Re: [PATCH] blk-mq: plug request for shared sbitmap

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On 14/05/2021 03:20, Ming Lei wrote:
In case of shared sbitmap, request won't be held in plug list any more
sine commit 32bc15afed04 ("blk-mq: Facilitate a shared sbitmap per
tagset"), this way makes request merge from flush plug list & batching
submission not possible, so cause performance regression.

Yanhui reports performance regression when running sequential IO
test(libaio, 16 jobs, 8 depth for each job) in VM, and the VM disk
is emulated with image stored on xfs/megaraid_sas.

Fix the issue by recovering original behavior to allow to hold request
in plug list.

Hi Ming,

Since testing v5.13-rc2, I noticed that this patch made the hang I was seeing disappear:
https://lore.kernel.org/linux-scsi/3d72d64d-314f-9d34-e039-7e508b2abe1b@xxxxxxxxxx/

I don't think that problem is solved, though.

So I wonder about throughput performance (I had hoped to test before it was merged). I only have 6x SAS SSDs at hand, but I see some significant changes (good and bad) for mq-deadline for hisi_sas:
Before 620K (read), 300K IOPs (randread)
After 430K (read), 460-490K IOPs (randread)

none IO sched is always about 450K (read) and 500K (randread)

Do you guys have any figures? Are my results as expected?

Thanks,
John


Cc: Yanhui Ma <yama@xxxxxxxxxx>
Cc: John Garry <john.garry@xxxxxxxxxx>
Cc: Bart Van Assche <bvanassche@xxxxxxx>
Cc: kashyap.desai@xxxxxxxxxxxx
Fixes: 32bc15afed04 ("blk-mq: Facilitate a shared sbitmap per tagset")
Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx>
---
  block/blk-mq.c | 5 +++--
  1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index ae7f5ee41cd3..baf7a9546068 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2236,8 +2236,9 @@ blk_qc_t blk_mq_submit_bio(struct bio *bio)
  		/* Bypass scheduler for flush requests */
  		blk_insert_flush(rq);
  		blk_mq_run_hw_queue(data.hctx, true);
-	} else if (plug && (q->nr_hw_queues == 1 || q->mq_ops->commit_rqs ||
-				!blk_queue_nonrot(q))) {
+	} else if (plug && (q->nr_hw_queues == 1 ||
+		   blk_mq_is_sbitmap_shared(rq->mq_hctx->flags) ||
+		   q->mq_ops->commit_rqs || !blk_queue_nonrot(q))) {
  		/*
  		 * Use plugging if we have a ->commit_rqs() hook as well, as
  		 * we know the driver uses bd->last in a smart fashion.





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