cfq-iosched: fix no-idle preemption logic

An incoming no-idle queue should preempt the active no-idle queue
 only if the active queue is idling due to service tree empty.
 Previous code was buggy in two ways:
 * it relied on service_tree field to be set on the active queue, while
   it is not set when the code is idling for a new request
 * it didn't check for the service tree empty condition, so could lead to
   LIFO behaviour if multiple queues with depth > 1 were preempting each
   other on an non-NCQ device.

Reported-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Corrado Zoccolo <czoccolo@gmail.com>
Acked-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

1 files changed, 3 insertions, 2 deletions

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index 47abd24617be..2c1086acddfa 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -2392,8 +2392,9 @@ cfq_should_preempt(struct cfq_data *cfqd, struct cfq_queue *new_cfqq,
         if (cfq_class_idle(cfqq))
                 return true;
 
-        if (cfqd->serving_type == SYNC_NOIDLE_WORKLOAD
-            && new_cfqq->service_tree == cfqq->service_tree)
+        if (cfqd->serving_type == SYNC_NOIDLE_WORKLOAD &&
+            cfqq_type(new_cfqq) == SYNC_NOIDLE_WORKLOAD &&
+            new_cfqq->service_tree->count == 1)
                 return true;
 
         /*