zvol: Support blk-mq for better performance

Add support for the kernel's block multiqueue (blk-mq) interface in
the zvol block driver.  blk-mq creates multiple request queues on
different CPUs rather than having a single request queue.  This can
improve zvol performance with multithreaded reads/writes.

This implementation uses the blk-mq interfaces on 4.13 or newer
kernels.  Building against older kernels will fall back to the
older BIO interfaces.

Note that you must set the `zvol_use_blk_mq` module param to
enable the blk-mq API.  It is disabled by default.

Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Issue #12483

config/kernel-blk-queue.m4

@@ -315,6 +315,36 @@ AC_DEFUN([ZFS_AC_KERNEL_BLK_QUEUE_MAX_SEGMENTS], [
 	])
 ])
 
+dnl #
+dnl # See if kernel supports block multi-queue and blk_status_t.
+dnl # blk_status_t represents the new status codes introduced in the 4.13
+dnl # kernel patch:
+dnl #
+dnl #  block: introduce new block status code type
+dnl #
+dnl # We do not currently support the "old" block multi-queue interfaces from
+dnl # prior kernels.
+dnl #
+AC_DEFUN([ZFS_AC_KERNEL_SRC_BLK_MQ], [
+	ZFS_LINUX_TEST_SRC([blk_mq], [
+		#include <linux/blk-mq.h>
+	], [
+		struct blk_mq_tag_set tag_set = {0};
+		(void) blk_mq_alloc_tag_set(&tag_set);
+		return BLK_STS_OK;
+	], [$NO_UNUSED_BUT_SET_VARIABLE])
+])
+
+AC_DEFUN([ZFS_AC_KERNEL_BLK_MQ], [
+	AC_MSG_CHECKING([whether block multiqueue with blk_status_t is available])
+	ZFS_LINUX_TEST_RESULT([blk_mq], [
+		AC_MSG_RESULT(yes)
+		AC_DEFINE(HAVE_BLK_MQ, 1, [block multiqueue is available])
+	], [
+		AC_MSG_RESULT(no)
+	])
+])
+
 AC_DEFUN([ZFS_AC_KERNEL_SRC_BLK_QUEUE], [
 	ZFS_AC_KERNEL_SRC_BLK_QUEUE_PLUG
 	ZFS_AC_KERNEL_SRC_BLK_QUEUE_BDI
@@ -326,6 +356,7 @@ AC_DEFUN([ZFS_AC_KERNEL_SRC_BLK_QUEUE], [
 	ZFS_AC_KERNEL_SRC_BLK_QUEUE_FLUSH
 	ZFS_AC_KERNEL_SRC_BLK_QUEUE_MAX_HW_SECTORS
 	ZFS_AC_KERNEL_SRC_BLK_QUEUE_MAX_SEGMENTS
+	ZFS_AC_KERNEL_SRC_BLK_MQ
 ])
 
 AC_DEFUN([ZFS_AC_KERNEL_BLK_QUEUE], [
@@ -339,4 +370,5 @@ AC_DEFUN([ZFS_AC_KERNEL_BLK_QUEUE], [
 	ZFS_AC_KERNEL_BLK_QUEUE_FLUSH
 	ZFS_AC_KERNEL_BLK_QUEUE_MAX_HW_SECTORS
 	ZFS_AC_KERNEL_BLK_QUEUE_MAX_SEGMENTS
+	ZFS_AC_KERNEL_BLK_MQ
 ])

configure.ac

@@ -404,6 +404,7 @@ AC_CONFIG_FILES([
 	tests/zfs-tests/tests/functional/zpool_influxdb/Makefile
 	tests/zfs-tests/tests/functional/zvol/Makefile
 	tests/zfs-tests/tests/functional/zvol/zvol_ENOSPC/Makefile
+	tests/zfs-tests/tests/functional/zvol/zvol_stress/Makefile
 	tests/zfs-tests/tests/functional/zvol/zvol_cli/Makefile
 	tests/zfs-tests/tests/functional/zvol/zvol_misc/Makefile
 	tests/zfs-tests/tests/functional/zvol/zvol_swap/Makefile

man/man4/zfs.4

@@ -2183,9 +2183,55 @@ for each I/O submitter.
 When unset, requests are handled asynchronously by a thread pool.
 The number of requests which can be handled concurrently is controlled by
 .Sy zvol_threads .
-.
-.It Sy zvol_threads Ns = Ns Sy 32 Pq uint
-Max number of threads which can handle zvol I/O requests concurrently.
+.Sy zvol_request_sync
+is ignored when running on a kernel that supports block multiqueue
+.Pq Li blk-mq .
+.
+.It Sy zvol_threads Ns = Ns Sy 0 Pq uint
+The number of threads to use for processing zvol block IOs.
+On older
+.No non- Ns Li blk-mq
+kernels,
+.Sy zvol_threads
+is the total number of threads to use for all zvols.
+On kernels that support
+.Li blk-mq
+.Sy zvol_threads
+is the total number of threads per zvol.
+If
+.Sy 0
+(the default) then internally set
+.Sy zvol_threads
+to the number of CPUs present.
+.It Sy zvol_use_blk_mq Ns = Ns Sy 0 Ns | Ns 1 Pq uint
+Set to
+.Sy 1
+to use the
+.Li blk-mq
+API for zvols.
+Set to
+.Sy 0
+(the default) to use the legacy zvol APIs.
+This setting can give better or worse zvol performance depending on
+the workload.
+This parameter will only appear if your kernel supports
+.Li blk-mq
+and is only read and assigned to a zvol at zvol load time.
+.
+.It Sy zvol_blk_mq_queue_depth Ns = Ns Sy 0 Pq uint
+The queue_depth value for the zvol
+.Li blk-mq
+interface.
+This parameter will only appear if your kernel supports
+.Li blk-mq
+and is only read at zvol load time.
+If
+.Sy 0
+(the default) then use the kernel's default queue depth.
+If you set
+.Sy zvol_blk_mq_queue_depth
+lower than the kernel's minimum queue depth, it will be internally
+capped to the kernel's minimum queue depth (currently 4 on 5.15 kernels).
 .
 .It Sy zvol_volmode Ns = Ns Sy 1 Pq uint
 Defines zvol block devices behaviour when