[PATCH 3/8] block/mq-deadline: skip expensive merge lookups if contended

[Jens Axboe <axboe@kernel.dk>]

We do several stages of merging in the block layer - the most likely one
to work is also the cheap one, merging direct in the per-task plug when
IO is submitted. Getting merges outside of that is a lot less likely,
but IO schedulers may still maintain internal data structures to
facilitate merge lookups outside of the plug.

Make mq-deadline skip expensive merge lookups if the queue lock is
already contended. The likelihood of getting a merge here is not very
high, hence it should not be a problem skipping the attempt in the also
unlikely event that the queue is already contended.

Perf diff shows the difference between a random read/write workload
with 4 threads doing IO, with expensive merges turned on and off:

    25.00%    +61.94%  [kernel.kallsyms]  [k] queued_spin_lock_slowpath

where we almost quadruple the lock contention by attempting these
expensive merges.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 block/mq-deadline.c | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/block/mq-deadline.c b/block/mq-deadline.c
index 79bc3b6784b3..740b94f36cac 100644
--- a/block/mq-deadline.c
+++ b/block/mq-deadline.c
@@ -800,7 +800,19 @@ static bool dd_bio_merge(struct request_queue *q, struct bio *bio,
 	struct request *free = NULL;
 	bool ret;
 
-	spin_lock(&dd->lock);
+	/*
+	 * bio merging is called for every bio queued, and it's very easy
+	 * to run into contention because of that. If we fail getting
+	 * the dd lock, just skip this merge attempt. For related IO, the
+	 * plug will be the successful merging point. If we get here, we
+	 * already failed doing the obvious merge. Chances of actually
+	 * getting a merge off this path is a lot slimmer, so skipping an
+	 * occassional lookup that will most likely not succeed anyway should
+	 * not be a problem.
+	 */
+	if (!spin_trylock(&dd->lock))
+		return false;
+
 	ret = blk_mq_sched_try_merge(q, bio, nr_segs, &free);
 	spin_unlock(&dd->lock);
 
-- 
2.43.0