cfq-iosched: Fix regression in bonnie++ rewrite performance

Commit 9a7f38c42c2 (cfq-iosched: Convert from jiffies to nanoseconds)
broke the condition for detecting starved sync IO in
cfq_completed_request() because rq->start_time remained in jiffies but
we compared it with nanosecond values. This manifested as a regression
in bonnie++ rewrite performance because we always ended up considering
sync IO starved and thus never increased async IO queue depth.

Since rq->start_time is used in a lot of places, converting it to ns
values would be non-trivial. So just revert the condition in CFQ to use
comparison with jiffies. This will lead to suboptimal results if
cfq_fifo_expire[1] will ever come close to 1 jiffie but so far we are
relatively far from that with the storage used with CFQ (the default
value is 128 ms).

Fixes: 9a7f38c42c2b92391d9dabaf9f51df7cfe5608e4
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@fb.com>

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index c1adbd01d0fa30e9d45e26d5912479eb614bf13c..2d03004dc77cf1bc776ff4139352e6f8a86da8f7 100644 (file)
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -4243,7 +4243,16 @@ static void cfq_completed_request(struct request_queue *q, struct request *rq)
                                        cfqq_type(cfqq));
 
                st->ttime.last_end_request = now;
-               if (!(rq->start_time + cfqd->cfq_fifo_expire[1] > now))
+               /*
+                * We have to do this check in jiffies since start_time is in
+                * jiffies and it is not trivial to convert to ns. If
+                * cfq_fifo_expire[1] ever comes close to 1 jiffie, this test
+                * will become problematic but so far we are fine (the default
+                * is 128 ms).
+                */
+               if (!time_after(rq->start_time +
+                                 nsecs_to_jiffies(cfqd->cfq_fifo_expire[1]),
+                               jiffies))
                        cfqd->last_delayed_sync = now;
        }