cfq-iosched: avoid probable slice overrun when idling

If the average think time is larger than the remaining time slice
for any given queue, don't allow it to idle. A succesful idle also
means that we need to dispatch and complete a request, so if we don't
even have time left for the idle process, we would overrun the slice
in any case.

Signed-off-by: Corrado Zoccolo <czoccolo@gmail.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

1 files changed, 9 insertions, 0 deletions

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index a592afcf1e6d..069a61017c02 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -1093,6 +1093,15 @@ static void cfq_arm_slice_timer(struct cfq_data *cfqd)
         if (!cic || !atomic_read(&cic->ioc->nr_tasks))
                 return;
 
+        /*
+         * If our average think time is larger than the remaining time
+         * slice, then don't idle. This avoids overrunning the allotted
+         * time slice.
+         */
+        if (sample_valid(cic->ttime_samples) &&
+            (cfqq->slice_end - jiffies < cic->ttime_mean))
+                return;
+
         cfq_mark_cfqq_wait_request(cfqq);
 
         /*