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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.
*/
typedef struct xfs_mru_cache_elem
{
struct list_head list_node;
unsigned long key;
void *value;
} xfs_mru_cache_elem_t;
static kmem_zone_t *xfs_mru_elem_zone;
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(
xfs_mru_cache_t *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(
xfs_mru_cache_t *mru,
xfs_mru_cache_elem_t *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(
xfs_mru_cache_t *mru) __releases(mru->lock) __acquires(mru->lock)
{
xfs_mru_cache_elem_t *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) {
/* Remove the element from the reap list. */
list_del_init(&elem->list_node);
/* Call the client's free function with the key and value pointer. */
mru->free_func(elem->key, elem->value);
/* Free the element structure. */
kmem_zone_free(xfs_mru_elem_zone, 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)
{
xfs_mru_cache_t *mru = container_of(work, xfs_mru_cache_t, 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_elem_zone = kmem_zone_init(sizeof(xfs_mru_cache_elem_t),
"xfs_mru_cache_elem");
if (!xfs_mru_elem_zone)
goto out;
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