block, bfq: decrease burst size when queues in burst exit

If many queues belonging to the same group happen to be created
shortly after each other, then the concurrent processes associated
with these queues have typically a common goal, and they get it done
as soon as possible if not hampered by device idling.  Examples are
processes spawned by git grep, or by systemd during boot. As for
device idling, this mechanism is currently necessary for weight
raising to succeed in its goal: privileging I/O.  In view of these
facts, BFQ does not provide the above queues with either weight
raising or device idling.

On the other hand, a burst of queue creations may be caused also by
the start-up of a complex application. In this case, these queues need
usually to be served one after the other, and as quickly as possible,
to maximise responsiveness. Therefore, in this case the best strategy
is to weight-raise all the queues created during the burst, i.e., the
exact opposite of the strategy for the above case.

To distinguish between the two cases, BFQ uses an empirical burst-size
threshold, found through extensive tests and monitoring of daily
usage. Only large bursts, i.e., burst with a size above this
threshold, are considered as generated by a high number of parallel
processes. In this respect, upstart-based boot proved to be rather
hard to detect as generating a large burst of queue creations, because
with upstart most of the queues created in a burst exit *before* the
next queues in the same burst are created. To address this issue, I
changed the burst-detection mechanism so as to not decrease the size
of the current burst even if one of the queues in the burst is
eliminated.

Unfortunately, this missing decrease causes false positives on very
fast systems: on the start-up of a complex application, such as
libreoffice writer, so many queues are created, served and exited
shortly after each other, that a large burst of queue creations is
wrongly detected as occurring. These false positives just disappear if
the size of a burst is decreased when one of the queues in the burst
exits. This commit restores the missing burst-size decrease, relying
of the fact that upstart is apparently unlikely to be used on systems
running this and future versions of the kernel.

Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Mauro Andreolini <mauro.andreolini@unimore.it>
Signed-off-by: Angelo Ruocco <angeloruocco90@gmail.com>
Tested-by: Mirko Montanari <mirkomontanari91@gmail.com>
Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name>
Tested-by: Lee Tibbert <lee.tibbert@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/bfq-iosched.c

@@ -3725,16 +3725,10 @@ void bfq_put_queue(struct bfq_queue *bfqq)
 	if (bfqq->ref)
 		return;
 
-	if (bfq_bfqq_sync(bfqq))
-		/*
-		 * The fact that this queue is being destroyed does not
-		 * invalidate the fact that this queue may have been
-		 * activated during the current burst. As a consequence,
-		 * although the queue does not exist anymore, and hence
-		 * needs to be removed from the burst list if there,
-		 * the burst size has not to be decremented.
-		 */
+	if (bfq_bfqq_sync(bfqq) && !hlist_unhashed(&bfqq->burst_list_node)) {
 		hlist_del_init(&bfqq->burst_list_node);
+		bfqq->bfqd->burst_size--;
+	}
 
 	kmem_cache_free(bfq_pool, bfqq);
 #ifdef CONFIG_BFQ_GROUP_IOSCHED