[PATCH v2 1/1] nvme-multipath: Skip nr_active increments in RETRY disposition

[Amit Chaudhary <achaudhary@purestorage.com>]

For queue-depth I/O policy, this patch fixes unbalanced I/Os across
nvme multipaths.

Issue Description:

The RETRY disposition incorrectly increments ns->ctrl->nr_active
counter and reinitializes iostat start-time. In such cases nr_active
counter never goes back to zero until that path disconnects and
reconnects.

Such a path is not chosen for new I/Os if multiple RETRY cases on a given
a path cause its queue-depth counter to be artificially higher compared
to other paths. This leads to unbalanced I/Os across paths.

The patch skips incrementing nr_active if NVME_MPATH_CNT_ACTIVE is already
set. And it skips restarting io stats if NVME_MPATH_IO_STATS is already set.

base-commit: e989a3da2d371a4b6597ee8dee5c72e407b4db7a
Fixes: d4d957b53d91eeb ("nvme-multipath: support io stats on the mpath device")
Signed-off-by: Amit Chaudhary <achaudhary@purestorage.com>
Reviewed-by: Randy Jennings <randyj@purestorage.com>
---
V1 -> V2: Moved check to mpath start and check relevant path bits

 drivers/nvme/host/multipath.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/nvme/host/multipath.c b/drivers/nvme/host/multipath.c
index 3da980dc60d9..543e17aead12 100644
--- a/drivers/nvme/host/multipath.c
+++ b/drivers/nvme/host/multipath.c
@@ -182,12 +182,14 @@ void nvme_mpath_start_request(struct request *rq)
 	struct nvme_ns *ns = rq->q->queuedata;
 	struct gendisk *disk = ns->head->disk;
 
-	if (READ_ONCE(ns->head->subsys->iopolicy) == NVME_IOPOLICY_QD) {
+	if ((READ_ONCE(ns->head->subsys->iopolicy) == NVME_IOPOLICY_QD) &&
+	    !(nvme_req(rq)->flags & NVME_MPATH_CNT_ACTIVE)) {
 		atomic_inc(&ns->ctrl->nr_active);
 		nvme_req(rq)->flags |= NVME_MPATH_CNT_ACTIVE;
 	}
 
-	if (!blk_queue_io_stat(disk->queue) || blk_rq_is_passthrough(rq))
+	if (!blk_queue_io_stat(disk->queue) || blk_rq_is_passthrough(rq) ||
+	    (nvme_req(rq)->flags & NVME_MPATH_IO_STATS))
 		return;
 
 	nvme_req(rq)->flags |= NVME_MPATH_IO_STATS;
-- 
2.43.0