xref: /linux/block/elevator.c (revision d163d60258c755845cbc9cfe0e45fca71e649488)
1 // SPDX-License-Identifier: GPL-2.0
2 /*
3  *  Block device elevator/IO-scheduler.
4  *
5  *  Copyright (C) 2000 Andrea Arcangeli <andrea@suse.de> SuSE
6  *
7  * 30042000 Jens Axboe <axboe@kernel.dk> :
8  *
9  * Split the elevator a bit so that it is possible to choose a different
10  * one or even write a new "plug in". There are three pieces:
11  * - elevator_fn, inserts a new request in the queue list
12  * - elevator_merge_fn, decides whether a new buffer can be merged with
13  *   an existing request
14  * - elevator_dequeue_fn, called when a request is taken off the active list
15  *
16  * 20082000 Dave Jones <davej@suse.de> :
17  * Removed tests for max-bomb-segments, which was breaking elvtune
18  *  when run without -bN
19  *
20  * Jens:
21  * - Rework again to work with bio instead of buffer_heads
22  * - loose bi_dev comparisons, partition handling is right now
23  * - completely modularize elevator setup and teardown
24  *
25  */
26 #include <linux/kernel.h>
27 #include <linux/fs.h>
28 #include <linux/blkdev.h>
29 #include <linux/bio.h>
30 #include <linux/module.h>
31 #include <linux/slab.h>
32 #include <linux/init.h>
33 #include <linux/compiler.h>
34 #include <linux/blktrace_api.h>
35 #include <linux/hash.h>
36 #include <linux/uaccess.h>
37 #include <linux/pm_runtime.h>
38 
39 #include <trace/events/block.h>
40 
41 #include "elevator.h"
42 #include "blk.h"
43 #include "blk-mq-sched.h"
44 #include "blk-pm.h"
45 #include "blk-wbt.h"
46 #include "blk-cgroup.h"
47 
48 static DEFINE_SPINLOCK(elv_list_lock);
49 static LIST_HEAD(elv_list);
50 
51 /*
52  * Merge hash stuff.
53  */
54 #define rq_hash_key(rq)		(blk_rq_pos(rq) + blk_rq_sectors(rq))
55 
56 /*
57  * Query io scheduler to see if the current process issuing bio may be
58  * merged with rq.
59  */
60 static bool elv_iosched_allow_bio_merge(struct request *rq, struct bio *bio)
61 {
62 	struct request_queue *q = rq->q;
63 	struct elevator_queue *e = q->elevator;
64 
65 	if (e->type->ops.allow_merge)
66 		return e->type->ops.allow_merge(q, rq, bio);
67 
68 	return true;
69 }
70 
71 /*
72  * can we safely merge with this request?
73  */
74 bool elv_bio_merge_ok(struct request *rq, struct bio *bio)
75 {
76 	if (!blk_rq_merge_ok(rq, bio))
77 		return false;
78 
79 	if (!elv_iosched_allow_bio_merge(rq, bio))
80 		return false;
81 
82 	return true;
83 }
84 EXPORT_SYMBOL(elv_bio_merge_ok);
85 
86 /**
87  * elevator_match - Check whether @e's name or alias matches @name
88  * @e: Scheduler to test
89  * @name: Elevator name to test
90  *
91  * Return true if the elevator @e's name or alias matches @name.
