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	"0.3.0"
23 
24 struct selector {
25 	struct list_head valid_paths;
26 	struct list_head failed_paths;
27 	spinlock_t lock;
28 };
29 
30 struct path_info {
31 	struct list_head list;
32 	struct dm_path *path;
33 	unsigned repeat_count;
34 	unsigned relative_throughput;
35 	atomic_t in_flight_size;	/* Total size of in-flight I/Os */
36 };
37 
38 static struct selector *alloc_selector(void)
39 {
40 	struct selector *s = kmalloc(sizeof(*s), GFP_KERNEL);
41 
42 	if (s) {
43 		INIT_LIST_HEAD(&s->valid_paths);
44 		INIT_LIST_HEAD(&s->failed_paths);
45 		spin_lock_init(&s->lock);
46 	}
47 
48 	return s;
49 }
50 
51 static int st_create(struct path_selector *ps, unsigned argc, char **argv)
52 {
53 	struct selector *s = alloc_selector();
54 
55 	if (!s)
56 		return -ENOMEM;
57 
58 	ps->context = s;
59 	return 0;
60 }
61 
62 static void free_paths(struct list_head *paths)
63 {
64 	struct path_info *pi, *next;
65 
66 	list_for_each_entry_safe(pi, next, paths, list) {
67 		list_del(&pi->list);
68 		kfree(pi);
69 	}
70 }
71 
72 static void st_destroy(struct path_selector *ps)
73 {
74 	struct selector *s = ps->context;
75 
76 	free_paths(&s->valid_paths);
77 	free_paths(&s->failed_paths);
78 	kfree(s);
79 	ps->context = NULL;
80 }
81 
82 static int st_status(struct path_selector *ps, struct dm_path *path,
83 		     status_type_t type, char *result, unsigned maxlen)
84 {
85 	unsigned sz = 0;
86 	struct path_info *pi;
87 
88 	if (!path)
89 		DMEMIT("0 ");
90 	else {
91 		pi = path->pscontext;
92 
93 		switch (type) {
94 		case STATUSTYPE_INFO:
95 			DMEMIT("%d %u ", atomic_read(&pi->in_flight_size),
96 			       pi->relative_throughput);
97 			break;
98 		case STATUSTYPE_TABLE:
99 			DMEMIT("%u %u ", pi->repeat_count,
100 			       pi->relative_throughput);
101 			break;
102 		case STATUSTYPE_IMA:
103 			result[0] = '\0';
104 			break;
105 		}
106 	}
107 
108 	return sz;
109 }
110 
111 static int st_add_path(struct path_selector *ps, struct dm_path *path,
112 		       int argc, char **argv, char **error)
113 {
114 	struct selector *s = ps->context;
115 	struct path_info *pi;
116 	unsigned repeat_count = ST_MIN_IO;
117 	unsigned relative_throughput = 1;
118 	char dummy;
119 	unsigned long flags;
120 
121 	/*
122 	 * Arguments: [<repeat_count> [<relative_throughput>]]
123 	 * 	<repeat_count>: The number of I/Os before switching path.
124 	 * 			If not given, default (ST_MIN_IO) is used.
125 	 * 	<relative_throughput>: The relative throughput value of
126 	 *			the path among all paths in the path-group.
127 	 * 			The valid range: 0-<ST_MAX_RELATIVE_THROUGHPUT>
128 	 *			If not given, minimum value '1' is used.
129 	 *			If '0' is given, the path isn't selected while
130 	 * 			other paths having a positive value are
131 	 * 			available.
