[go: nahoru, domu]
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