92  */
93 static bool elevator_match(const struct elevator_type *e, const char *name)
94 {
95 	return !strcmp(e->elevator_name, name) ||
96 		(e->elevator_alias && !strcmp(e->elevator_alias, name));
97 }
98 
99 static struct elevator_type *__elevator_find(const char *name)
100 {
101 	struct elevator_type *e;
102 
103 	list_for_each_entry(e, &elv_list, list)
104 		if (elevator_match(e, name))
105 			return e;
106 	return NULL;
107 }
108 
109 static struct elevator_type *elevator_find_get(struct request_queue *q,
110 		const char *name)
111 {
112 	struct elevator_type *e;
113 
114 	spin_lock(&elv_list_lock);
115 	e = __elevator_find(name);
116 	if (e && (!elevator_tryget(e)))
117 		e = NULL;
118 	spin_unlock(&elv_list_lock);
119 	return e;
120 }
121 
122 static const struct kobj_type elv_ktype;
123 
124 struct elevator_queue *elevator_alloc(struct request_queue *q,
125 				  struct elevator_type *e)
126 {
127 	struct elevator_queue *eq;
128 
129 	eq = kzalloc_node(sizeof(*eq), GFP_KERNEL, q->node);
130 	if (unlikely(!eq))
131 		return NULL;
132 
133 	__elevator_get(e);
134 	eq->type = e;
135 	kobject_init(&eq->kobj, &elv_ktype);
136 	mutex_init(&eq->sysfs_lock);
137 	hash_init(eq->hash);
138 
139 	return eq;
140 }
141 EXPORT_SYMBOL(elevator_alloc);
142 
143 static void elevator_release(struct kobject *kobj)
144 {
145 	struct elevator_queue *e;
146 
147 	e = container_of(kobj, struct elevator_queue, kobj);
148 	elevator_put(e->type);
149 	kfree(e);
150 }
151 
152 void elevator_exit(struct request_queue *q)
153 {
154 	struct elevator_queue *e = q->elevator;
155 
156 	ioc_clear_queue(q);
157 	blk_mq_sched_free_rqs(q);
158 
159 	mutex_lock(&e->sysfs_lock);
160 	blk_mq_exit_sched(q, e);
161 	mutex_unlock(&e->sysfs_lock);
162 
163 	kobject_put(&e->kobj);
164 }
165 
166 static inline void __elv_rqhash_del(struct request *rq)
167 {
168 	hash_del(&rq->hash);
169 	rq->rq_flags &= ~RQF_HASHED;
170 }
171 
172 void elv_rqhash_del(struct request_queue *q, struct request *rq)
173 {
174 	if (ELV_ON_HASH(rq))
175 		__elv_rqhash_del(rq);
176 }
177 EXPORT_SYMBOL_GPL(elv_rqhash_del);
178 
179 void elv_rqhash_add(struct request_queue *q, struct request *rq)
180 {
181 	struct elevator_queue *e = q->elevator;
182 
183 	BUG_ON(ELV_ON_HASH(rq));
184 	hash_add(e->hash, &rq->hash, rq_hash_key(rq));
185 	rq->rq_flags |= RQF_HASHED;
186 }
187 EXPORT_SYMBOL_GPL(elv_rqhash_add);
188 
189 void elv_rqhash_reposition(struct request_queue *q, struct request *rq)
190 {
191 	__elv_rqhash_del(rq);
192 	elv_rqhash_add(q, rq);
193 }
194 
195 struct request *elv_rqhash_find(struct request_queue *q, sector_t offset)
196 {
197 	struct elevator_queue *e = q->elevator;
198 	struct hlist_node *next;
199 	struct request *rq;
200 
201 	hash_for_each_possible_safe(e->hash, rq, next, hash, offset) {
202 		BUG_ON(!ELV_ON_HASH(rq));
203 
204 		if (unlikely(!rq_mergeable(rq))) {
205 			__elv_rqhash_del(rq);
206 			continue;
207 		}
208 
209 		if (rq_hash_key(rq) == offset)
210 			return rq;
211 	}
212 
213 	return NULL;
214 }
215 
216 /*
217  * RB-tree support functions for inserting/lookup/removal of requests
218  * in a sorted RB tree.
