Re: [PATCH v4] zswap: replace RB tree with xarray

From: Chengming Zhou
Date: Mon Mar 04 2024 - 21:52:41 EST

Hi Chris,

On 2024/3/5 05:32, Chris Li wrote:
> Very deep RB tree requires rebalance at times. That
> contributes to the zswap fault latencies. Xarray does not
> need to perform tree rebalance. Replacing RB tree to xarray
> can have some small performance gain.
>
> One small difference is that xarray insert might fail with
> ENOMEM, while RB tree insert does not allocate additional
> memory.
>
> The zswap_entry size will reduce a bit due to removing the
> RB node, which has two pointers and a color field. Xarray
> store the pointer in the xarray tree rather than the
> zswap_entry. Every entry has one pointer from the xarray
> tree. Overall, switching to xarray should save some memory,
> if the swap entries are densely packed.
>
> Notice the zswap_rb_search and zswap_rb_insert always
> followed by zswap_rb_erase. Use xa_erase directly. The entry
> erase into zswap_xa_insert as well. That saves one tree
> lookup as well.
>
> Remove zswap_invalidate_entry due to no need to call
> zswap_rb_erase any more. Use zswap_free_entry instead.
>
> The "struct zswap_tree" has been replaced by "struct xarray".
> The tree spin lock has transferred to the xarray lock.
>
> Run the kernel build testing 10 times for each version, averages:
> (memory.max=2GB, zswap shrinker and writeback enabled, one 50GB swapfile.)
>
> mm-9a0181a3710eb xarray v4
> user 3526.829 3526.930
> sys 532.754 526.525
> real 198.748 198.850
>
> ---
>
>
> Signed-off-by: Chris Li <chrisl@xxxxxxxxxx>
> ---
> Changes in v4:
> - Remove zswap_xa_search_and_earse, use xa_erase directly.
> - Move charge of objcg after zswap_xa_insert.
> - Avoid erase old entry on insert fail error path.
> - Remove not needed swap_zswap_tree change
> - Link to v3: https://lore.kernel.org/r/20240302-zswap-xarray-v3-1-5900252f2302@xxxxxxxxxx
>
> Changes in v3:
> - Use xa_cmpxchg instead of zswap_xa_search_and_delete in zswap_writeback_entry.
> - Use xa_store in zswap_xa_insert directly. Reduce the scope of spinlock.
> - Fix xa_store error handling for same page fill case.
> - Link to v2: https://lore.kernel.org/r/20240229-zswap-xarray-v2-1-e50284dfcdb1@xxxxxxxxxx
>
> Changes in v2:
> - Replace struct zswap_tree with struct xarray.
> - Remove zswap_tree spinlock, use xarray lock instead.
> - Fold zswap_rb_erase() into zswap_xa_search_and_delete() and zswap_xa_insert().
> - Delete zswap_invalidate_entry(), use zswap_free_entry() instead.
> - Link to v1: https://lore.kernel.org/r/20240117-zswap-xarray-v1-0-6daa86c08fae@xxxxxxxxxx
> ---
> mm/zswap.c | 186 ++++++++++++++++++++++++-------------------------------------
> 1 file changed, 72 insertions(+), 114 deletions(-)
>
> diff --git a/mm/zswap.c b/mm/zswap.c
> index 011e068eb355..4f4a3f452b76 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -20,7 +20,6 @@
> #include <linux/spinlock.h>
> #include <linux/types.h>
> #include <linux/atomic.h>
> -#include <linux/rbtree.h>
> #include <linux/swap.h>
> #include <linux/crypto.h>
> #include <linux/scatterlist.h>
> @@ -71,6 +70,8 @@ static u64 zswap_reject_compress_poor;
> static u64 zswap_reject_alloc_fail;
> /* Store failed because the entry metadata could not be allocated (rare) */
> static u64 zswap_reject_kmemcache_fail;
> +/* Store failed because xarray can't insert the entry*/
> +static u64 zswap_reject_xarray_fail;
>
> /* Shrinker work queue */
> static struct workqueue_struct *shrink_wq;
> @@ -196,7 +197,6 @@ static struct {
> * This structure contains the metadata for tracking a single compressed
> * page within zswap.
> *
> - * rbnode - links the entry into red-black tree for the appropriate swap type
> * swpentry - associated swap entry, the offset indexes into the red-black tree
> * length - the length in bytes of the compressed page data. Needed during
> * decompression. For a same value filled page length is 0, and both
> @@ -208,7 +208,6 @@ static struct {
> * lru - handle to the pool's lru used to evict pages.
> */
> struct zswap_entry {
> - struct rb_node rbnode;
> swp_entry_t swpentry;
> unsigned int length;
> struct zswap_pool *pool;
> @@ -220,12 +219,7 @@ struct zswap_entry {
> struct list_head lru;
> };
>
> -struct zswap_tree {
> - struct rb_root rbroot;
> - spinlock_t lock;
> -};
> -
> -static struct zswap_tree *zswap_trees[MAX_SWAPFILES];
