xfs_mru_cache.c 17.7 KB
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552
/*
 * Copyright (c) 2006-2007 Silicon Graphics, Inc.
 * All Rights Reserved.
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License as
 * published by the Free Software Foundation.
 *
 * This program is distributed in the hope that it would be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write the Free Software Foundation,
 * Inc.,  51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
 */
#include "xfs.h"
#include "xfs_mru_cache.h"

/*
 * The MRU Cache data structure consists of a data store, an array of lists and
 * a lock to protect its internal state.  At initialisation time, the client
 * supplies an element lifetime in milliseconds and a group count, as well as a
 * function pointer to call when deleting elements.  A data structure for
 * queueing up work in the form of timed callbacks is also included.
 *
 * The group count controls how many lists are created, and thereby how finely
 * the elements are grouped in time.  When reaping occurs, all the elements in
 * all the lists whose time has expired are deleted.
 *
 * To give an example of how this works in practice, consider a client that
 * initialises an MRU Cache with a lifetime of ten seconds and a group count of
 * five.  Five internal lists will be created, each representing a two second
 * period in time.  When the first element is added, time zero for the data
 * structure is initialised to the current time.
 *
 * All the elements added in the first two seconds are appended to the first
 * list.  Elements added in the third second go into the second list, and so on.
 * If an element is accessed at any point, it is removed from its list and
 * inserted at the head of the current most-recently-used list.
 *
 * The reaper function will have nothing to do until at least twelve seconds
 * have elapsed since the first element was added.  The reason for this is that
 * if it were called at t=11s, there could be elements in the first list that
 * have only been inactive for nine seconds, so it still does nothing.  If it is
 * called anywhere between t=12 and t=14 seconds, it will delete all the
 * elements that remain in the first list.  It's therefore possible for elements
 * to remain in the data store even after they've been inactive for up to
 * (t + t/g) seconds, where t is the inactive element lifetime and g is the
 * number of groups.
 *
 * The above example assumes that the reaper function gets called at least once
 * every (t/g) seconds.  If it is called less frequently, unused elements will
 * accumulate in the reap list until the reaper function is eventually called.
 * The current implementation uses work queue callbacks to carefully time the
 * reaper function calls, so this should happen rarely, if at all.
 *
 * From a design perspective, the primary reason for the choice of a list array
 * representing discrete time intervals is that it's only practical to reap
 * expired elements in groups of some appreciable size.  This automatically
 * introduces a granularity to element lifetimes, so there's no point storing an
 * individual timeout with each element that specifies a more precise reap time.
 * The bonus is a saving of sizeof(long) bytes of memory per element stored.
 *
 * The elements could have been stored in just one list, but an array of
 * counters or pointers would need to be maintained to allow them to be divided
 * up into discrete time groups.  More critically, the process of touching or
 * removing an element would involve walking large portions of the entire list,
 * which would have a detrimental effect on performance.  The additional memory
 * requirement for the array of list heads is minimal.
 *
 * When an element is touched or deleted, it needs to be removed from its
 * current list.  Doubly linked lists are used to make the list maintenance
 * portion of these operations O(1).  Since reaper timing can be imprecise,
 * inserts and lookups can occur when there are no free lists available.  When
 * this happens, all the elements on the LRU list need to be migrated to the end
 * of the reap list.  To keep the list maintenance portion of these operations
 * O(1) also, list tails need to be accessible without walking the entire list.
 * This is the reason why doubly linked list heads are used.
 */

/*
 * An MRU Cache is a dynamic data structure that stores its elements in a way
 * that allows efficient lookups, but also groups them into discrete time
 * intervals based on insertion time.  This allows elements to be efficiently
 * and automatically reaped after a fixed period of inactivity.
 *
 * When a client data pointer is stored in the MRU Cache it needs to be added to
 * both the data store and to one of the lists.  It must also be possible to
 * access each of these entries via the other, i.e. to:
 *
 *    a) Walk a list, removing the corresponding data store entry for each item.
 *    b) Look up a data store entry, then access its list entry directly.
 *
 * To achieve both of these goals, each entry must contain both a list entry and
 * a key, in addition to the user's data pointer.  Note that it's not a good
 * idea to have the client embed one of these structures at the top of their own
 * data structure, because inserting the same item more than once would most
 * likely result in a loop in one of the lists.  That's a sure-fire recipe for
 * an infinite loop in the code.
 */
struct xfs_mru_cache {
	struct radix_tree_root	store;     /* Core storage data structure.  */
	struct list_head	*lists;    /* Array of lists, one per grp.  */
	struct list_head	reap_list; /* Elements overdue for reaping. */
	spinlock_t		lock;      /* Lock to protect this struct.  */
	unsigned int		grp_count; /* Number of discrete groups.    */
	unsigned int		grp_time;  /* Time period spanned by grps.  */
	unsigned int		lru_grp;   /* Group containing time zero.   */
	unsigned long		time_zero; /* Time first element was added. */
	xfs_mru_cache_free_func_t free_func; /* Function pointer for freeing. */
	struct delayed_work	work;      /* Workqueue data for reaping.   */
	unsigned int		queued;	   /* work has been queued */
};

static struct workqueue_struct	*xfs_mru_reap_wq;