219  */
220 void elv_rb_add(struct rb_root *root, struct request *rq)
221 {
222 	struct rb_node **p = &root->rb_node;
223 	struct rb_node *parent = NULL;
224 	struct request *__rq;
225 
226 	while (*p) {
227 		parent = *p;
228 		__rq = rb_entry(parent, struct request, rb_node);
229 
230 		if (blk_rq_pos(rq) < blk_rq_pos(__rq))
231 			p = &(*p)->rb_left;
232 		else if (blk_rq_pos(rq) >= blk_rq_pos(__rq))
233 			p = &(*p)->rb_right;
234 	}
235 
236 	rb_link_node(&rq->rb_node, parent, p);
237 	rb_insert_color(&rq->rb_node, root);
238 }
239 EXPORT_SYMBOL(elv_rb_add);
240 
241 void elv_rb_del(struct rb_root *root, struct request *rq)
242 {
243 	BUG_ON(RB_EMPTY_NODE(&rq->rb_node));
244 	rb_erase(&rq->rb_node, root);
245 	RB_CLEAR_NODE(&rq->rb_node);
246 }
247 EXPORT_SYMBOL(elv_rb_del);
248 
249 struct request *elv_rb_find(struct rb_root *root, sector_t sector)
250 {
251 	struct rb_node *n = root->rb_node;
252 	struct request *rq;
253 
254 	while (n) {
255 		rq = rb_entry(n, struct request, rb_node);
256 
257 		if (sector < blk_rq_pos(rq))
258 			n = n->rb_left;
259 		else if (sector > blk_rq_pos(rq))
260 			n = n->rb_right;
261 		else
262 			return rq;
263 	}
264 
265 	return NULL;
266 }
267 EXPORT_SYMBOL(elv_rb_find);
268 
269 enum elv_merge elv_merge(struct request_queue *q, struct request **req,
270 		struct bio *bio)
271 {
272 	struct elevator_queue *e = q->elevator;
273 	struct request *__rq;
274 
275 	/*
276 	 * Levels of merges:
277 	 * 	nomerges:  No merges at all attempted
278 	 * 	noxmerges: Only simple one-hit cache try
279 	 * 	merges:	   All merge tries attempted
280 	 */
281 	if (blk_queue_nomerges(q) || !bio_mergeable(bio))
282 		return ELEVATOR_NO_MERGE;
283 
284 	/*
285 	 * First try one-hit cache.
286 	 */
287 	if (q->last_merge && elv_bio_merge_ok(q->last_merge, bio)) {
288 		enum elv_merge ret = blk_try_merge(q->last_merge, bio);
289 
290 		if (ret != ELEVATOR_NO_MERGE) {
291 			*req = q->last_merge;
292 			return ret;
293 		}
294 	}
295 
296 	if (blk_queue_noxmerges(q))
297 		return ELEVATOR_NO_MERGE;
298 
299 	/*
300 	 * See if our hash lookup can find a potential backmerge.
301 	 */
302 	__rq = elv_rqhash_find(q, bio->bi_iter.bi_sector);
303 	if (__rq && elv_bio_merge_ok(__rq, bio)) {
304 		*req = __rq;
305 
306 		if (blk_discard_mergable(__rq))
307 			return ELEVATOR_DISCARD_MERGE;
308 		return ELEVATOR_BACK_MERGE;
309 	}
310 
311 	if (e->type->ops.request_merge)
312 		return e->type->ops.request_merge(q, req, bio);
313 
314 	return ELEVATOR_NO_MERGE;
315 }
316 
317 /*
318  * Attempt to do an insertion back merge. Only check for the case where
319  * we can append 'rq' to an existing request, so we can throw 'rq' away
320  * afterwards.
321  *
322  * Returns true if we merged, false otherwise. 'free' will contain all
323  * requests that need to be freed.
324  */
325 bool elv_attempt_insert_merge(struct request_queue *q, struct request *rq,
326 			      struct list_head *free)
327 {
328 	struct request *__rq;
329 	bool ret;
330 
331 	if (blk_queue_nomerges(q))
332 		return false;
333 
334 	/*
335 	 * First try one-hit cache.
