[PATCH 2/3] nvme: multipath: only update ctrl->nr_active when using queue-depth iopolicy

["Ewan D. Milne" <emilne@redhat.com>]

The atomic updates of ctrl->nr_active are unnecessary when using
numa or round-robin iopolicy, so avoid that cost on a per-request basis.
Clear nr_active when changing iopolicy and do not decrement below zero.
(This handles changing the iopolicy while requests are in flight.)

Signed-off-by: Ewan D. Milne <emilne@redhat.com>
---
 drivers/nvme/host/core.c      |  2 +-
 drivers/nvme/host/multipath.c | 21 ++++++++++++++++++---
 drivers/nvme/host/nvme.h      |  2 ++
 3 files changed, 21 insertions(+), 4 deletions(-)

diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
index 75a1b58a7a43..9bc19755be77 100644
--- a/drivers/nvme/host/core.c
+++ b/drivers/nvme/host/core.c
@@ -110,7 +110,7 @@ struct workqueue_struct *nvme_delete_wq;
 EXPORT_SYMBOL_GPL(nvme_delete_wq);
 
 static LIST_HEAD(nvme_subsystems);
-static DEFINE_MUTEX(nvme_subsystems_lock);
+DEFINE_MUTEX(nvme_subsystems_lock);
 
 static DEFINE_IDA(nvme_instance_ida);
 static dev_t nvme_ctrl_base_chr_devt;
diff --git a/drivers/nvme/host/multipath.c b/drivers/nvme/host/multipath.c
index 4c2690cddef3..e184e7c377bc 100644
--- a/drivers/nvme/host/multipath.c
+++ b/drivers/nvme/host/multipath.c
@@ -133,7 +133,8 @@ void nvme_mpath_start_request(struct request *rq)
 	if (!blk_queue_io_stat(disk->queue) || blk_rq_is_passthrough(rq))
 		return;
 
-	atomic_inc(&ns->ctrl->nr_active);
+	if (READ_ONCE(ns->head->subsys->iopolicy) == NVME_IOPOLICY_QD)
+		atomic_inc(&ns->ctrl->nr_active);
 	nvme_req(rq)->flags |= NVME_MPATH_IO_STATS;
 	nvme_req(rq)->start_time = bdev_start_io_acct(disk->part0, req_op(rq),
 						      jiffies);
@@ -147,7 +148,8 @@ void nvme_mpath_end_request(struct request *rq)
 	if (!(nvme_req(rq)->flags & NVME_MPATH_IO_STATS))
 		return;
 
-	atomic_dec(&ns->ctrl->nr_active);
+	if (READ_ONCE(ns->head->subsys->iopolicy) == NVME_IOPOLICY_QD)
+		atomic_dec_if_positive(&ns->ctrl->nr_active);
 	bdev_end_io_acct(ns->head->disk->part0, req_op(rq),
 			 blk_rq_bytes(rq) >> SECTOR_SHIFT,
 			 nvme_req(rq)->start_time);
@@ -848,6 +850,19 @@ static ssize_t nvme_subsys_iopolicy_show(struct device *dev,
 			  nvme_iopolicy_names[READ_ONCE(subsys->iopolicy)]);
 }
 
+void nvme_subsys_iopolicy_update(struct nvme_subsystem *subsys, int iopolicy)
+{
+	struct nvme_ctrl *ctrl;
+
+	WRITE_ONCE(subsys->iopolicy, iopolicy);
+
+	mutex_lock(&nvme_subsystems_lock);
+	list_for_each_entry(ctrl, &subsys->ctrls, subsys_entry) {
+		atomic_set(&ctrl->nr_active, 0);
+	}
+	mutex_unlock(&nvme_subsystems_lock);
+}
+
 static ssize_t nvme_subsys_iopolicy_store(struct device *dev,
 		struct device_attribute *attr, const char *buf, size_t count)
 {
@@ -857,7 +872,7 @@ static ssize_t nvme_subsys_iopolicy_store(struct device *dev,
 
 	for (i = 0; i < ARRAY_SIZE(nvme_iopolicy_names); i++) {
 		if (sysfs_streq(buf, nvme_iopolicy_names[i])) {
-			WRITE_ONCE(subsys->iopolicy, i);
+			nvme_subsys_iopolicy_update(subsys, i);
 			return count;
 		}
 	}
diff --git a/drivers/nvme/host/nvme.h b/drivers/nvme/host/nvme.h
index f0f3fd8b4197..c4469bc38d89 100644
--- a/drivers/nvme/host/nvme.h
+++ b/drivers/nvme/host/nvme.h
@@ -49,6 +49,8 @@ extern struct workqueue_struct *nvme_wq;
 extern struct workqueue_struct *nvme_reset_wq;
 extern struct workqueue_struct *nvme_delete_wq;
 
+extern struct mutex nvme_subsystems_lock;
+
 /*
  * List of workarounds for devices that required behavior not specified in
  * the standard.
-- 
2.20.1