Il giorno 26 ago 2021, alle ore 11:16, Paolo Valente [off-list ref] ha scritto:

quoted

Il giorno 2 ago 2021, alle ore 16:13, Paolo Valente [off-list ref] ha scritto:

The function bfq_setup_merge prepares the merging between two
bfq_queues, say bfqq and new_bfqq. To this goal, it assigns
bfqq->new_bfqq = new_bfqq. Then, each time some I/O for bfqq arrives,
the process that generated that I/O is disassociated from bfqq and
associated with new_bfqq (merging is actually a redirection). In this
respect, bfq_setup_merge increases new_bfqq->ref in advance, adding
the number of processes that are expected to be associated with
new_bfqq.

Unfortunately, the stable-merging mechanism interferes with this
setup. After bfqq->new_bfqq has been set by bfq_setup_merge, and
before all the expected processes have been associated with
bfqq->new_bfqq, bfqq may happen to be stably merged with a different
queue than the current bfqq->new_bfqq. In this case, bfqq->new_bfqq
gets changed. So, some of the processes that have been already
accounted for in the ref counter of the previous new_bfqq will not be
associated with that queue.  This creates an unbalance, because those
references will never be decremented.

This commit fixes this issue by reestablishing the previous, natural
behaviour: once bfqq->new_bfqq has been set, it will not be changed
until all expected redirections have occurred.

Hi Jens,
did you have time to look at this fix?

ping ...

Thanks,
Paolo

quoted

Signed-off-by: Davide Zini <redacted>
Signed-off-by: Paolo Valente <redacted>
---
block/bfq-iosched.c | 16 +++++++++++++---
1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index 727955918563..08d9122dd4c0 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c

@@ -2659,6 +2659,15 @@ bfq_setup_merge(struct bfq_queue *bfqq, struct bfq_queue *new_bfqq)

	 * are likely to increase the throughput.
	 */
	bfqq->new_bfqq = new_bfqq;
+	/*
+	 * The above assignment schedules the following redirections:
+	 * each time some I/O for bfqq arrives, the process that
+	 * generated that I/O is disassociated from bfqq and
+	 * associated with new_bfqq. Here we increases new_bfqq->ref
+	 * in advance, adding the number of processes that are
+	 * expected to be associated with new_bfqq as they happen to
+	 * issue I/O.
+	 */
	new_bfqq->ref += process_refs;
	return new_bfqq;
}

@@ -2721,6 +2730,10 @@ bfq_setup_cooperator(struct bfq_data *bfqd, struct bfq_queue *bfqq,

{
	struct bfq_queue *in_service_bfqq, *new_bfqq;

+	/* if a merge has already been setup, then proceed with that first */
+	if (bfqq->new_bfqq)
+		return bfqq->new_bfqq;
+
	/*
	 * Check delayed stable merge for rotational or non-queueing
	 * devs. For this branch to be executed, bfqq must not be

@@ -2822,9 +2835,6 @@ bfq_setup_cooperator(struct bfq_data *bfqd, struct bfq_queue *bfqq,

	if (bfq_too_late_for_merging(bfqq))
		return NULL;

-	if (bfqq->new_bfqq)
-		return bfqq->new_bfqq;
-
	if (!io_struct || unlikely(bfqq == &bfqd->oom_bfqq))
		return NULL;

-- 
2.20.1