dm mpath: add service time load balancer

This patch adds a service time oriented dynamic load balancer,
dm-service-time, which selects the path with the shortest estimated
service time for the incoming I/O.
The service time is estimated by dividing the in-flight I/O size
by a performance value of each path.

The performance value can be given as a table argument at the table
loading time.  If no performance value is given, all paths are
considered equal.

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>

1 files changed, 339 insertions, 0 deletions

diff --git a/drivers/md/dm-service-time.c b/drivers/md/dm-service-time.c
new file mode 100644
index 000000000000..cfa668f46c40
--- /dev/null
+++ b/drivers/md/dm-service-time.c
@@ -0,0 +1,339 @@
+/*
+ * Copyright (C) 2007-2009 NEC Corporation.  All Rights Reserved.
+ *
+ * Module Author: Kiyoshi Ueda
+ *
+ * This file is released under the GPL.
+ *
+ * Throughput oriented path selector.
+ */
+
+#include "dm.h"
+#include "dm-path-selector.h"
+
+#define DM_MSG_PREFIX   "multipath service-time"
+#define ST_MIN_IO       1
+#define ST_MAX_RELATIVE_THROUGHPUT      100
+#define ST_MAX_RELATIVE_THROUGHPUT_SHIFT        7
+#define ST_MAX_INFLIGHT_SIZE    ((size_t)-1 >> ST_MAX_RELATIVE_THROUGHPUT_SHIFT)
+#define ST_VERSION      "0.2.0"
+
+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;
+        unsigned relative_throughput;
+        atomic_t in_flight_size;        /* Total size 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 st_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 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 st_destroy(struct path_selector *ps)
+{
+        struct selector *s = ps->context;
+
+        free_paths(&s->valid_paths);
+        free_paths(&s->failed_paths);
+        kfree(s);
+        ps->context = NULL;
+}
+
+static int st_status(struct path_selector *ps, struct dm_path *path,
+                     status_type_t type, char *result, unsigned maxlen)
+{
+        unsigned sz = 0;
+        struct path_info *pi;
+
+        if (!path)
+                DMEMIT("0 ");
+        else {
+                pi = path->pscontext;
+
+                switch (type) {
+                case STATUSTYPE_INFO:
+                        DMEMIT("%d %u ", atomic_read(&pi->in_flight_size),
+                               pi->relative_throughput);
+                        break;
+                case STATUSTYPE_TABLE:
+                        DMEMIT("%u %u ", pi->repeat_count,
+                               pi->relative_throughput);
+                        break;
+                }
+        }
+
+        return sz;
+}
+
+static int st_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 = ST_MIN_IO;
+        unsigned relative_throughput = 1;
+
+        /*
+         * Arguments: [<repeat_count> [<relative_throughput>]]
+         *      <repeat_count>: The number of I/Os before switching path.
+         *                      If not given, default (ST_MIN_IO) is used.
+         *      <relative_throughput>: The relative throughput value of
+         *                      the path among all paths in the path-group.
+         *                      The valid range: 0-<ST_MAX_RELATIVE_THROUGHPUT>
+         *                      If not given, minimum value '1' is used.
+         *                      If '0' is given, the path isn't selected while
+         *                      other paths having a positive value are
+         *                      available.
+         */
+        if (argc > 2) {
+                *error = "service-time ps: incorrect number of arguments";
+                return -EINVAL;
+        }
+
+        if (argc && (sscanf(argv[0], "%u", &repeat_count) != 1)) {
+                *error = "service-time ps: invalid repeat count";
+                return -EINVAL;
+        }
+
+        if ((argc == 2) &&
+            (sscanf(argv[1], "%u", &relative_throughput) != 1 ||
+             relative_throughput > ST_MAX_RELATIVE_THROUGHPUT)) {
+                *error = "service-time ps: invalid relative_throughput value";
+                return -EINVAL;
+        }
+
+        /* allocate the path */
+        pi = kmalloc(sizeof(*pi), GFP_KERNEL);
+        if (!pi) {
+                *error = "service-time ps: Error allocating path context";
+                return -ENOMEM;
+        }
+
+        pi->path = path;
+        pi->repeat_count = repeat_count;
+        pi->relative_throughput = relative_throughput;
+        atomic_set(&pi->in_flight_size, 0);
+
+        path->pscontext = pi;
+
+        list_add_tail(&pi->list, &s->valid_paths);
+
+        return 0;
+}
+
+static void st_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 st_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;
+}
+
+/*
+ * Compare the estimated service time of 2 paths, pi1 and pi2,
+ * for the incoming I/O.
+ *
+ * Returns:
