nvme_core: scan namespaces asynchronously

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@gmail.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>

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>
@@ -4040,6 +4041,35 @@ 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_nsid:	Index of next NSID to scan in ns_list
+ * @ns_list:	Pointer to list of NSIDs to scan
+ *
+ * Note: There is a single async_scan_info structure shared by all instances
+ * of nvme_scan_ns_async() scanning a given controller, so the atomic
+ * operations on next_nsid are critical to ensure each instance scans a unique
+ * NSID.
+ */
+struct async_scan_info {
+	struct nvme_ctrl *ctrl;
+	atomic_t next_nsid;
+	__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_inc(&scan_info->next_nsid);
+	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)
 {
@@ -4066,11 +4096,15 @@ static int nvme_scan_ns_list(struct nvme_ctrl *ctrl)
 	__le32 *ns_list;
 	u32 prev = 0;
 	int ret = 0, i;
+	ASYNC_DOMAIN(domain);
+	struct async_scan_info scan_info;
 
 	ns_list = kzalloc(NVME_IDENTIFY_DATA_SIZE, GFP_KERNEL);
 	if (!ns_list)
 		return -ENOMEM;
 
+	scan_info.ctrl = ctrl;
+	scan_info.ns_list = ns_list;
 	for (;;) {
 		struct nvme_command cmd = {
 			.identify.opcode	= nvme_admin_identify,
@@ -4086,19 +4120,23 @@ static int nvme_scan_ns_list(struct nvme_ctrl *ctrl)
 			goto free;
 		}
 
+		atomic_set(&scan_info.next_nsid, 0);
 		for (i = 0; i < nr_entries; i++) {
 			u32 nsid = le32_to_cpu(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:
+	async_synchronize_full_domain(&domain);
 	kfree(ns_list);
 	return ret;
 }