Re: [PATCH v8] zswap: replace RB tree with xarray

[Chengming Zhou]

On 2024/3/21 03:31, 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 and xa_store
> directly. 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 The "struct zswap_tree" has been replaced by "struct xarray".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, 24 HT core, 32 jobs)
> 
>            mm-unstable-a824831a082f     xarray v7
> user       3547.264			3541.509
> sys        531.176                      526.111
> real       200.752                      201.334
> 
> Reviewed-by: Nhat Pham <nphamcs@xxxxxxxxx>
> Acked-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
> Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> Signed-off-by: Chris Li <chrisl@xxxxxxxxxx>

Very nice!

Reviewed-by: Chengming Zhou <chengming.zhou@xxxxxxxxx>

Thanks.

> ---
> Changes in v8:
> - Fix some white space damage discovered by Yosry. No actual code change.
> - Collect review tags.
> - Link to v7: https://lore.kernel.org/r/20240319-zswap-xarray-v7-1-e9a03a049e86@xxxxxxxxxx
> 
> Changes in v7:
> - update comment suggested by Johannes and Yosry
> - Simplify some error handling code, suggested by Johannes.
> - Link to v6: https://lore.kernel.org/r/20240312-zswap-xarray-v6-1-1b82027d7082@xxxxxxxxxx
> 
> Changes in v6:
> - Add WARN_ONCE() for xa_store failed other than -ENOMEM.
> - Collect review tags.
> - Link to v5: https://lore.kernel.org/r/20240311-zswap-xarray-v5-1-a3031feb9c85@xxxxxxxxxx
> 
> Changes in v5:
> - Remove zswap_xa_insert(), call xa_store and xa_erase directly.
> - Remove zswap_reject_xarray_fail. 
> - Link to v4: https://lore.kernel.org/r/20240304-zswap-xarray-v4-1-c4b45670cc30@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 | 178 ++++++++++++++++++-------------------------------------------
>  1 file changed, 53 insertions(+), 125 deletions(-)
> 
> diff --git a/mm/zswap.c b/mm/zswap.c
> index b31c977f53e9..a97de9e547df 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>
> @@ -196,7 +195,6 @@ static struct shrinker *zswap_shrinker;
>   * 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 +206,6 @@ static struct shrinker *zswap_shrinker;
>   * 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 +217,7 @@ 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 +245,7 @@ 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];
> @@ -792,63 +784,6 @@ void zswap_memcg_offline_cleanup(struct mem_cgroup *memcg)
>  	spin_unlock(&zswap_shrink_lock);
>  }
>  
> -/*********************************
> -* rbtree 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
> - */
> -static int zswap_rb_insert(struct rb_root *root, struct zswap_entry *entry,
> -			struct zswap_entry **dupentry)
> -{
> -	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;
> -}
> -
> -static void zswap_rb_erase(struct rb_root *root, struct zswap_entry *entry)
> -{
> -	rb_erase(&entry->rbnode, root);
> -	RB_CLEAR_NODE(&entry->rbnode);
> -}
> -
>  /*********************************
>  * zswap entry functions
>  **********************************/
> @@ -860,7 +795,6 @@ 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;
>  }
>  
> @@ -896,17 +830,6 @@ static void zswap_entry_free(struct zswap_entry *entry)
>  	atomic_dec(&zswap_stored_pages);
>  }
>  
> -/*
> - * 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
>  **********************************/
> @@ -1106,7 +1029,8 @@ 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 folio *folio;
>  	struct mempolicy *mpol;
>  	bool folio_was_allocated;
> @@ -1143,19 +1067,13 @@ 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);
> +	if (entry != xa_cmpxchg(tree, offset, entry, NULL, GFP_KERNEL)) {
>  		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);
> @@ -1467,8 +1385,8 @@ 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;
>  	unsigned long max_pages, cur_pages;
> @@ -1556,28 +1474,44 @@ bool zswap_store(struct folio *folio)
>  insert_entry:
>  	entry->swpentry = swp;
>  	entry->objcg = objcg;
> +
> +	old = xa_store(tree, offset, entry, GFP_KERNEL);
> +	if (xa_is_err(old)) {
> +		int err = xa_err(old);
> +
> +		WARN_ONCE(err != -ENOMEM, "unexpected xarray error: %d\n", err);
> +		zswap_reject_alloc_fail++;
> +		goto store_failed;
> +	}
> +
> +	/*
> +	 * We may have had an existing entry that became stale when
> +	 * the folio was redirtied and now the new version is being
> +	 * swapped out. Get rid of the old.
> +	 */
> +	if (old)
> +		zswap_entry_free(old);
> +
>  	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.
> +	 * We finish initializing the entry while it's already in xarray.
> +	 * This is safe because:
> +	 *
> +	 * 1. Concurrent stores and invalidations are excluded by folio lock.
> +	 *
> +	 * 2. Writeback is excluded by the entry not being on the LRU yet.
> +	 *    The publishing order matters to prevent writeback from seeing
> +	 *    an incoherent entry.
>  	 */
> -	if (zswap_rb_insert(&tree->rbroot, entry, &dupentry) == -EEXIST) {
> -		zswap_invalidate_entry(tree, dupentry);
> -		WARN_ON(zswap_rb_insert(&tree->rbroot, entry, &dupentry));
> -	}
>  	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);
> @@ -1585,8 +1519,14 @@ bool zswap_store(struct folio *folio)
>  
>  	return true;
>  
> +store_failed:
> +	if (!entry->length)
> +		atomic_dec(&zswap_same_filled_pages);
> +	else {
> +		zpool_free(zswap_find_zpool(entry), entry->handle);
>  put_pool:
> -	zswap_pool_put(entry->pool);
> +		zswap_pool_put(entry->pool);
> +	}
>  freepage:
>  	zswap_entry_cache_free(entry);
>  reject:
> @@ -1597,11 +1537,9 @@ bool zswap_store(struct folio *folio)
>  	 * 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);
>  	return false;
>  
>  shrink:
> @@ -1614,20 +1552,15 @@ 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);
> @@ -1651,19 +1584,17 @@ 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);
> @@ -1673,11 +1604,8 @@ 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;
> @@ -1686,7 +1614,7 @@ 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)
> @@ -1694,7 +1622,7 @@ 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;
> 
> ---
> base-commit: a824831a082f1d8f9b51a4c0598e633d38555fcf
> change-id: 20240104-zswap-xarray-716260e541e3
> 
> Best regards,