+ * < 0 : pi1 is better
+ * 0   : no difference between pi1 and pi2
+ * > 0 : pi2 is better
+ *
+ * Description:
+ * Basically, the service time is estimated by:
+ *     ('pi->in-flight-size' + 'incoming') / 'pi->relative_throughput'
+ * To reduce the calculation, some optimizations are made.
+ * (See comments inline)
+ */
+static int st_compare_load(struct path_info *pi1, struct path_info *pi2,
+                           size_t incoming)
+{
+        size_t sz1, sz2, st1, st2;
+
+        sz1 = atomic_read(&pi1->in_flight_size);
+        sz2 = atomic_read(&pi2->in_flight_size);
+
+        /*
+         * Case 1: Both have same throughput value. Choose less loaded path.
+         */
+        if (pi1->relative_throughput == pi2->relative_throughput)
+                return sz1 - sz2;
+
+        /*
+         * Case 2a: Both have same load. Choose higher throughput path.
+         * Case 2b: One path has no throughput value. Choose the other one.
+         */
+        if (sz1 == sz2 ||
+            !pi1->relative_throughput || !pi2->relative_throughput)
+                return pi2->relative_throughput - pi1->relative_throughput;
+
+        /*
+         * Case 3: Calculate service time. Choose faster path.
+         *         Service time using pi1:
+         *             st1 = (sz1 + incoming) / pi1->relative_throughput
+         *         Service time using pi2:
+         *             st2 = (sz2 + incoming) / pi2->relative_throughput
+         *
+         *         To avoid the division, transform the expression to use
+         *         multiplication.
+         *         Because ->relative_throughput > 0 here, if st1 < st2,
+         *         the expressions below are the same meaning:
+         *             (sz1 + incoming) / pi1->relative_throughput <
+         *                 (sz2 + incoming) / pi2->relative_throughput
+         *             (sz1 + incoming) * pi2->relative_throughput <
+         *                 (sz2 + incoming) * pi1->relative_throughput
+         *         So use the later one.
+         */
+        sz1 += incoming;
+        sz2 += incoming;
+        if (unlikely(sz1 >= ST_MAX_INFLIGHT_SIZE ||
+                     sz2 >= ST_MAX_INFLIGHT_SIZE)) {
+                /*
+                 * Size may be too big for multiplying pi->relative_throughput
+                 * and overflow.
+                 * To avoid the overflow and mis-selection, shift down both.
+                 */
+                sz1 >>= ST_MAX_RELATIVE_THROUGHPUT_SHIFT;
+                sz2 >>= ST_MAX_RELATIVE_THROUGHPUT_SHIFT;
+        }
+        st1 = sz1 * pi2->relative_throughput;
+        st2 = sz2 * pi1->relative_throughput;
+        if (st1 != st2)
+                return st1 - st2;
+
+        /*
+         * Case 4: Service time is equal. Choose higher throughput path.
+         */
+        return pi2->relative_throughput - pi1->relative_throughput;
+}
+
+static struct dm_path *st_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 || (st_compare_load(pi, best, nr_bytes) < 0))
+                        best = pi;
+
+        if (!best)
+                return NULL;
+
+        *repeat_count = best->repeat_count;
+
+        return best->path;
+}
+
+static int st_start_io(struct path_selector *ps, struct dm_path *path,
+                       size_t nr_bytes)
+{
+        struct path_info *pi = path->pscontext;
+
+        atomic_add(nr_bytes, &pi->in_flight_size);
+
+        return 0;
+}
+
+static int st_end_io(struct path_selector *ps, struct dm_path *path,
+                     size_t nr_bytes)
+{
+        struct path_info *pi = path->pscontext;
+
+        atomic_sub(nr_bytes, &pi->in_flight_size);
+
+        return 0;
+}
+
+static struct path_selector_type st_ps = {
+        .name           = "service-time",
+        .module         = THIS_MODULE,
+        .table_args     = 2,
+        .info_args      = 2,
+        .create         = st_create,
+        .destroy        = st_destroy,
+        .status         = st_status,
+        .add_path       = st_add_path,
+        .fail_path      = st_fail_path,
+        .reinstate_path = st_reinstate_path,
+        .select_path    = st_select_path,
+        .start_io       = st_start_io,
+        .end_io         = st_end_io,
+};
+
+static int __init dm_st_init(void)
+{
+        int r = dm_register_path_selector(&st_ps);
+
+        if (r < 0)
+                DMERR("register failed %d", r);
+
+        DMINFO("version " ST_VERSION " loaded");
+
+        return r;
+}
+
+static void __exit dm_st_exit(void)
+{
+        int r = dm_unregister_path_selector(&st_ps);
+
+        if (r < 0)
+                DMERR("unregister failed %d", r);
+}
+
+module_init(dm_st_init);
+module_exit(dm_st_exit);
+
+MODULE_DESCRIPTION(DM_NAME " throughput oriented path selector");
+MODULE_AUTHOR("Kiyoshi Ueda <k-ueda@ct.jp.nec.com>");
+MODULE_LICENSE("GPL");