132 	 */
133 	if (argc > 2) {
134 		*error = "service-time ps: incorrect number of arguments";
135 		return -EINVAL;
136 	}
137 
138 	if (argc && (sscanf(argv[0], "%u%c", &repeat_count, &dummy) != 1)) {
139 		*error = "service-time ps: invalid repeat count";
140 		return -EINVAL;
141 	}
142 
143 	if (repeat_count > 1) {
144 		DMWARN_LIMIT("repeat_count > 1 is deprecated, using 1 instead");
145 		repeat_count = 1;
146 	}
147 
148 	if ((argc == 2) &&
149 	    (sscanf(argv[1], "%u%c", &relative_throughput, &dummy) != 1 ||
150 	     relative_throughput > ST_MAX_RELATIVE_THROUGHPUT)) {
151 		*error = "service-time ps: invalid relative_throughput value";
152 		return -EINVAL;
153 	}
154 
155 	/* allocate the path */
156 	pi = kmalloc(sizeof(*pi), GFP_KERNEL);
157 	if (!pi) {
158 		*error = "service-time ps: Error allocating path context";
159 		return -ENOMEM;
160 	}
161 
162 	pi->path = path;
163 	pi->repeat_count = repeat_count;
164 	pi->relative_throughput = relative_throughput;
165 	atomic_set(&pi->in_flight_size, 0);
166 
167 	path->pscontext = pi;
168 
169 	spin_lock_irqsave(&s->lock, flags);
170 	list_add_tail(&pi->list, &s->valid_paths);
171 	spin_unlock_irqrestore(&s->lock, flags);
172 
173 	return 0;
174 }
175 
176 static void st_fail_path(struct path_selector *ps, struct dm_path *path)
177 {
178 	struct selector *s = ps->context;
179 	struct path_info *pi = path->pscontext;
180 	unsigned long flags;
181 
182 	spin_lock_irqsave(&s->lock, flags);
183 	list_move(&pi->list, &s->failed_paths);
184 	spin_unlock_irqrestore(&s->lock, flags);
185 }
186 
187 static int st_reinstate_path(struct path_selector *ps, struct dm_path *path)
188 {
189 	struct selector *s = ps->context;
190 	struct path_info *pi = path->pscontext;
191 	unsigned long flags;
192 
193 	spin_lock_irqsave(&s->lock, flags);
194 	list_move_tail(&pi->list, &s->valid_paths);
195 	spin_unlock_irqrestore(&s->lock, flags);
196 
197 	return 0;
198 }
199 
200 /*
201  * Compare the estimated service time of 2 paths, pi1 and pi2,
202  * for the incoming I/O.
203  *
204  * Returns:
205  * < 0 : pi1 is better
206  * 0   : no difference between pi1 and pi2
207  * > 0 : pi2 is better
208  *
209  * Description:
210  * Basically, the service time is estimated by:
211  *     ('pi->in-flight-size' + 'incoming') / 'pi->relative_throughput'
212  * To reduce the calculation, some optimizations are made.
213  * (See comments inline)
214  */
215 static int st_compare_load(struct path_info *pi1, struct path_info *pi2,
216 			   size_t incoming)
217 {
218 	size_t sz1, sz2, st1, st2;
219 
220 	sz1 = atomic_read(&pi1->in_flight_size);
221 	sz2 = atomic_read(&pi2->in_flight_size);
222 
223 	/*
224 	 * Case 1: Both have same throughput value. Choose less loaded path.
225 	 */
226 	if (pi1->relative_throughput == pi2->relative_throughput)
227 		return sz1 - sz2;
228 
229 	/*
230 	 * Case 2a: Both have same load. Choose higher throughput path.
231 	 * Case 2b: One path has no throughput value. Choose the other one.
232 	 */
233 	if (sz1 == sz2 ||
234 	    !pi1->relative_throughput || !pi2->relative_throughput)
235 		return pi2->relative_throughput - pi1->relative_throughput;
236 
237 	/*
238 	 * Case 3: Calculate service time. Choose faster path.
239 	 *         Service time using pi1:
240 	 *             st1 = (sz1 + incoming) / pi1->relative_throughput
241 	 *         Service time using pi2:
242 	 *             st2 = (sz2 + incoming) / pi2->relative_throughput
243 	 *
244 	 *         To avoid the division, transform the expression to use
245 	 *         multiplication.
246 	 *         Because ->relative_throughput > 0 here, if st1 < st2,
247 	 *         the expressions below are the same meaning:
248 	 *             (sz1 + incoming) / pi1->relative_throughput <
249 	 *                 (sz2 + incoming) / pi2->relative_throughput
250 	 *             (sz1 + incoming) * pi2->relative_throughput <
251 	 *                 (sz2 + incoming) * pi1->relative_throughput
252 	 *         So use the later one.