> +static struct xarray *zswap_trees[MAX_SWAPFILES];
> static unsigned int nr_zswap_trees[MAX_SWAPFILES];
>
> /* RCU-protected iteration */
> @@ -253,7 +247,7 @@ static bool zswap_has_pool;
> * helpers and fwd declarations
> **********************************/
>
> -static inline struct zswap_tree *swap_zswap_tree(swp_entry_t swp)
> +static inline struct xarray *swap_zswap_tree(swp_entry_t swp)
> {
> return &zswap_trees[swp_type(swp)][swp_offset(swp)
> >> SWAP_ADDRESS_SPACE_SHIFT];
> @@ -805,60 +799,33 @@ void zswap_memcg_offline_cleanup(struct mem_cgroup *memcg)
> }
>
> /*********************************
> -* rbtree functions
> +* xarray functions
> **********************************/
> -static struct zswap_entry *zswap_rb_search(struct rb_root *root, pgoff_t offset)
> -{
> - struct rb_node *node = root->rb_node;
> - struct zswap_entry *entry;
> - pgoff_t entry_offset;
> -
> - while (node) {
> - entry = rb_entry(node, struct zswap_entry, rbnode);
> - entry_offset = swp_offset(entry->swpentry);
> - if (entry_offset > offset)
> - node = node->rb_left;
> - else if (entry_offset < offset)
> - node = node->rb_right;
> - else
> - return entry;
> - }
> - return NULL;
> -}
>
> /*
> * In the case that a entry with the same offset is found, a pointer to
> - * the existing entry is stored in dupentry and the function returns -EEXIST
> + * the existing entry is stored in old and erased from the tree.
> + * Function return error on insert.
> */
> -static int zswap_rb_insert(struct rb_root *root, struct zswap_entry *entry,
> - struct zswap_entry **dupentry)
> +static int zswap_xa_insert(struct xarray *tree, struct zswap_entry *entry,
> + struct zswap_entry **old)
> {
> - struct rb_node **link = &root->rb_node, *parent = NULL;
> - struct zswap_entry *myentry;
> - pgoff_t myentry_offset, entry_offset = swp_offset(entry->swpentry);
> -
> - while (*link) {
> - parent = *link;
> - myentry = rb_entry(parent, struct zswap_entry, rbnode);
> - myentry_offset = swp_offset(myentry->swpentry);
> - if (myentry_offset > entry_offset)
> - link = &(*link)->rb_left;
> - else if (myentry_offset < entry_offset)
> - link = &(*link)->rb_right;
> - else {
> - *dupentry = myentry;
> - return -EEXIST;
> - }
> - }
> - rb_link_node(&entry->rbnode, parent, link);
> - rb_insert_color(&entry->rbnode, root);
> - return 0;
> -}
> + int err;
> + struct zswap_entry *e;
> + pgoff_t offset = swp_offset(entry->swpentry);
>
> -static void zswap_rb_erase(struct rb_root *root, struct zswap_entry *entry)
> -{
> - rb_erase(&entry->rbnode, root);
> - RB_CLEAR_NODE(&entry->rbnode);
> + e = xa_store(tree, offset, entry, GFP_KERNEL);
> + err = xa_err(e);
> +
> + if (err) {
> + e = xa_erase(tree, offset);
> + if (err == -ENOMEM)
> + zswap_reject_alloc_fail++;
> + else
> + zswap_reject_xarray_fail++;
> + }
> + *old = e;
> + return err;
> }
>
> /*********************************
> @@ -872,7 +839,6 @@ static struct zswap_entry *zswap_entry_cache_alloc(gfp_t gfp, int nid)
> entry = kmem_cache_alloc_node(zswap_entry_cache, gfp, nid);
> if (!entry)
> return NULL;
> - RB_CLEAR_NODE(&entry->rbnode);
> return entry;
> }
>
> @@ -914,17 +880,6 @@ static void zswap_entry_free(struct zswap_entry *entry)
> zswap_update_total_size();
> }
>
> -/*
> - * The caller hold the tree lock and search the entry from the tree,
> - * so it must be on the tree, remove it from the tree and free it.
> - */
> -static void zswap_invalidate_entry(struct zswap_tree *tree,
> - struct zswap_entry *entry)
> -{
> - zswap_rb_erase(&tree->rbroot, entry);
> - zswap_entry_free(entry);
> -}
> -
> /*********************************
> * compressed storage functions
> **********************************/
> @@ -1113,7 +1068,9 @@ static void zswap_decompress(struct zswap_entry *entry, struct page *page)
> static int zswap_writeback_entry(struct zswap_entry *entry,
> swp_entry_t swpentry)
> {
> - struct zswap_tree *tree;
> + struct xarray *tree;
> + pgoff_t offset = swp_offset(swpentry);
> + struct zswap_entry *e;
> struct folio *folio;
> struct mempolicy *mpol;
> bool folio_was_allocated;
> @@ -1150,19 +1107,14 @@ static int zswap_writeback_entry(struct zswap_entry *entry,
> * be dereferenced.
> */
> tree = swap_zswap_tree(swpentry);
> - spin_lock(&tree->lock);
> - if (zswap_rb_search(&tree->rbroot, swp_offset(swpentry)) != entry) {
> - spin_unlock(&tree->lock);
> + e = xa_cmpxchg(tree, offset, entry, NULL, GFP_KERNEL);
> + if (e != entry) {

