[PATCH v2] nvme: core: shorten duration of multipath namespace rescan

[Martin Wilck]

For multipath devices, nvme_update_ns_info() needs to freeze both
the queue of the path and the queue of the multipath device. For
both operations, it waits for one RCU grace period to pass, ~25ms
on my test system. By calling blk_freeze_queue_start() for the
multipath queue early, we avoid waiting twice; tests using ftrace
have shown that the second blk_mq_freeze_queue_wait() call finishes
in just a few microseconds. The path queue is unfrozen before
calling blk_mq_freeze_queue_wait() on the multipath queue, so that
possibly outstanding IO in the multipath queue can be flushed.

I tested this using the "controller rescan under I/O load" test
I submitted recently [1].

[1] https://lore.kernel.org/linux-nvme/20240822193814.106111-3-mwilck@suse.com/T/#u

Signed-off-by: Martin Wilck <mwilck at suse.com>
---
v2: (all changes suggested by Sagi Grimberg)
 - patch subject changed from "nvme: core: freeze multipath queue early in
   nvme_update_ns_info()" to "nvme: core: shorten duration of multipath
   namespace rescan"
 - inserted comment explaining why blk_freeze_queue_start() is called early
 - wait for queue to be frozen even if ret != 0
 - make code structure more obvious vs. freeze_start / freeze_wait / unfreeze

Hannes and Daniel had already added Reviewed-by: tags to the v1 patch, but
I didn't add them above, because the patch looks quite different now.

---
 drivers/nvme/host/core.c | 21 ++++++++++++++++-----
 1 file changed, 16 insertions(+), 5 deletions(-)

diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
index 0dc8bcc664f2..1ea052450846 100644
--- a/drivers/nvme/host/core.c
+++ b/drivers/nvme/host/core.c
@@ -2217,6 +2217,16 @@ static int nvme_update_ns_info(struct nvme_ns *ns, struct nvme_ns_info *info)
 	bool unsupported = false;
 	int ret;
 
+	/*
+	 * The controller queue is going to be frozen in
+	 * nvme_update_ns_info_{generic,block}(). Every freeze implies waiting
+	 * for an RCU grace period to pass. For multipath devices, we
+	 * need to freeze the multipath queue, too. Start freezing the
+	 * multipath queue now, lest we need to wait for two grace periods.
+	 */
+	if (nvme_ns_head_multipath(ns->head))
+		blk_freeze_queue_start(ns->head->disk->queue);
+
 	switch (info->ids.csi) {
 	case NVME_CSI_ZNS:
 		if (!IS_ENABLED(CONFIG_BLK_DEV_ZONED)) {
@@ -2250,11 +2260,13 @@ static int nvme_update_ns_info(struct nvme_ns *ns, struct nvme_ns_info *info)
 		ret = 0;
 	}
 
-	if (!ret && nvme_ns_head_multipath(ns->head)) {
+	if (!nvme_ns_head_multipath(ns->head))
+		return ret;
+
+	blk_mq_freeze_queue_wait(ns->head->disk->queue);
+	if (!ret) {
 		struct queue_limits *ns_lim = &ns->disk->queue->limits;
 		struct queue_limits lim;
-
-		blk_mq_freeze_queue(ns->head->disk->queue);
 		/*
 		 * queue_limits mixes values that are the hardware limitations
 		 * for bio splitting with what is the device configuration.
@@ -2286,9 +2298,8 @@ static int nvme_update_ns_info(struct nvme_ns *ns, struct nvme_ns_info *info)
 		set_capacity_and_notify(ns->head->disk, get_capacity(ns->disk));
 		set_disk_ro(ns->head->disk, nvme_ns_is_readonly(ns, info));
 		nvme_mpath_revalidate_paths(ns);
-
-		blk_mq_unfreeze_queue(ns->head->disk->queue);
 	}
+	blk_mq_unfreeze_queue(ns->head->disk->queue);
 
 	return ret;
 }
-- 
2.46.0