On Wed, Mar 6, 2024 at 6:17 PM Chengming Zhou <zhouchengming@xxxxxxxxxxxxx> wrote: > > On 2024/3/7 05:12, Chris Li wrote: > > Hi Chengming, > > > > On Mon, Mar 4, 2024 at 6:52 PM Chengming Zhou > > <zhouchengming@xxxxxxxxxxxxx> wrote: > >> > >> 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 The "struct zswap_tree" has been replaced by "struct xarray".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. > > > > I thought about that. The main reason "if (xa_cmpxchg(tree, offset, > > entry, NULL, GFP_KERNEL) != entry)" is a long expression. Having > > != entry at the end of the expression makes it harder to read. Have > > the temporary variable IMHO help reading the comparison. > > > > Not sure I understand the motivation to save a temporary variable. The > > compiler always generates temporary variables internally. Having one > > here does not affect the compiling result at all. It is just there to > > help reading. If you still think it has the reverse effect on reading > > I can of course remove it. It generates the same code anyway. > You are right, just a minor type suggestion, no much difference if you keep it. Thanks for the understanding and flexibility. I agree this is more a personal flavor kind of thing, Doesn't have a big enough impact on readability which has to flip one way or the other. Chris