Maybe "if (xa_cmpxchg() != entry)" look better, so "e" variable can be removed,
since we don't use it.

> delete_from_swap_cache(folio);
> folio_unlock(folio);
> folio_put(folio);
> return -ENOMEM;
> }
>
> - /* Safe to deref entry after the entry is verified above. */
> - zswap_rb_erase(&tree->rbroot, entry);
> - spin_unlock(&tree->lock);
> -
> zswap_decompress(entry, &folio->page);
>
> count_vm_event(ZSWPWB);
> @@ -1471,10 +1423,12 @@ bool zswap_store(struct folio *folio)
> {
> swp_entry_t swp = folio->swap;
> pgoff_t offset = swp_offset(swp);
> - struct zswap_tree *tree = swap_zswap_tree(swp);
> - struct zswap_entry *entry, *dupentry;
> + struct xarray *tree = swap_zswap_tree(swp);
> + struct zswap_entry *entry, *old;
> struct obj_cgroup *objcg = NULL;
> struct mem_cgroup *memcg = NULL;
> + int err;
> + bool old_erased = false;
>
> VM_WARN_ON_ONCE(!folio_test_locked(folio));
> VM_WARN_ON_ONCE(!folio_test_swapcache(folio));
> @@ -1526,6 +1480,7 @@ bool zswap_store(struct folio *folio)
> kunmap_local(src);
> entry->length = 0;
> entry->value = value;
> + entry->pool = NULL;
> atomic_inc(&zswap_same_filled_pages);
> goto insert_entry;
> }
> @@ -1555,28 +1510,31 @@ bool zswap_store(struct folio *folio)
> insert_entry:
> entry->swpentry = swp;
> entry->objcg = objcg;
> - if (objcg) {
> - obj_cgroup_charge_zswap(objcg, entry->length);
> - /* Account before objcg ref is moved to tree */
> - count_objcg_event(objcg, ZSWPOUT);
> - }
>
> /* map */
> - spin_lock(&tree->lock);
> /*
> * The folio may have been dirtied again, invalidate the
> * possibly stale entry before inserting the new entry.
> */
> - if (zswap_rb_insert(&tree->rbroot, entry, &dupentry) == -EEXIST) {
> - zswap_invalidate_entry(tree, dupentry);
> - WARN_ON(zswap_rb_insert(&tree->rbroot, entry, &dupentry));
> + err = zswap_xa_insert(tree, entry, &old);
> + if (old)
> + zswap_entry_free(old);
> + if (err) {
> + old_erased = true;
> + goto insert_failed;
> }

It looks a little complicated for me :( How do you think this like below?

old = xa_store(tree, offset, entry, GFP_KERNEL);
if (xa_is_err(old))
goto store_failed;

if (old)
zswap_entry_free(old);

Then zswap_xa_insert() wrapper can be removed since no use elsewhere.
So the error handling path is kept much the same as before and simpler.

> +
> + if (objcg) {
> + obj_cgroup_charge_zswap(objcg, entry->length);
> + /* Account before objcg ref is moved to tree */
> + count_objcg_event(objcg, ZSWPOUT);
> + }
> +
> if (entry->length) {
> INIT_LIST_HEAD(&entry->lru);
> zswap_lru_add(&zswap.list_lru, entry);
> atomic_inc(&zswap.nr_stored);
> }
> - spin_unlock(&tree->lock);
>
> /* update stats */
> atomic_inc(&zswap_stored_pages);
> @@ -1585,6 +1543,12 @@ bool zswap_store(struct folio *folio)
>
> return true;
>
> +insert_failed:
> + if (!entry->length) {
> + atomic_dec(&zswap_same_filled_pages);
> + goto freepage;
> + }
> + zpool_free(zswap_find_zpool(entry), entry->handle);

entry->pool can be used here instead of zswap_find_zpool().

Thanks!

