- 根目录:
- fs
- xfs
- xfs_mru_cache.c
/*
* 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);
}