/*
 * When inserting, destroying or reaping, it's first necessary to update the
 * lists relative to a particular time.  In the case of destroying, that time
 * will be well in the future to ensure that all items are moved to the reap
 * list.  In all other cases though, the time will be the current time.
 *
 * This function enters a loop, moving the contents of the LRU list to the reap
 * list again and again until either a) the lists are all empty, or b) time zero
 * has been advanced sufficiently to be within the immediate element lifetime.
 *
 * Case a) above is detected by counting how many groups are migrated and
 * stopping when they've all been moved.  Case b) is detected by monitoring the
 * time_zero field, which is updated as each group is migrated.
 *
 * The return value is the earliest time that more migration could be needed, or
 * zero if there's no need to schedule more work because the lists are empty.
 */
STATIC unsigned long
_xfs_mru_cache_migrate(
	struct xfs_mru_cache	*mru,
	unsigned long		now)
{
	unsigned int		grp;
	unsigned int		migrated = 0;
	struct list_head	*lru_list;

	/* Nothing to do if the data store is empty. */
	if (!mru->time_zero)
		return 0;

	/* While time zero is older than the time spanned by all the lists. */
	while (mru->time_zero <= now - mru->grp_count * mru->grp_time) {

		/*
		 * If the LRU list isn't empty, migrate its elements to the tail
		 * of the reap list.
		 */
		lru_list = mru->lists + mru->lru_grp;
		if (!list_empty(lru_list))
			list_splice_init(lru_list, mru->reap_list.prev);

		/*
		 * Advance the LRU group number, freeing the old LRU list to
		 * become the new MRU list; advance time zero accordingly.
		 */
		mru->lru_grp = (mru->lru_grp + 1) % mru->grp_count;
		mru->time_zero += mru->grp_time;

		/*
		 * If reaping is so far behind that all the elements on all the
		 * lists have been migrated to the reap list, it's now empty.
		 */
		if (++migrated == mru->grp_count) {
			mru->lru_grp = 0;
			mru->time_zero = 0;
			return 0;
		}
	}

	/* Find the first non-empty list from the LRU end. */
	for (grp = 0; grp < mru->grp_count; grp++) {

		/* Check the grp'th list from the LRU end. */
		lru_list = mru->lists + ((mru->lru_grp + grp) % mru->grp_count);
		if (!list_empty(lru_list))
			return mru->time_zero +
			       (mru->grp_count + grp) * mru->grp_time;
	}

	/* All the lists must be empty. */
	mru->lru_grp = 0;
	mru->time_zero = 0;
	return 0;
}

/*
 * When inserting or doing a lookup, an element needs to be inserted into the
 * MRU list.  The lists must be migrated first to ensure that they're
 * up-to-date, otherwise the new element could be given a shorter lifetime in
 * the cache than it should.
 */
STATIC void
_xfs_mru_cache_list_insert(
	struct xfs_mru_cache	*mru,
	struct xfs_mru_cache_elem *elem)
{
	unsigned int		grp = 0;
	unsigned long		now = jiffies;

	/*
	 * If the data store is empty, initialise time zero, leave grp set to
	 * zero and start the work queue timer if necessary.  Otherwise, set grp
	 * to the number of group times that have elapsed since time zero.
	 */
	if (!_xfs_mru_cache_migrate(mru, now)) {
		mru->time_zero = now;
		if (!mru->queued) {
			mru->queued = 1;
			queue_delayed_work(xfs_mru_reap_wq, &mru->work,
			                   mru->grp_count * mru->grp_time);
		}
	} else {
		grp = (now - mru->time_zero) / mru->grp_time;
		grp = (mru->lru_grp + grp) % mru->grp_count;
	}

	/* Insert the element at the tail of the corresponding list. */
	list_add_tail(&elem->list_node, mru->lists + grp);
}

/*
 * When destroying or reaping, all the elements that were migrated to the reap
 * list need to be deleted.  For each element this involves removing it from the
 * data store, removing it from the reap list, calling the client's free
 * function and deleting the element from the element zone.
 *
 * We get called holding the mru->lock, which we drop and then reacquire.
 * Sparse need special help with this to tell it we know what we are doing.
 */
STATIC void
_xfs_mru_cache_clear_reap_list(
	struct xfs_mru_cache	*mru)
		__releases(mru->lock) __acquires(mru->lock)
{
	struct xfs_mru_cache_elem *elem, *next;
	struct list_head	tmp;

