dm mpath: add queue length load balancer

This patch adds a dynamic load balancer, dm-queue-length, which
balances the number of in-flight I/Os across the paths.

The code is based on the patch posted by Stefan Bader:
https://www.redhat.com/archives/dm-devel/2005-October/msg00050.html

Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Kiyoshi Ueda <k-ueda@ct.jp.nec.com>
Signed-off-by: Jun'ichi Nomura <j-nomura@ce.jp.nec.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

3 files changed, 273 insertions, 0 deletions

diff --git a/drivers/md/Kconfig b/drivers/md/Kconfig
index 36e0675be9f7..3b311d273346 100644
--- a/drivers/md/Kconfig
+++ b/drivers/md/Kconfig
@@ -249,6 +249,15 @@ config DM_MULTIPATH
         ---help---
           Allow volume managers to support multipath hardware.
 
+config DM_MULTIPATH_QL
+        tristate "I/O Path Selector based on the number of in-flight I/Os"
+        depends on DM_MULTIPATH
+        ---help---
+          This path selector is a dynamic load balancer which selects
+          the path with the least number of in-flight I/Os.
+
+          If unsure, say N.
+
 config DM_DELAY
         tristate "I/O delaying target (EXPERIMENTAL)"
         depends on BLK_DEV_DM && EXPERIMENTAL
diff --git a/drivers/md/Makefile b/drivers/md/Makefile
index 45cc5951d928..ff9f545dd516 100644
--- a/drivers/md/Makefile
+++ b/drivers/md/Makefile
@@ -36,6 +36,7 @@ obj-$(CONFIG_BLK_DEV_DM)	+= dm-mod.o
 obj-$(CONFIG_DM_CRYPT)          += dm-crypt.o
 obj-$(CONFIG_DM_DELAY)          += dm-delay.o
 obj-$(CONFIG_DM_MULTIPATH)      += dm-multipath.o dm-round-robin.o
+obj-$(CONFIG_DM_MULTIPATH_QL)   += dm-queue-length.o
 obj-$(CONFIG_DM_SNAPSHOT)       += dm-snapshot.o
 obj-$(CONFIG_DM_MIRROR)         += dm-mirror.o dm-log.o dm-region-hash.o
 obj-$(CONFIG_DM_ZERO)           += dm-zero.o
diff --git a/drivers/md/dm-queue-length.c b/drivers/md/dm-queue-length.c
new file mode 100644
index 000000000000..f92b6cea9d9c
--- /dev/null
+++ b/drivers/md/dm-queue-length.c
@@ -0,0 +1,263 @@
+/*
+ * Copyright (C) 2004-2005 IBM Corp.  All Rights Reserved.
+ * Copyright (C) 2006-2009 NEC Corporation.
+ *
+ * dm-queue-length.c
+ *
+ * Module Author: Stefan Bader, IBM
+ * Modified by: Kiyoshi Ueda, NEC
+ *
+ * This file is released under the GPL.
+ *
+ * queue-length path selector - choose a path with the least number of
+ * in-flight I/Os.
+ */
+
+#include "dm.h"
+#include "dm-path-selector.h"
+
+#include <linux/slab.h>
+#include <linux/ctype.h>
+#include <linux/errno.h>
+#include <linux/module.h>
+#include <asm/atomic.h>
+
+#define DM_MSG_PREFIX   "multipath queue-length"
+#define QL_MIN_IO       128
+#define QL_VERSION      "0.1.0"
+
+struct selector {
+        struct list_head        valid_paths;
+        struct list_head        failed_paths;
+};
+
+struct path_info {
+        struct list_head        list;
+        struct dm_path          *path;
+        unsigned                repeat_count;
+        atomic_t                qlen;   /* the number of in-flight I/Os */
+};
+
+static struct selector *alloc_selector(void)
+{
+        struct selector *s = kmalloc(sizeof(*s), GFP_KERNEL);
+
+        if (s) {
+                INIT_LIST_HEAD(&s->valid_paths);
+                INIT_LIST_HEAD(&s->failed_paths);
+        }
+
+        return s;
+}
+
+static int ql_create(struct path_selector *ps, unsigned argc, char **argv)
+{
+        struct selector *s = alloc_selector();
+
+        if (!s)
+                return -ENOMEM;
+
+        ps->context = s;
+        return 0;
+}
+
+static void ql_free_paths(struct list_head *paths)
+{
+        struct path_info *pi, *next;
+
+        list_for_each_entry_safe(pi, next, paths, list) {
+                list_del(&pi->list);
+                kfree(pi);
+        }
+}
+
+static void ql_destroy(struct path_selector *ps)
+{
+        struct selector *s = ps->context;
+
+        ql_free_paths(&s->valid_paths);
+        ql_free_paths(&s->failed_paths);
+        kfree(s);
+        ps->context = NULL;
+}
+
+static int ql_status(struct path_selector *ps, struct dm_path *path,
+                     status_type_t type, char *result, unsigned maxlen)
+{
+        unsigned sz = 0;
+        struct path_info *pi;
+
+        /* When called with NULL path, return selector status/args. */
+        if (!path)
+                DMEMIT("0 ");
+        else {
+                pi = path->pscontext;
+
+                switch (type) {
+                case STATUSTYPE_INFO:
+                        DMEMIT("%d ", atomic_read(&pi->qlen));
+                        break;
+                case STATUSTYPE_TABLE:
