block, bfq: use half slice_idle as a threshold to check short ttime

The value of the I/O plugging (idling) timeout is used also as the
think-time threshold to decide whether a process has a short think
time.  In this respect, a good value of this timeout for rotational
drives is un the order of several ms. Yet, this is often too long a
time interval to be effective as a think-time threshold. This commit
mitigates this problem (by a lot, according to tests), by halving the
threshold.
Tested-by: Jan Kara <jack@suse.cz>
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/bfq-iosched.c

@@ -5238,12 +5238,13 @@ static void bfq_update_has_short_ttime(struct bfq_data *bfqd,
 		return;
 
 	/* Think time is infinite if no process is linked to
-	 * bfqq. Otherwise check average think time to
-	 * decide whether to mark as has_short_ttime
+	 * bfqq. Otherwise check average think time to decide whether
+	 * to mark as has_short_ttime. To this goal, compare average
+	 * think time with half the I/O-plugging timeout.
 	 */
 	if (atomic_read(&bic->icq.ioc->active_ref) == 0 ||
 	    (bfq_sample_valid(bfqq->ttime.ttime_samples) &&
-	     bfqq->ttime.ttime_mean > bfqd->bfq_slice_idle))
+	     bfqq->ttime.ttime_mean > bfqd->bfq_slice_idle>>1))
 		has_short_ttime = false;
 
 	state_changed = has_short_ttime != bfq_bfqq_has_short_ttime(bfqq);