On 02/16/2017 03:46 AM, Paolo Valente wrote:

quoted

Il giorno 17 dic 2016, alle ore 01:12, Jens Axboe [off-list ref] ha scritto:

This is basically identical to deadline-iosched, except it registers
as a MQ capable scheduler. This is still a single queue design.

Signed-off-by: Jens Axboe <axboe@fb.com>

...

quoted

+
+static void dd_merged_requests(struct request_queue *q, struct request *req,
+			       struct request *next)
+{
+	/*
+	 * if next expires before rq, assign its expire time to rq
+	 * and move into next position (next will be deleted) in fifo
+	 */
+	if (!list_empty(&req->queuelist) && !list_empty(&next->queuelist)) {
+		if (time_before((unsigned long)next->fifo_time,
+				(unsigned long)req->fifo_time)) {
+			list_move(&req->queuelist, &next->queuelist);
+			req->fifo_time = next->fifo_time;
+		}
+	}
+

Jens,
while trying to imagine the possible causes of Bart's hang with
bfq-mq, I've bumped into the following doubt: in the above function
(in my case, in bfq-mq-'s equivalent of the above function), are
we sure that neither req or next could EVER be in dd->dispatch instead
of dd->fifo_list?  I've tried to verify it, but, although I think it has never
happened in my tests, I was not able to make sure that no unlucky
combination may ever happen (considering also the use of
blk_rq_is_passthrough too, to decide where to put a new request).

I'm making a blunder, right?

If a request goes into dd->dispatch, it's going to be found for merging.
Hence we can never call the above on the request.

-- 
Jens Axboe