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authorAndré Fabian Silva Delgado <emulatorman@parabola.nu>2015-08-05 17:04:01 -0300
committerAndré Fabian Silva Delgado <emulatorman@parabola.nu>2015-08-05 17:04:01 -0300
commit57f0f512b273f60d52568b8c6b77e17f5636edc0 (patch)
tree5e910f0e82173f4ef4f51111366a3f1299037a7b /fs/xfs/xfs_mru_cache.c
Initial import
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+/*
+ * 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);
+}