[PATCH v3] nvme_core: scan namespaces asynchronously

[Stuart Hayes]

Use async function calls to make namespace scanning happen in parallel.

Without the patch, NVME namespaces are scanned serially, so it can take
a long time for all of a controller's namespaces to become available,
especially with a slower (TCP) interface with large number of
namespaces.

It is not uncommon to have large numbers (hundreds or thousands) of
namespaces on nvme-of with storage servers.

The time it took for all namespaces to show up after connecting (via
TCP) to a controller with 1002 namespaces was measured on one system:

network latency   without patch   with patch
     0                 6s            1s
    50ms             210s           10s
   100ms             417s           18s

Measurements taken on another system show the effect of the patch on the
time nvme_scan_work() took to complete, when connecting to a linux
nvme-of target with varying numbers of namespaces, on a network of
400us.

namespaces    without patch   with patch
     1            16ms           14ms
     2            24ms           16ms
     4            49ms           22ms
     8           101ms           33ms
    16           207ms           56ms
   100           1.4s           0.6s
  1000          12.9s           2.0s

On the same system, connecting to a local PCIe NVMe drive (a Samsung
PM1733) instead of a network target:

namespaces    without patch   with patch
     1            13ms           12ms
     2            41ms           13ms

Signed-off-by: Stuart Hayes <stuart.w.hayes@xxxxxxxxx>
---
changes from V2:
  * make a separate function nvme_scan_ns_async() that calls
    nvme_scan_ns(), instead of modifying nvme_scan_ns()
  * only scan asynchronously from nvme_scan_ns_list(), not from
    nvme_scan_ns_sequential()
  * provide more timing data in the commit message

changes from V1:
  * remove module param to enable/disable async scanning
  * add scan time measurements to commit message


 drivers/nvme/host/core.c | 48 +++++++++++++++++++++++++++++++++-------
 1 file changed, 40 insertions(+), 8 deletions(-)

diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
index 782090ce0bc1..dbf05cfea063 100644
--- a/drivers/nvme/host/core.c
+++ b/drivers/nvme/host/core.c
@@ -4,6 +4,7 @@
  * Copyright (c) 2011-2014, Intel Corporation.
  */
 
+#include <linux/async.h>
 #include <linux/blkdev.h>
 #include <linux/blk-mq.h>
 #include <linux/blk-integrity.h>
@@ -3952,6 +3953,30 @@ static void nvme_scan_ns(struct nvme_ctrl *ctrl, unsigned nsid)
 	}
 }
 
+/*
+ * struct async_scan_info - keeps track of controller & NSIDs to scan
+ * @ctrl:	Controller on which namespaces are being scanned
+ * @next_idx:	Index of next NSID to scan in ns_list
+ * @ns_list:	Pointer to list of NSIDs to scan
+ */
+struct async_scan_info {
+	struct nvme_ctrl *ctrl;
+	atomic_t next_idx;
+	__le32 *ns_list;
+};
+
+static void nvme_scan_ns_async(void *data, async_cookie_t cookie)
+{
+	struct async_scan_info *scan_info = data;
+	int idx;
+	u32 nsid;
+
+	idx = (u32)atomic_fetch_add(1, &scan_info->next_idx);
+	nsid = le32_to_cpu(scan_info->ns_list[idx]);
+
+	nvme_scan_ns(scan_info->ctrl, nsid);
+}
+
 static void nvme_remove_invalid_namespaces(struct nvme_ctrl *ctrl,
 					unsigned nsid)
 {
@@ -3975,12 +4000,14 @@ static void nvme_remove_invalid_namespaces(struct nvme_ctrl *ctrl,
 static int nvme_scan_ns_list(struct nvme_ctrl *ctrl)
 {
 	const int nr_entries = NVME_IDENTIFY_DATA_SIZE / sizeof(__le32);
-	__le32 *ns_list;
+	struct async_scan_info scan_info;
 	u32 prev = 0;
 	int ret = 0, i;
+	ASYNC_DOMAIN(domain);
 
-	ns_list = kzalloc(NVME_IDENTIFY_DATA_SIZE, GFP_KERNEL);
-	if (!ns_list)
+	scan_info.ctrl = ctrl;
+	scan_info.ns_list = kzalloc(NVME_IDENTIFY_DATA_SIZE, GFP_KERNEL);
+	if (!scan_info.ns_list)
 		return -ENOMEM;
 
 	for (;;) {
@@ -3990,28 +4017,33 @@ static int nvme_scan_ns_list(struct nvme_ctrl *ctrl)
 			.identify.nsid		= cpu_to_le32(prev),
 		};
 
-		ret = nvme_submit_sync_cmd(ctrl->admin_q, &cmd, ns_list,
-					    NVME_IDENTIFY_DATA_SIZE);
+		ret = nvme_submit_sync_cmd(ctrl->admin_q, &cmd,
+					   scan_info.ns_list,
+					   NVME_IDENTIFY_DATA_SIZE);
 		if (ret) {
 			dev_warn(ctrl->device,
 				"Identify NS List failed (status=0x%x)\n", ret);
 			goto free;
 		}
 
+		atomic_set(&scan_info.next_idx, 0);
 		for (i = 0; i < nr_entries; i++) {
-			u32 nsid = le32_to_cpu(ns_list[i]);
+			u32 nsid = le32_to_cpu(scan_info.ns_list[i]);
 
 			if (!nsid)	/* end of the list? */
 				goto out;
-			nvme_scan_ns(ctrl, nsid);
+			async_schedule_domain(nvme_scan_ns_async, &scan_info,
+						&domain);
 			while (++prev < nsid)
 				nvme_ns_remove_by_nsid(ctrl, prev);
 		}
+		async_synchronize_full_domain(&domain);
 	}
  out:
 	nvme_remove_invalid_namespaces(ctrl, prev);
  free:
-	kfree(ns_list);
+	async_synchronize_full_domain(&domain);
+	kfree(scan_info.ns_list);
 	return ret;
 }
 
-- 
2.39.3