+                        DMEMIT("%u ", pi->repeat_count);
+                        break;
+                }
+        }
+
+        return sz;
+}
+
+static int ql_add_path(struct path_selector *ps, struct dm_path *path,
+                       int argc, char **argv, char **error)
+{
+        struct selector *s = ps->context;
+        struct path_info *pi;
+        unsigned repeat_count = QL_MIN_IO;
+
+        /*
+         * Arguments: [<repeat_count>]
+         *      <repeat_count>: The number of I/Os before switching path.
+         *                      If not given, default (QL_MIN_IO) is used.
+         */
+        if (argc > 1) {
+                *error = "queue-length ps: incorrect number of arguments";
+                return -EINVAL;
+        }
+
+        if ((argc == 1) && (sscanf(argv[0], "%u", &repeat_count) != 1)) {
+                *error = "queue-length ps: invalid repeat count";
+                return -EINVAL;
+        }
+
+        /* Allocate the path information structure */
+        pi = kmalloc(sizeof(*pi), GFP_KERNEL);
+        if (!pi) {
+                *error = "queue-length ps: Error allocating path information";
+                return -ENOMEM;
+        }
+
+        pi->path = path;
+        pi->repeat_count = repeat_count;
+        atomic_set(&pi->qlen, 0);
+
+        path->pscontext = pi;
+
+        list_add_tail(&pi->list, &s->valid_paths);
+
+        return 0;
+}
+
+static void ql_fail_path(struct path_selector *ps, struct dm_path *path)
+{
+        struct selector *s = ps->context;
+        struct path_info *pi = path->pscontext;
+
+        list_move(&pi->list, &s->failed_paths);
+}
+
+static int ql_reinstate_path(struct path_selector *ps, struct dm_path *path)
+{
+        struct selector *s = ps->context;
+        struct path_info *pi = path->pscontext;
+
+        list_move_tail(&pi->list, &s->valid_paths);
+
+        return 0;
+}
+
+/*
+ * Select a path having the minimum number of in-flight I/Os
+ */
+static struct dm_path *ql_select_path(struct path_selector *ps,
+                                      unsigned *repeat_count, size_t nr_bytes)
+{
+        struct selector *s = ps->context;
+        struct path_info *pi = NULL, *best = NULL;
+
+        if (list_empty(&s->valid_paths))
+                return NULL;
+
+        /* Change preferred (first in list) path to evenly balance. */
+        list_move_tail(s->valid_paths.next, &s->valid_paths);
+
+        list_for_each_entry(pi, &s->valid_paths, list) {
+                if (!best ||
+                    (atomic_read(&pi->qlen) < atomic_read(&best->qlen)))
+                        best = pi;
+
+                if (!atomic_read(&best->qlen))
+                        break;
+        }
+
+        if (!best)
+                return NULL;
+
+        *repeat_count = best->repeat_count;
+
+        return best->path;
+}
+
+static int ql_start_io(struct path_selector *ps, struct dm_path *path,
+                       size_t nr_bytes)
+{
+        struct path_info *pi = path->pscontext;
+
+        atomic_inc(&pi->qlen);
+
+        return 0;
+}
+
+static int ql_end_io(struct path_selector *ps, struct dm_path *path,
+                     size_t nr_bytes)
+{
+        struct path_info *pi = path->pscontext;
+
+        atomic_dec(&pi->qlen);
+
+        return 0;
+}
+
+static struct path_selector_type ql_ps = {
+        .name           = "queue-length",
+        .module         = THIS_MODULE,
+        .table_args     = 1,
+        .info_args      = 1,
+        .create         = ql_create,
+        .destroy        = ql_destroy,
+        .status         = ql_status,
+        .add_path       = ql_add_path,
+        .fail_path      = ql_fail_path,
+        .reinstate_path = ql_reinstate_path,
+        .select_path    = ql_select_path,
+        .start_io       = ql_start_io,
+        .end_io         = ql_end_io,
+};
+
+static int __init dm_ql_init(void)
+{
+        int r = dm_register_path_selector(&ql_ps);
+
+        if (r < 0)
+                DMERR("register failed %d", r);
+
+        DMINFO("version " QL_VERSION " loaded");
+
+        return r;
+}
+
+static void __exit dm_ql_exit(void)
+{
+        int r = dm_unregister_path_selector(&ql_ps);
+
+        if (r < 0)
+                DMERR("unregister failed %d", r);
+}
+
+module_init(dm_ql_init);
+module_exit(dm_ql_exit);
+
+MODULE_AUTHOR("Stefan Bader <Stefan.Bader at de.ibm.com>");
+MODULE_DESCRIPTION(
+        "(C) Copyright IBM Corp. 2004,2005   All Rights Reserved.\n"
+        DM_NAME " path selector to balance the number of in-flight I/Os"
+);
+MODULE_LICENSE("GPL");