253 	 */
254 	sz1 += incoming;
255 	sz2 += incoming;
256 	if (unlikely(sz1 >= ST_MAX_INFLIGHT_SIZE ||
257 		     sz2 >= ST_MAX_INFLIGHT_SIZE)) {
258 		/*
259 		 * Size may be too big for multiplying pi->relative_throughput
260 		 * and overflow.
261 		 * To avoid the overflow and mis-selection, shift down both.
262 		 */
263 		sz1 >>= ST_MAX_RELATIVE_THROUGHPUT_SHIFT;
264 		sz2 >>= ST_MAX_RELATIVE_THROUGHPUT_SHIFT;
265 	}
266 	st1 = sz1 * pi2->relative_throughput;
267 	st2 = sz2 * pi1->relative_throughput;
268 	if (st1 != st2)
269 		return st1 - st2;
270 
271 	/*
272 	 * Case 4: Service time is equal. Choose higher throughput path.
273 	 */
274 	return pi2->relative_throughput - pi1->relative_throughput;
275 }
276 
277 static struct dm_path *st_select_path(struct path_selector *ps, size_t nr_bytes)
278 {
279 	struct selector *s = ps->context;
280 	struct path_info *pi = NULL, *best = NULL;
281 	struct dm_path *ret = NULL;
282 	unsigned long flags;
283 
284 	spin_lock_irqsave(&s->lock, flags);
285 	if (list_empty(&s->valid_paths))
286 		goto out;
287 
288 	list_for_each_entry(pi, &s->valid_paths, list)
289 		if (!best || (st_compare_load(pi, best, nr_bytes) < 0))
290 			best = pi;
291 
292 	if (!best)
293 		goto out;
294 
295 	/* Move most recently used to least preferred to evenly balance. */
296 	list_move_tail(&best->list, &s->valid_paths);
297 
298 	ret = best->path;
299 out:
300 	spin_unlock_irqrestore(&s->lock, flags);
301 	return ret;
302 }
303 
304 static int st_start_io(struct path_selector *ps, struct dm_path *path,
305 		       size_t nr_bytes)
306 {
307 	struct path_info *pi = path->pscontext;
308 
309 	atomic_add(nr_bytes, &pi->in_flight_size);
310 
311 	return 0;
312 }
313 
314 static int st_end_io(struct path_selector *ps, struct dm_path *path,
315 		     size_t nr_bytes, u64 start_time)
316 {
317 	struct path_info *pi = path->pscontext;
318 
319 	atomic_sub(nr_bytes, &pi->in_flight_size);
320 
321 	return 0;
322 }
323 
324 static struct path_selector_type st_ps = {
325 	.name		= "service-time",
326 	.module		= THIS_MODULE,
327 	.table_args	= 2,
328 	.info_args	= 2,
329 	.create		= st_create,
330 	.destroy	= st_destroy,
331 	.status		= st_status,
332 	.add_path	= st_add_path,
333 	.fail_path	= st_fail_path,
334 	.reinstate_path	= st_reinstate_path,
335 	.select_path	= st_select_path,
336 	.start_io	= st_start_io,
337 	.end_io		= st_end_io,
338 };
339 
340 static int __init dm_st_init(void)
341 {
342 	int r = dm_register_path_selector(&st_ps);
343 
344 	if (r < 0)
345 		DMERR("register failed %d", r);
346 
347 	DMINFO("version " ST_VERSION " loaded");
348 
349 	return r;
350 }
351 
352 static void __exit dm_st_exit(void)
353 {
354 	int r = dm_unregister_path_selector(&st_ps);
355 
356 	if (r < 0)
357 		DMERR("unregister failed %d", r);
358 }
359 
360 module_init(dm_st_init);
361 module_exit(dm_st_exit);
362 
363 MODULE_DESCRIPTION(DM_NAME " throughput oriented path selector");
364 MODULE_AUTHOR("Kiyoshi Ueda <k-ueda@ct.jp.nec.com>");
365 MODULE_LICENSE("GPL");
366