336 	 */
337 	if (q->last_merge && blk_attempt_req_merge(q, q->last_merge, rq)) {
338 		list_add(&rq->queuelist, free);
339 		return true;
340 	}
341 
342 	if (blk_queue_noxmerges(q))
343 		return false;
344 
345 	ret = false;
346 	/*
347 	 * See if our hash lookup can find a potential backmerge.
348 	 */
349 	while (1) {
350 		__rq = elv_rqhash_find(q, blk_rq_pos(rq));
351 		if (!__rq || !blk_attempt_req_merge(q, __rq, rq))
352 			break;
353 
354 		list_add(&rq->queuelist, free);
355 		/* The merged request could be merged with others, try again */
356 		ret = true;
357 		rq = __rq;
358 	}
359 
360 	return ret;
361 }
362 
363 void elv_merged_request(struct request_queue *q, struct request *rq,
364 		enum elv_merge type)
365 {
366 	struct elevator_queue *e = q->elevator;
367 
368 	if (e->type->ops.request_merged)
369 		e->type->ops.request_merged(q, rq, type);
370 
371 	if (type == ELEVATOR_BACK_MERGE)
372 		elv_rqhash_reposition(q, rq);
373 
374 	q->last_merge = rq;
375 }
376 
377 void elv_merge_requests(struct request_queue *q, struct request *rq,
378 			     struct request *next)
379 {
380 	struct elevator_queue *e = q->elevator;
381 
382 	if (e->type->ops.requests_merged)
383 		e->type->ops.requests_merged(q, rq, next);
384 
385 	elv_rqhash_reposition(q, rq);
386 	q->last_merge = rq;
387 }
388 
389 struct request *elv_latter_request(struct request_queue *q, struct request *rq)
390 {
391 	struct elevator_queue *e = q->elevator;
392 
393 	if (e->type->ops.next_request)
394 		return e->type->ops.next_request(q, rq);
395 
396 	return NULL;
397 }
398 
399 struct request *elv_former_request(struct request_queue *q, struct request *rq)
400 {
401 	struct elevator_queue *e = q->elevator;
402 
403 	if (e->type->ops.former_request)
404 		return e->type->ops.former_request(q, rq);
405 
406 	return NULL;
407 }
408 
409 #define to_elv(atr) container_of((atr), struct elv_fs_entry, attr)
410 
411 static ssize_t
412 elv_attr_show(struct kobject *kobj, struct attribute *attr, char *page)
413 {
414 	struct elv_fs_entry *entry = to_elv(attr);
415 	struct elevator_queue *e;
416 	ssize_t error;
417 
418 	if (!entry->show)
419 		return -EIO;
420 
421 	e = container_of(kobj, struct elevator_queue, kobj);
422 	mutex_lock(&e->sysfs_lock);
423 	error = e->type ? entry->show(e, page) : -ENOENT;
424 	mutex_unlock(&e->sysfs_lock);
425 	return error;
426 }
427 
428 static ssize_t
429 elv_attr_store(struct kobject *kobj, struct attribute *attr,
430 	       const char *page, size_t length)
431 {
432 	struct elv_fs_entry *entry = to_elv(attr);
433 	struct elevator_queue *e;
434 	ssize_t error;
435 
436 	if (!entry->store)
437 		return -EIO;
438 
439 	e = container_of(kobj, struct elevator_queue, kobj);
440 	mutex_lock(&e->sysfs_lock);
441 	error = e->type ? entry->store(e, page, length) : -ENOENT;
442 	mutex_unlock(&e->sysfs_lock);
443 	return error;
444 }
445 
446 static const struct sysfs_ops elv_sysfs_ops = {
447 	.show	= elv_attr_show,
448 	.store	= elv_attr_store,
449 };
450 
451 static const struct kobj_type elv_ktype = {
452 	.sysfs_ops	= &elv_sysfs_ops,
453 	.release	= elevator_release,
