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

[Chris Li <chrisl@kernel.org>]

On Mon, Mar 4, 2024 at 6:52 PM Chengming Zhou
<zhouchengming@bytedance.com> 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 The "struct zswap_tree" has been replaced by "struct xarray".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@kernel.org>
> > ---
> > 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@kernel.org
> >
> > 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@kernel.org
> >
> > 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@kernel.org
> > ---
> >  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().

Actually you can't use the entry->pool there. I get a compile error.

static struct zpool *zswap_find_zpool(struct zswap_entry *entry)
{
        int i = 0;

        if (ZSWAP_NR_ZPOOLS > 1)
                i = hash_ptr(entry, ilog2(ZSWAP_NR_ZPOOLS));

        return entry->pool->zpools[i];
}

Find zpool will return one of the entry->pool->zpools[i]. It is not
the same as entry->pool.

Chris