cfq-iosched: Respect ioprio_class when preempting

In cfq_should_preempt(), we currently allow some cases where a non-RT request
can preempt an ongoing RT cfqq timeslice. This should not happen.
Examples include:

o A sync_noidle wl type non-RT request pre-empting a sync_noidle wl type cfqq
  on which we are idling.
o Once we have per-cgroup async queues, a non-RT sync request pre-empting a RT
  async cfqq.

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

1 files changed, 6 insertions, 0 deletions

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index 918c7fd9aeb1..ee130f14d1fc 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -3077,6 +3077,12 @@ cfq_should_preempt(struct cfq_data *cfqd, struct cfq_queue *new_cfqq,
                 return true;
 
         /*
+         * Don't allow a non-RT request to preempt an ongoing RT cfqq timeslice.
+         */
+        if (cfq_class_rt(cfqq) && !cfq_class_rt(new_cfqq))
+                return false;
+
+        /*
          * if the new request is sync, but the currently running queue is
          * not, let the sync request have priority.
          */