454 };
455 
456 int elv_register_queue(struct request_queue *q, bool uevent)
457 {
458 	struct elevator_queue *e = q->elevator;
459 	int error;
460 
461 	lockdep_assert_held(&q->sysfs_lock);
462 
463 	error = kobject_add(&e->kobj, &q->disk->queue_kobj, "iosched");
464 	if (!error) {
465 		struct elv_fs_entry *attr = e->type->elevator_attrs;
466 		if (attr) {
467 			while (attr->attr.name) {
468 				if (sysfs_create_file(&e->kobj, &attr->attr))
469 					break;
470 				attr++;
471 			}
472 		}
473 		if (uevent)
474 			kobject_uevent(&e->kobj, KOBJ_ADD);
475 
476 		set_bit(ELEVATOR_FLAG_REGISTERED, &e->flags);
477 	}
478 	return error;
479 }
480 
481 void elv_unregister_queue(struct request_queue *q)
482 {
483 	struct elevator_queue *e = q->elevator;
484 
485 	lockdep_assert_held(&q->sysfs_lock);
486 
487 	if (e && test_and_clear_bit(ELEVATOR_FLAG_REGISTERED, &e->flags)) {
488 		kobject_uevent(&e->kobj, KOBJ_REMOVE);
489 		kobject_del(&e->kobj);
490 	}
491 }
492 
493 int elv_register(struct elevator_type *e)
494 {
495 	/* finish request is mandatory */
496 	if (WARN_ON_ONCE(!e->ops.finish_request))
497 		return -EINVAL;
498 	/* insert_requests and dispatch_request are mandatory */
499 	if (WARN_ON_ONCE(!e->ops.insert_requests || !e->ops.dispatch_request))
500 		return -EINVAL;
501 
502 	/* create icq_cache if requested */
503 	if (e->icq_size) {
504 		if (WARN_ON(e->icq_size < sizeof(struct io_cq)) ||
505 		    WARN_ON(e->icq_align < __alignof__(struct io_cq)))
506 			return -EINVAL;
507 
508 		snprintf(e->icq_cache_name, sizeof(e->icq_cache_name),
509 			 "%s_io_cq", e->elevator_name);
510 		e->icq_cache = kmem_cache_create(e->icq_cache_name, e->icq_size,
511 						 e->icq_align, 0, NULL);
512 		if (!e->icq_cache)
513 			return -ENOMEM;
514 	}
515 
516 	/* register, don't allow duplicate names */
517 	spin_lock(&elv_list_lock);
518 	if (__elevator_find(e->elevator_name)) {
519 		spin_unlock(&elv_list_lock);
520 		kmem_cache_destroy(e->icq_cache);
521 		return -EBUSY;
522 	}
523 	list_add_tail(&e->list, &elv_list);
524 	spin_unlock(&elv_list_lock);
525 
526 	printk(KERN_INFO "io scheduler %s registered\n", e->elevator_name);
527 
528 	return 0;
529 }
530 EXPORT_SYMBOL_GPL(elv_register);
531 
532 void elv_unregister(struct elevator_type *e)
533 {
534 	/* unregister */
535 	spin_lock(&elv_list_lock);
536 	list_del_init(&e->list);
537 	spin_unlock(&elv_list_lock);
538 
539 	/*
540 	 * Destroy icq_cache if it exists.  icq's are RCU managed.  Make
541 	 * sure all RCU operations are complete before proceeding.
542 	 */
543 	if (e->icq_cache) {
544 		rcu_barrier();
545 		kmem_cache_destroy(e->icq_cache);
546 		e->icq_cache = NULL;
547 	}
548 }
549 EXPORT_SYMBOL_GPL(elv_unregister);
550 
551 static inline bool elv_support_iosched(struct request_queue *q)
552 {
553 	if (!queue_is_mq(q) ||
554 	    (q->tag_set && (q->tag_set->flags & BLK_MQ_F_NO_SCHED)))
555 		return false;
556 	return true;
557 }
558 
559 /*
560  * For single queue devices, default to using mq-deadline. If we have multiple
561  * queues or mq-deadline is not available, default to "none".
