Root/drivers/md/dm-service-time.c

1/*
2 * Copyright (C) 2007-2009 NEC Corporation. All Rights Reserved.
3 *
4 * Module Author: Kiyoshi Ueda
5 *
6 * This file is released under the GPL.
7 *
8 * Throughput oriented path selector.
9 */
10
11#include "dm.h"
12#include "dm-path-selector.h"
13
14#include <linux/slab.h>
15#include <linux/module.h>
16
17#define DM_MSG_PREFIX "multipath service-time"
18#define ST_MIN_IO 1
19#define ST_MAX_RELATIVE_THROUGHPUT 100
20#define ST_MAX_RELATIVE_THROUGHPUT_SHIFT 7
21#define ST_MAX_INFLIGHT_SIZE ((size_t)-1 >> ST_MAX_RELATIVE_THROUGHPUT_SHIFT)
22#define ST_VERSION "0.2.0"
23
24struct selector {
25    struct list_head valid_paths;
26    struct list_head failed_paths;
27};
28
29struct path_info {
30    struct list_head list;
31    struct dm_path *path;
32    unsigned repeat_count;
33    unsigned relative_throughput;
34    atomic_t in_flight_size; /* Total size of in-flight I/Os */
35};
36
37static struct selector *alloc_selector(void)
38{
39    struct selector *s = kmalloc(sizeof(*s), GFP_KERNEL);
40
41    if (s) {
42        INIT_LIST_HEAD(&s->valid_paths);
43        INIT_LIST_HEAD(&s->failed_paths);
44    }
45
46    return s;
47}
48
49static int st_create(struct path_selector *ps, unsigned argc, char **argv)
50{
51    struct selector *s = alloc_selector();
52
53    if (!s)
54        return -ENOMEM;
55
56    ps->context = s;
57    return 0;
58}
59
60static void free_paths(struct list_head *paths)
61{
62    struct path_info *pi, *next;
63
64    list_for_each_entry_safe(pi, next, paths, list) {
65        list_del(&pi->list);
66        kfree(pi);
67    }
68}
69
70static void st_destroy(struct path_selector *ps)
71{
72    struct selector *s = ps->context;
73
74    free_paths(&s->valid_paths);
75    free_paths(&s->failed_paths);
76    kfree(s);
77    ps->context = NULL;
78}
79
80static int st_status(struct path_selector *ps, struct dm_path *path,
81             status_type_t type, char *result, unsigned maxlen)
82{
83    unsigned sz = 0;
84    struct path_info *pi;
85
86    if (!path)
87        DMEMIT("0 ");
88    else {
89        pi = path->pscontext;
90
91        switch (type) {
92        case STATUSTYPE_INFO:
93            DMEMIT("%d %u ", atomic_read(&pi->in_flight_size),
94                   pi->relative_throughput);
95            break;
96        case STATUSTYPE_TABLE:
97            DMEMIT("%u %u ", pi->repeat_count,
98                   pi->relative_throughput);
99            break;
100        }
101    }
102
103    return sz;
104}
105
106static int st_add_path(struct path_selector *ps, struct dm_path *path,
107               int argc, char **argv, char **error)
108{
109    struct selector *s = ps->context;
110    struct path_info *pi;
111    unsigned repeat_count = ST_MIN_IO;
112    unsigned relative_throughput = 1;
113    char dummy;
114
115    /*
116     * Arguments: [<repeat_count> [<relative_throughput>]]
117     * <repeat_count>: The number of I/Os before switching path.
118     * If not given, default (ST_MIN_IO) is used.
119     * <relative_throughput>: The relative throughput value of
120     * the path among all paths in the path-group.
121     * The valid range: 0-<ST_MAX_RELATIVE_THROUGHPUT>
122     * If not given, minimum value '1' is used.
123     * If '0' is given, the path isn't selected while
124     * other paths having a positive value are
125     * available.
126     */
127    if (argc > 2) {
128        *error = "service-time ps: incorrect number of arguments";
129        return -EINVAL;
130    }
131
132    if (argc && (sscanf(argv[0], "%u%c", &repeat_count, &dummy) != 1)) {
133        *error = "service-time ps: invalid repeat count";
134        return -EINVAL;
135    }
136
137    if ((argc == 2) &&
138        (sscanf(argv[1], "%u%c", &relative_throughput, &dummy) != 1 ||
139         relative_throughput > ST_MAX_RELATIVE_THROUGHPUT)) {
140        *error = "service-time ps: invalid relative_throughput value";
141        return -EINVAL;
142    }
143
144    /* allocate the path */
145    pi = kmalloc(sizeof(*pi), GFP_KERNEL);
146    if (!pi) {
147        *error = "service-time ps: Error allocating path context";
148        return -ENOMEM;
149    }
150
151    pi->path = path;
152    pi->repeat_count = repeat_count;
153    pi->relative_throughput = relative_throughput;
154    atomic_set(&pi->in_flight_size, 0);
155
156    path->pscontext = pi;
157
158    list_add_tail(&pi->list, &s->valid_paths);
159
160    return 0;
161}
162
163static void st_fail_path(struct path_selector *ps, struct dm_path *path)
164{
165    struct selector *s = ps->context;
166    struct path_info *pi = path->pscontext;
167
168    list_move(&pi->list, &s->failed_paths);
169}
170
171static int st_reinstate_path(struct path_selector *ps, struct dm_path *path)
172{
173    struct selector *s = ps->context;
174    struct path_info *pi = path->pscontext;
175
176    list_move_tail(&pi->list, &s->valid_paths);
177
178    return 0;
179}
180
181/*
182 * Compare the estimated service time of 2 paths, pi1 and pi2,
183 * for the incoming I/O.
