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>

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index a592afc..069a610 100644 (file)
--- 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);
 
        /*