562  */
563 static struct elevator_type *elevator_get_default(struct request_queue *q)
564 {
565 	if (q->tag_set && q->tag_set->flags & BLK_MQ_F_NO_SCHED_BY_DEFAULT)
566 		return NULL;
567 
568 	if (q->nr_hw_queues != 1 &&
569 	    !blk_mq_is_shared_tags(q->tag_set->flags))
570 		return NULL;
571 
572 	return elevator_find_get(q, "mq-deadline");
573 }
574 
575 /*
576  * Use the default elevator settings. If the chosen elevator initialization
577  * fails, fall back to the "none" elevator (no elevator).
578  */
579 void elevator_init_mq(struct request_queue *q)
580 {
581 	struct elevator_type *e;
582 	int err;
583 
584 	if (!elv_support_iosched(q))
585 		return;
586 
587 	WARN_ON_ONCE(blk_queue_registered(q));
588 
589 	if (unlikely(q->elevator))
590 		return;
591 
592 	e = elevator_get_default(q);
593 	if (!e)
594 		return;
595 
596 	/*
597 	 * We are called before adding disk, when there isn't any FS I/O,
598 	 * so freezing queue plus canceling dispatch work is enough to
599 	 * drain any dispatch activities originated from passthrough
600 	 * requests, then no need to quiesce queue which may add long boot
601 	 * latency, especially when lots of disks are involved.
602 	 */
603 	blk_mq_freeze_queue(q);
604 	blk_mq_cancel_work_sync(q);
605 
606 	err = blk_mq_init_sched(q, e);
607 
608 	blk_mq_unfreeze_queue(q);
609 
610 	if (err) {
611 		pr_warn("\"%s\" elevator initialization failed, "
612 			"falling back to \"none\"\n", e->elevator_name);
613 	}
614 
615 	elevator_put(e);
616 }
617 
618 /*
619  * Switch to new_e io scheduler.
620  *
621  * If switching fails, we are most likely running out of memory and not able
622  * to restore the old io scheduler, so leaving the io scheduler being none.
623  */
624 int elevator_switch(struct request_queue *q, struct elevator_type *new_e)
625 {
626 	int ret;
627 
628 	lockdep_assert_held(&q->sysfs_lock);
629 
630 	blk_mq_freeze_queue(q);
631 	blk_mq_quiesce_queue(q);
632 
633 	if (q->elevator) {
634 		elv_unregister_queue(q);
635 		elevator_exit(q);
636 	}
637 
638 	ret = blk_mq_init_sched(q, new_e);
639 	if (ret)
640 		goto out_unfreeze;
641 
642 	ret = elv_register_queue(q, true);
643 	if (ret) {
644 		elevator_exit(q);
645 		goto out_unfreeze;
646 	}
647 	blk_add_trace_msg(q, "elv switch: %s", new_e->elevator_name);
648 
649 out_unfreeze:
650 	blk_mq_unquiesce_queue(q);
651 	blk_mq_unfreeze_queue(q);
652 
653 	if (ret) {
654 		pr_warn("elv: switch to \"%s\" failed, falling back to \"none\"\n",
655 			new_e->elevator_name);
656 	}
657 
658 	return ret;
659 }
660 
661 void elevator_disable(struct request_queue *q)
662 {
663 	lockdep_assert_held(&q->sysfs_lock);
664 
665 	blk_mq_freeze_queue(q);
666 	blk_mq_quiesce_queue(q);
667 
668 	elv_unregister_queue(q);
669 	elevator_exit(q);
670 	blk_queue_flag_clear(QUEUE_FLAG_SQ_SCHED, q);
671 	q->elevator = NULL;
672 	q->nr_requests = q->tag_set->queue_depth;
673 	blk_add_trace_msg(q, "elv switch: none");
674 
675 	blk_mq_unquiesce_queue(q);
676 	blk_mq_unfreeze_queue(q);
677 }
678 
679 /*
680  * Switch this queue to the given IO scheduler.