	INIT_LIST_HEAD(&tmp);
	list_for_each_entry_safe(elem, next, &mru->reap_list, list_node) {

		/* Remove the element from the data store. */
		radix_tree_delete(&mru->store, elem->key);

		/*
		 * remove to temp list so it can be freed without
		 * needing to hold the lock
		 */
		list_move(&elem->list_node, &tmp);
	}
	spin_unlock(&mru->lock);

	list_for_each_entry_safe(elem, next, &tmp, list_node) {
		list_del_init(&elem->list_node);
		mru->free_func(elem);
	}

	spin_lock(&mru->lock);
}

/*
 * We fire the reap timer every group expiry interval so
 * we always have a reaper ready to run. This makes shutdown
 * and flushing of the reaper easy to do. Hence we need to
 * keep when the next reap must occur so we can determine
 * at each interval whether there is anything we need to do.
 */
STATIC void
_xfs_mru_cache_reap(
	struct work_struct	*work)
{
	struct xfs_mru_cache	*mru =
		container_of(work, struct xfs_mru_cache, work.work);
	unsigned long		now, next;

	ASSERT(mru && mru->lists);
	if (!mru || !mru->lists)
		return;

	spin_lock(&mru->lock);
	next = _xfs_mru_cache_migrate(mru, jiffies);
	_xfs_mru_cache_clear_reap_list(mru);

	mru->queued = next;
	if ((mru->queued > 0)) {
		now = jiffies;
		if (next <= now)
			next = 0;
		else
			next -= now;
		queue_delayed_work(xfs_mru_reap_wq, &mru->work, next);
	}

	spin_unlock(&mru->lock);
}

int
xfs_mru_cache_init(void)
{
	xfs_mru_reap_wq = alloc_workqueue("xfs_mru_cache",
				WQ_MEM_RECLAIM|WQ_FREEZABLE, 1);
	if (!xfs_mru_reap_wq)
		return -ENOMEM;
	return 0;
}

void
xfs_mru_cache_uninit(void)
{
	destroy_workqueue(xfs_mru_reap_wq);
}

/*
 * To initialise a struct xfs_mru_cache pointer, call xfs_mru_cache_create()
 * with the address of the pointer, a lifetime value in milliseconds, a group
 * count and a free function to use when deleting elements.  This function
 * returns 0 if the initialisation was successful.
 */
int
xfs_mru_cache_create(
	struct xfs_mru_cache	**mrup,
	unsigned int		lifetime_ms,
	unsigned int		grp_count,
	xfs_mru_cache_free_func_t free_func)
{
	struct xfs_mru_cache	*mru = NULL;
	int			err = 0, grp;
	unsigned int		grp_time;

	if (mrup)
		*mrup = NULL;

	if (!mrup || !grp_count || !lifetime_ms || !free_func)
		return -EINVAL;

	if (!(grp_time = msecs_to_jiffies(lifetime_ms) / grp_count))
		return -EINVAL;

	if (!(mru = kmem_zalloc(sizeof(*mru), KM_SLEEP)))
		return -ENOMEM;

	/* An extra list is needed to avoid reaping up to a grp_time early. */
	mru->grp_count = grp_count + 1;
	mru->lists = kmem_zalloc(mru->grp_count * sizeof(*mru->lists), KM_SLEEP);

	if (!mru->lists) {
		err = -ENOMEM;
		goto exit;
	}

	for (grp = 0; grp < mru->grp_count; grp++)
		INIT_LIST_HEAD(mru->lists + grp);

	/*
	 * We use GFP_KERNEL radix tree preload and do inserts under a
	 * spinlock so GFP_ATOMIC is appropriate for the radix tree itself.
	 */
	INIT_RADIX_TREE(&mru->store, GFP_ATOMIC);
	INIT_LIST_HEAD(&mru->reap_list);
	spin_lock_init(&mru->lock);
	INIT_DELAYED_WORK(&mru->work, _xfs_mru_cache_reap);

	mru->grp_time  = grp_time;
	mru->free_func = free_func;

	*mrup = mru;

exit:
	if (err && mru && mru->lists)
		kmem_free(mru->lists);
	if (err && mru)
		kmem_free(mru);

	return err;
}

/*
 * Call xfs_mru_cache_flush() to flush out all cached entries, calling their
 * free functions as they're deleted.  When this function returns, the caller is
 * guaranteed that all the free functions for all the elements have finished
 * executing and the reaper is not running.
 */
static void
xfs_mru_cache_flush(
	struct xfs_mru_cache	*mru)
{
	if (!mru || !mru->lists)
		return;

	spin_lock(&mru->lock);
	if (mru->queued) {
		spin_unlock(&mru->lock);
		cancel_delayed_work_sync(&mru->work);
		spin_lock(&mru->lock);
	}

