Commit 2c1b1848 authored Aug 02, 2021 by Paolo Valente Committed by Greg Kroah-Hartman Sep 22, 2021

block, bfq: honor already-setup queue merges



[ Upstream commit 2d52c58b9c9bdae0ca3df6a1eab5745ab3f7d80b ]

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.

Signed-off-by: Davide Zini <davidezini2@gmail.com>
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Link: https://lore.kernel.org/r/20210802141352.74353-2-paolo.valente@linaro.org


Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Sasha Levin <sashal@kernel.org>

parent 052447e9

block/bfq-iosched.c

+13 −3

Original line number	Diff line number	Diff line
		@@ -2137,6 +2137,15 @@ 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;
		}
		@@ -2196,6 +2205,10 @@ 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;

		/*
		* Prevent bfqq from being merged if it has been created too
		* long ago. The idea is that true cooperating processes, and
		@@ -2210,9 +2223,6 @@ 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;