681  */
682 static int elevator_change(struct request_queue *q, const char *elevator_name)
683 {
684 	struct elevator_type *e;
685 	int ret;
686 
687 	/* Make sure queue is not in the middle of being removed */
688 	if (!blk_queue_registered(q))
689 		return -ENOENT;
690 
691 	if (!strncmp(elevator_name, "none", 4)) {
692 		if (q->elevator)
693 			elevator_disable(q);
694 		return 0;
695 	}
696 
697 	if (q->elevator && elevator_match(q->elevator->type, elevator_name))
698 		return 0;
699 
700 	e = elevator_find_get(q, elevator_name);
701 	if (!e) {
702 		request_module("%s-iosched", elevator_name);
703 		e = elevator_find_get(q, elevator_name);
704 		if (!e)
705 			return -EINVAL;
706 	}
707 	ret = elevator_switch(q, e);
708 	elevator_put(e);
709 	return ret;
710 }
711 
712 ssize_t elv_iosched_store(struct request_queue *q, const char *buf,
713 			  size_t count)
714 {
715 	char elevator_name[ELV_NAME_MAX];
716 	int ret;
717 
718 	if (!elv_support_iosched(q))
719 		return count;
720 
721 	strscpy(elevator_name, buf, sizeof(elevator_name));
722 	ret = elevator_change(q, strstrip(elevator_name));
723 	if (!ret)
724 		return count;
725 	return ret;
726 }
727 
728 ssize_t elv_iosched_show(struct request_queue *q, char *name)
729 {
730 	struct elevator_queue *eq = q->elevator;
731 	struct elevator_type *cur = NULL, *e;
732 	int len = 0;
733 
734 	if (!elv_support_iosched(q))
735 		return sprintf(name, "none\n");
736 
737 	if (!q->elevator) {
738 		len += sprintf(name+len, "[none] ");
739 	} else {
740 		len += sprintf(name+len, "none ");
741 		cur = eq->type;
742 	}
743 
744 	spin_lock(&elv_list_lock);
745 	list_for_each_entry(e, &elv_list, list) {
746 		if (e == cur)
747 			len += sprintf(name+len, "[%s] ", e->elevator_name);
748 		else
749 			len += sprintf(name+len, "%s ", e->elevator_name);
750 	}
751 	spin_unlock(&elv_list_lock);
752 
753 	len += sprintf(name+len, "\n");
754 	return len;
755 }
756 
757 struct request *elv_rb_former_request(struct request_queue *q,
758 				      struct request *rq)
759 {
760 	struct rb_node *rbprev = rb_prev(&rq->rb_node);
761 
762 	if (rbprev)
763 		return rb_entry_rq(rbprev);
764 
765 	return NULL;
766 }
767 EXPORT_SYMBOL(elv_rb_former_request);
768 
769 struct request *elv_rb_latter_request(struct request_queue *q,
770 				      struct request *rq)
771 {
772 	struct rb_node *rbnext = rb_next(&rq->rb_node);
773 
774 	if (rbnext)
775 		return rb_entry_rq(rbnext);
776 
777 	return NULL;
778 }
779 EXPORT_SYMBOL(elv_rb_latter_request);
780 
781 static int __init elevator_setup(char *str)
782 {
783 	pr_warn("Kernel parameter elevator= does not have any effect anymore.\n"
784 		"Please use sysfs to set IO scheduler for individual devices.\n");
785 	return 1;
786 }
787 
788 __setup("elevator=", elevator_setup);
789