[PATCH v2] nvme: remove namespaces in parallel

[Qiliang Yuan <odys.yuan@gmail.com>]

nvme_remove_namespaces() removes the namespaces of a controller one
after another. Each removal waits for RCU grace periods in
del_gendisk() for the namespace and for its multipath head, about
20 ms in all.

Deleting a controller, such as disconnecting from a fabrics target,
thus takes time proportional to its namespace count, over a second for
64 namespaces. Scanning had the same problem until commit 4e893ca81170
("nvme_core: scan namespaces asynchronously") made it add namespaces
in parallel.

Remove the namespaces through an async domain the same way, and wait
for all of them before returning. Each removal still waits about
20 ms, but the waits now overlap: a grace period is system-wide, so
removals that wait at the same time are released by the same grace
periods instead of each needing its own.

Disconnect time from a target on the same host, over tcp to 127.0.0.1
and over nvme-loop, with 4 I/O queues, median of 3 runs on each of two
boots per kernel:

  namespaces  transport  before              after
           8  tcp         170 /  171 ms        85 /   91 ms
          32  tcp         628 /  628 ms       294 /  293 ms
          64  tcp        1201 / 1308 ms       533 /  550 ms
          64  loop       1988 / 1913 ms      1283 / 1200 ms

Signed-off-by: Qiliang Yuan <odys.yuan@gmail.com>
---
V1 -> V2:
- Explain why removing the namespaces in parallel saves time
- Drop patch 2 until the elevator switch locking change settles:
  https://lore.kernel.org/r/20261031044141.5569-1-laveeshb@laveeshbansal.com

v1: https://lore.kernel.org/r/20261102-bug-nvme-remove-ns-async-v1-0-61780fee111c@gmail.com
---
 drivers/nvme/host/core.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
index a7a4d09644c13..18948c2ba218d 100644
--- a/drivers/nvme/host/core.c
+++ b/drivers/nvme/host/core.c
@@ -4716,6 +4716,11 @@ static void nvme_scan_work(struct work_struct *work)
 #endif
 }
 
+static void nvme_ns_remove_async(void *data, async_cookie_t cookie)
+{
+	nvme_ns_remove(data);
+}
+
 /*
  * This function iterates the namespace list unlocked to allow recovery from
  * controller failure. It is up to the caller to ensure the namespace list is
@@ -4725,6 +4730,7 @@ void nvme_remove_namespaces(struct nvme_ctrl *ctrl)
 {
 	struct nvme_ns *ns, *next;
 	LIST_HEAD(ns_list);
+	ASYNC_DOMAIN(domain);
 
 	/*
 	 * make sure to requeue I/O to all namespaces as these
@@ -4759,8 +4765,13 @@ void nvme_remove_namespaces(struct nvme_ctrl *ctrl)
 	mutex_unlock(&ctrl->namespaces_lock);
 	synchronize_srcu(&ctrl->srcu);
 
+	/*
+	 * Remove the namespaces in parallel, as each removal waits for several
+	 * RCU grace periods.
+	 */
 	list_for_each_entry_safe(ns, next, &ns_list, list)
-		nvme_ns_remove(ns);
+		async_schedule_domain(nvme_ns_remove_async, ns, &domain);
+	async_synchronize_full_domain(&domain);
 }
 EXPORT_SYMBOL_GPL(nvme_remove_namespaces);
 

---
base-commit: 6628e7d4c6b25c8d18f1345e3a20d21829aae7e4
change-id: 20261102-bug-nvme-remove-ns-async-ceaaa1eda6a7

Best regards,
-- 
Qiliang Yuan <odys.yuan@gmail.com>