> put_pool:
> zswap_pool_put(entry->pool);
> freepage:
> @@ -1592,17 +1556,19 @@ bool zswap_store(struct folio *folio)
> reject:
> if (objcg)
> obj_cgroup_put(objcg);
> +
> + if (old_erased)
> + goto failed;
> check_old:
> /*
> * If the zswap store fails or zswap is disabled, we must invalidate the
> * possibly stale entry which was previously stored at this offset.
> * Otherwise, writeback could overwrite the new data in the swapfile.
> */
> - spin_lock(&tree->lock);
> - entry = zswap_rb_search(&tree->rbroot, offset);
> + entry = xa_erase(tree, offset);
> if (entry)
> - zswap_invalidate_entry(tree, entry);
> - spin_unlock(&tree->lock);
> + zswap_entry_free(entry);
> +failed:
> return false;
>
> shrink:
> @@ -1615,20 +1581,15 @@ bool zswap_load(struct folio *folio)
> swp_entry_t swp = folio->swap;
> pgoff_t offset = swp_offset(swp);
> struct page *page = &folio->page;
> - struct zswap_tree *tree = swap_zswap_tree(swp);
> + struct xarray *tree = swap_zswap_tree(swp);
> struct zswap_entry *entry;
> u8 *dst;
>
> VM_WARN_ON_ONCE(!folio_test_locked(folio));
>
> - spin_lock(&tree->lock);
> - entry = zswap_rb_search(&tree->rbroot, offset);
> - if (!entry) {
> - spin_unlock(&tree->lock);
> + entry = xa_erase(tree, offset);
> + if (!entry)
> return false;
> - }
> - zswap_rb_erase(&tree->rbroot, entry);
> - spin_unlock(&tree->lock);
>
> if (entry->length)
> zswap_decompress(entry, page);
> @@ -1652,19 +1613,17 @@ bool zswap_load(struct folio *folio)
> void zswap_invalidate(swp_entry_t swp)
> {
> pgoff_t offset = swp_offset(swp);
> - struct zswap_tree *tree = swap_zswap_tree(swp);
> + struct xarray *tree = swap_zswap_tree(swp);
> struct zswap_entry *entry;
>
> - spin_lock(&tree->lock);
> - entry = zswap_rb_search(&tree->rbroot, offset);
> + entry = xa_erase(tree, offset);
> if (entry)
> - zswap_invalidate_entry(tree, entry);
> - spin_unlock(&tree->lock);
> + zswap_entry_free(entry);
> }
>
> int zswap_swapon(int type, unsigned long nr_pages)
> {
> - struct zswap_tree *trees, *tree;
> + struct xarray *trees, *tree;
> unsigned int nr, i;
>
> nr = DIV_ROUND_UP(nr_pages, SWAP_ADDRESS_SPACE_PAGES);
> @@ -1674,11 +1633,8 @@ int zswap_swapon(int type, unsigned long nr_pages)
> return -ENOMEM;
> }
>
> - for (i = 0; i < nr; i++) {
> - tree = trees + i;
> - tree->rbroot = RB_ROOT;
> - spin_lock_init(&tree->lock);
> - }
> + for (i = 0; i < nr; i++)
> + xa_init(trees + i);
>
> nr_zswap_trees[type] = nr;
> zswap_trees[type] = trees;
> @@ -1687,7 +1643,7 @@ int zswap_swapon(int type, unsigned long nr_pages)
>
> void zswap_swapoff(int type)
> {
> - struct zswap_tree *trees = zswap_trees[type];
> + struct xarray *trees = zswap_trees[type];
> unsigned int i;
>
> if (!trees)
> @@ -1695,7 +1651,7 @@ void zswap_swapoff(int type)
>
> /* try_to_unuse() invalidated all the entries already */
> for (i = 0; i < nr_zswap_trees[type]; i++)
> - WARN_ON_ONCE(!RB_EMPTY_ROOT(&trees[i].rbroot));
> + WARN_ON_ONCE(!xa_empty(trees + i));
>
> kvfree(trees);
> nr_zswap_trees[type] = 0;
> @@ -1727,6 +1683,8 @@ static int zswap_debugfs_init(void)
> zswap_debugfs_root, &zswap_reject_kmemcache_fail);
> debugfs_create_u64("reject_compress_fail", 0444,
> zswap_debugfs_root, &zswap_reject_compress_fail);
> + debugfs_create_u64("reject_xarray_fail", 0444,
> + zswap_debugfs_root, &zswap_reject_xarray_fail);
> debugfs_create_u64("reject_compress_poor", 0444,
> zswap_debugfs_root, &zswap_reject_compress_poor);
> debugfs_create_u64("written_back_pages", 0444,
>
> ---
> base-commit: 9a0181a3710eba1f5c6d19eadcca888be3d54e4f
> change-id: 20240104-zswap-xarray-716260e541e3
>
> Best regards,