31 @@ static void cfq_insert_request(struct request_queue *q, struct request *rq)
         cfq_rq_enqueued(cfqd, cfqq, rq);
 }
 
+/*
+ * Update hw_tag based on peak queue depth over 50 samples under
+ * sufficient load.
+ */
+static void cfq_update_hw_tag(struct cfq_data *cfqd)
+{
+        if (cfqd->rq_in_driver > cfqd->rq_in_driver_peak)
+                cfqd->rq_in_driver_peak = cfqd->rq_in_driver;
+
+        if (cfqd->rq_queued <= CFQ_HW_QUEUE_MIN &&
+            cfqd->rq_in_driver <= CFQ_HW_QUEUE_MIN)
+                return;
+
+        if (cfqd->hw_tag_samples++ < 50)
+                return;
+
+        if (cfqd->rq_in_driver_peak >= CFQ_HW_QUEUE_MIN)
+                cfqd->hw_tag = 1;
+        else
+                cfqd->hw_tag = 0;
+
+        cfqd->hw_tag_samples = 0;
+        cfqd->rq_in_driver_peak = 0;
+}
+
 static void cfq_completed_request(struct request_queue *q, struct request *rq)
 {
         struct cfq_queue *cfqq = RQ_CFQQ(rq);
@@ -1890,6 +1916,8 @@ static void cfq_completed_request(struct request_queue *q, struct request *rq)
         now = jiffies;
         cfq_log_cfqq(cfqd, cfqq, "complete");
 
+        cfq_update_hw_tag(cfqd);
+
         WARN_ON(!cfqd->rq_in_driver);
         WARN_ON(!cfqq->dispatched);
         cfqd->rq_in_driver--;
@@ -2200,6 +2228,7 @@ static void *cfq_init_queue(struct request_queue *q)
         cfqd->cfq_slice[1] = cfq_slice_sync;
         cfqd->cfq_slice_async_rq = cfq_slice_async_rq;
         cfqd->cfq_slice_idle = cfq_slice_idle;
+        cfqd->hw_tag = 1;
 
         return cfqd;
 }