block, bfq: let a queue be merged only shortly after starting I/O

In BFQ and CFQ, two processes are said to be cooperating if they do
I/O in such a way that the union of their I/O requests yields a
sequential I/O pattern. To get such a sequential I/O pattern out of
the non-sequential pattern of each cooperating process, BFQ and CFQ
merge the queues associated with these processes. In more detail,
cooperating processes, and thus their associated queues, usually
start, or restart, to do I/O shortly after each other. This is the
case, e.g., for the I/O threads of KVM/QEMU and of the dump
utility. Basing on this assumption, this commit allows a bfq_queue to
be merged only during a short time interval (100ms) after it starts,
or re-starts, to do I/O.  This filtering provides two important
benefits.

First, it greatly reduces the probability that two non-cooperating
processes have their queues merged by mistake, if they just happen to
do I/O close to each other for a short time interval. These spurious
merges cause loss of service guarantees. A low-weight bfq_queue may
unjustly get more than its expected share of the throughput: if such a
low-weight queue is merged with a high-weight queue, then the I/O for
the low-weight queue is served as if the queue had a high weight. This
may damage other high-weight queues unexpectedly.  For instance,
because of this issue, lxterminal occasionally took 7.5 seconds to
start, instead of 6.5 seconds, when some sequential readers and
writers did I/O in the background on a FUJITSU MHX2300BT HDD.  The
reason is that the bfq_queues associated with some of the readers or
the writers were merged with the high-weight queues of some processes
that had to do some urgent but little I/O. The readers then exploited
the inherited high weight for all or most of their I/O, during the
start-up of terminal. The filtering introduced by this commit
eliminated any outlier caused by spurious queue merges in our start-up
time tests.

This filtering also provides a little boost of the throughput
sustainable by BFQ: 3-4%, depending on the CPU. The reason is that,
once a bfq_queue cannot be merged any longer, this commit makes BFQ
stop updating the data needed to handle merging for the queue.
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Angelo Ruocco <angeloruocco90@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/bfq-iosched.c

@@ -166,6 +166,20 @@ static const int bfq_async_charge_factor = 10;
 /* Default timeout values, in jiffies, approximating CFQ defaults. */
 const int bfq_timeout = HZ / 8;
 
+/*
+ * Time limit for merging (see comments in bfq_setup_cooperator). Set
+ * to the slowest value that, in our tests, proved to be effective in
+ * removing false positives, while not causing true positives to miss
+ * queue merging.
+ *
+ * As can be deduced from the low time limit below, queue merging, if
+ * successful, happens at the very beggining of the I/O of the involved
+ * cooperating processes, as a consequence of the arrival of the very
+ * first requests from each cooperator.  After that, there is very
+ * little chance to find cooperators.
+ */
+static const unsigned long bfq_merge_time_limit = HZ/10;
+
 static struct kmem_cache *bfq_pool;
 
 /* Below this threshold (in ns), we consider thinktime immediate. */
@@ -444,6 +458,13 @@ bfq_rq_pos_tree_lookup(struct bfq_data *bfqd, struct rb_root *root,
 	return bfqq;
 }
 
+static bool bfq_too_late_for_merging(struct bfq_queue *bfqq)
+{
+	return bfqq->service_from_backlogged > 0 &&
+		time_is_before_jiffies(bfqq->first_IO_time +
+				       bfq_merge_time_limit);
+}
+
 void bfq_pos_tree_add_move(struct bfq_data *bfqd, struct bfq_queue *bfqq)
 {
 	struct rb_node **p, *parent;
@@ -454,6 +475,14 @@ void bfq_pos_tree_add_move(struct bfq_data *bfqd, struct bfq_queue *bfqq)
 		bfqq->pos_root = NULL;
 	}
 
+	/*
+	 * bfqq cannot be merged any longer (see comments in
+	 * bfq_setup_cooperator): no point in adding bfqq into the
+	 * position tree.
+	 */
+	if (bfq_too_late_for_merging(bfqq))
+		return;
+
 	if (bfq_class_idle(bfqq))
 		return;
 	if (!bfqq->next_rq)
@@ -1935,6 +1964,9 @@ bfq_setup_merge(struct bfq_queue *bfqq, struct bfq_queue *new_bfqq)
 static bool bfq_may_be_close_cooperator(struct bfq_queue *bfqq,
 					struct bfq_queue *new_bfqq)
 {
+	if (bfq_too_late_for_merging(new_bfqq))
+		return false;
+
 	if (bfq_class_idle(bfqq) || bfq_class_idle(new_bfqq) ||
 	    (bfqq->ioprio_class != new_bfqq->ioprio_class))
 		return false;
@@ -2003,6 +2035,20 @@ bfq_setup_cooperator(struct bfq_data *bfqd, struct bfq_queue *bfqq,
 {
 	struct bfq_queue *in_service_bfqq, *new_bfqq;
 
+	/*
+	 * Prevent bfqq from being merged if it has been created too
+	 * long ago. The idea is that true cooperating processes, and
+	 * thus their associated bfq_queues, are supposed to be
+	 * created shortly after each other. This is the case, e.g.,
+	 * for KVM/QEMU and dump I/O threads. Basing on this
+	 * assumption, the following filtering greatly reduces the
+	 * probability that two non-cooperating processes, which just
+	 * happen to do close I/O for some short time interval, have
+	 * their queues merged by mistake.
+	 */
+	if (bfq_too_late_for_merging(bfqq))
+		return NULL;
+
 	if (bfqq->new_bfqq)
 		return bfqq->new_bfqq;
 
@@ -3002,17 +3048,6 @@ void bfq_bfqq_expire(struct bfq_data *bfqd,
 	 */
 	slow = bfq_bfqq_is_slow(bfqd, bfqq, compensate, reason, &delta);
 
-	/*
-	 * Increase service_from_backlogged before next statement,
-	 * because the possible next invocation of
-	 * bfq_bfqq_charge_time would likely inflate
-	 * entity->service. In contrast, service_from_backlogged must
-	 * contain real service, to enable the soft real-time
-	 * heuristic to correctly compute the bandwidth consumed by
-	 * bfqq.
-	 */
-	bfqq->service_from_backlogged += entity->service;
-
 	/*
 	 * As above explained, charge slow (typically seeky) and
 	 * timed-out queues with the time and not the service

block/bfq-iosched.h

@@ -344,6 +344,8 @@ struct bfq_queue {
 	unsigned long wr_start_at_switch_to_srt;
 
 	unsigned long split_time; /* time of last split */
+
+	unsigned long first_IO_time; /* time of first I/O for this queue */
 };
 
 /**

block/bfq-wf2q.c

@@ -835,6 +835,10 @@ void bfq_bfqq_served(struct bfq_queue *bfqq, int served)
 	struct bfq_entity *entity = &bfqq->entity;
 	struct bfq_service_tree *st;
 
+	if (!bfqq->service_from_backlogged)
+		bfqq->first_IO_time = jiffies;
+
+	bfqq->service_from_backlogged += served;
 	for_each_entity(entity) {
 		st = bfq_entity_service_tree(entity);