184 *
185 * Returns:
186 * < 0 : pi1 is better
187 * 0 : no difference between pi1 and pi2
188 * > 0 : pi2 is better
189 *
190 * Description:
191 * Basically, the service time is estimated by:
192 * ('pi->in-flight-size' + 'incoming') / 'pi->relative_throughput'
193 * To reduce the calculation, some optimizations are made.
194 * (See comments inline)
195 */
196static int st_compare_load(struct path_info *pi1, struct path_info *pi2,
197               size_t incoming)
198{
199    size_t sz1, sz2, st1, st2;
200
201    sz1 = atomic_read(&pi1->in_flight_size);
202    sz2 = atomic_read(&pi2->in_flight_size);
203
204    /*
205     * Case 1: Both have same throughput value. Choose less loaded path.
206     */
207    if (pi1->relative_throughput == pi2->relative_throughput)
208        return sz1 - sz2;
209
210    /*
211     * Case 2a: Both have same load. Choose higher throughput path.
212     * Case 2b: One path has no throughput value. Choose the other one.
213     */
214    if (sz1 == sz2 ||
215        !pi1->relative_throughput || !pi2->relative_throughput)
216        return pi2->relative_throughput - pi1->relative_throughput;
217
218    /*
219     * Case 3: Calculate service time. Choose faster path.
220     * Service time using pi1:
221     * st1 = (sz1 + incoming) / pi1->relative_throughput
222     * Service time using pi2:
223     * st2 = (sz2 + incoming) / pi2->relative_throughput
224     *
225     * To avoid the division, transform the expression to use
226     * multiplication.
227     * Because ->relative_throughput > 0 here, if st1 < st2,
228     * the expressions below are the same meaning:
229     * (sz1 + incoming) / pi1->relative_throughput <
230     * (sz2 + incoming) / pi2->relative_throughput
231     * (sz1 + incoming) * pi2->relative_throughput <
232     * (sz2 + incoming) * pi1->relative_throughput
233     * So use the later one.
234     */
235    sz1 += incoming;
236    sz2 += incoming;
237    if (unlikely(sz1 >= ST_MAX_INFLIGHT_SIZE ||
238             sz2 >= ST_MAX_INFLIGHT_SIZE)) {
239        /*
240         * Size may be too big for multiplying pi->relative_throughput
241         * and overflow.
242         * To avoid the overflow and mis-selection, shift down both.
243         */
244        sz1 >>= ST_MAX_RELATIVE_THROUGHPUT_SHIFT;
245        sz2 >>= ST_MAX_RELATIVE_THROUGHPUT_SHIFT;
246    }
247    st1 = sz1 * pi2->relative_throughput;
248    st2 = sz2 * pi1->relative_throughput;
249    if (st1 != st2)
250        return st1 - st2;
251
252    /*
253     * Case 4: Service time is equal. Choose higher throughput path.
254     */
255    return pi2->relative_throughput - pi1->relative_throughput;
256}
257
258static struct dm_path *st_select_path(struct path_selector *ps,
259                      unsigned *repeat_count, size_t nr_bytes)
260{
261    struct selector *s = ps->context;
262    struct path_info *pi = NULL, *best = NULL;
263
264    if (list_empty(&s->valid_paths))
265        return NULL;
266
267    /* Change preferred (first in list) path to evenly balance. */
268    list_move_tail(s->valid_paths.next, &s->valid_paths);
269
270    list_for_each_entry(pi, &s->valid_paths, list)
271        if (!best || (st_compare_load(pi, best, nr_bytes) < 0))
272            best = pi;
273
274    if (!best)
275        return NULL;
276
277    *repeat_count = best->repeat_count;
278
279    return best->path;
280}
281
282static int st_start_io(struct path_selector *ps, struct dm_path *path,
283               size_t nr_bytes)
284{
285    struct path_info *pi = path->pscontext;
286
287    atomic_add(nr_bytes, &pi->in_flight_size);
288
289    return 0;
290}
291
292static int st_end_io(struct path_selector *ps, struct dm_path *path,
293             size_t nr_bytes)
294{
295    struct path_info *pi = path->pscontext;
296
297    atomic_sub(nr_bytes, &pi->in_flight_size);
298
299    return 0;
300}
301
302static struct path_selector_type st_ps = {
303    .name = "service-time",
304    .module = THIS_MODULE,
305    .table_args = 2,
306    .info_args = 2,
307    .create = st_create,
308    .destroy = st_destroy,
309    .status = st_status,
310    .add_path = st_add_path,
311    .fail_path = st_fail_path,
312    .reinstate_path = st_reinstate_path,
313    .select_path = st_select_path,
314    .start_io = st_start_io,
315    .end_io = st_end_io,
316};
317
318static int __init dm_st_init(void)
319{
320    int r = dm_register_path_selector(&st_ps);
321
322    if (r < 0)
323        DMERR("register failed %d", r);
324
325    DMINFO("version " ST_VERSION " loaded");
326
327    return r;
328}
329
330static void __exit dm_st_exit(void)
331{
332    int r = dm_unregister_path_selector(&st_ps);
333
334    if (r < 0)
335        DMERR("unregister failed %d", r);
336}
337
338module_init(dm_st_init);
339module_exit(dm_st_exit);
340
341MODULE_DESCRIPTION(DM_NAME " throughput oriented path selector");
342MODULE_AUTHOR("Kiyoshi Ueda <k-ueda@ct.jp.nec.com>");
343MODULE_LICENSE("GPL");
344

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