	_xfs_mru_cache_migrate(mru, jiffies + mru->grp_count * mru->grp_time);
	_xfs_mru_cache_clear_reap_list(mru);

	spin_unlock(&mru->lock);
}

void
xfs_mru_cache_destroy(
	struct xfs_mru_cache	*mru)
{
	if (!mru || !mru->lists)
		return;

	xfs_mru_cache_flush(mru);

	kmem_free(mru->lists);
	kmem_free(mru);
}

/*
 * To insert an element, call xfs_mru_cache_insert() with the data store, the
 * element's key and the client data pointer.  This function returns 0 on
 * success or ENOMEM if memory for the data element couldn't be allocated.
 */
int
xfs_mru_cache_insert(
	struct xfs_mru_cache	*mru,
	unsigned long		key,
	struct xfs_mru_cache_elem *elem)
{
	int			error;

	ASSERT(mru && mru->lists);
	if (!mru || !mru->lists)
		return -EINVAL;

	if (radix_tree_preload(GFP_NOFS))
		return -ENOMEM;

	INIT_LIST_HEAD(&elem->list_node);
	elem->key = key;

	spin_lock(&mru->lock);
	error = radix_tree_insert(&mru->store, key, elem);
	radix_tree_preload_end();
	if (!error)
		_xfs_mru_cache_list_insert(mru, elem);
	spin_unlock(&mru->lock);

	return error;
}

/*
 * To remove an element without calling the free function, call
 * xfs_mru_cache_remove() with the data store and the element's key.  On success
 * the client data pointer for the removed element is returned, otherwise this
 * function will return a NULL pointer.
 */
struct xfs_mru_cache_elem *
xfs_mru_cache_remove(
	struct xfs_mru_cache	*mru,
	unsigned long		key)
{
	struct xfs_mru_cache_elem *elem;

	ASSERT(mru && mru->lists);
	if (!mru || !mru->lists)
		return NULL;

	spin_lock(&mru->lock);
	elem = radix_tree_delete(&mru->store, key);
	if (elem)
		list_del(&elem->list_node);
	spin_unlock(&mru->lock);

	return elem;
}

/*
 * To remove and element and call the free function, call xfs_mru_cache_delete()
 * with the data store and the element's key.
 */
void
xfs_mru_cache_delete(
	struct xfs_mru_cache	*mru,
	unsigned long		key)
{
	struct xfs_mru_cache_elem *elem;

	elem = xfs_mru_cache_remove(mru, key);
	if (elem)
		mru->free_func(elem);
}

/*
 * To look up an element using its key, call xfs_mru_cache_lookup() with the
 * data store and the element's key.  If found, the element will be moved to the
 * head of the MRU list to indicate that it's been touched.
 *
 * The internal data structures are protected by a spinlock that is STILL HELD
 * when this function returns.  Call xfs_mru_cache_done() to release it.  Note
 * that it is not safe to call any function that might sleep in the interim.
 *
 * The implementation could have used reference counting to avoid this
 * restriction, but since most clients simply want to get, set or test a member
 * of the returned data structure, the extra per-element memory isn't warranted.
 *
 * If the element isn't found, this function returns NULL and the spinlock is
 * released.  xfs_mru_cache_done() should NOT be called when this occurs.
 *
 * Because sparse isn't smart enough to know about conditional lock return
 * status, we need to help it get it right by annotating the path that does
 * not release the lock.
 */
struct xfs_mru_cache_elem *
xfs_mru_cache_lookup(
	struct xfs_mru_cache	*mru,
	unsigned long		key)
{
	struct xfs_mru_cache_elem *elem;

	ASSERT(mru && mru->lists);
	if (!mru || !mru->lists)
		return NULL;

	spin_lock(&mru->lock);
	elem = radix_tree_lookup(&mru->store, key);
	if (elem) {
		list_del(&elem->list_node);
		_xfs_mru_cache_list_insert(mru, elem);
		__release(mru_lock); /* help sparse not be stupid */
	} else
		spin_unlock(&mru->lock);

	return elem;
}

/*
 * To release the internal data structure spinlock after having performed an
 * xfs_mru_cache_lookup() or an xfs_mru_cache_peek(), call xfs_mru_cache_done()
 * with the data store pointer.
 */
void
xfs_mru_cache_done(
	struct xfs_mru_cache	*mru)
		__releases(mru->lock)
{
	spin_unlock(&mru->lock);
}