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    SubjectRe: [PATCH 15/17] fs/ext4: use cached rbtrees
    On Tue 18-07-17 18:46:01, Davidlohr Bueso wrote:
    > This patch replaces two uses of rbtrees to the cached version for
    > better rb_first() performance during:
    >
    > * Tree traversals to mark blocks as used during ext4_mb_init_cache()
    > looping. (struct ext4_group_info)
    >
    > * Storing ->curr_node for dx_readdir(). (struct dir_private_info)
    >
    > Caching the leftmost node comes at negligible runtime cost, thus the
    > only downside is that the aforementioned structures are enlarged for
    > the extra pointer. No changes in semantics whatsoever.
    >
    > Cc: tytso@mit.edu
    > Signed-off-by: Davidlohr Bueso <dbueso@suse.de>

    Hum, so I'm somewhat undecided whether this is worth the churn. For free
    blocks rb_tree we use rb_first() only in ext4_mb_generate_from_freelist()
    which gets called only when generating new buddy bitmap from on-disk bitmap
    and we traverse the whole tree after that - thus the extra cost of
    rb_first() is a) well hidden in the total cost of iteration, b) rather rare
    anyway.

    Similarly for the H-tree directory code, we call rb_first() in
    ext4_dx_readdir() only to start an iteration over the whole B-tree and in
    such case I don't think optimizing rb_first() makes a big difference
    (maintaining cached value is going to have about the same cost as we save
    by using it).

    Honza

    > ---
    > This is part of the rbtree internal caching series:
    > https://lwn.net/Articles/726809/
    >
    > fs/ext4/dir.c | 24 ++++++++++++++----------
    > fs/ext4/ext4.h | 4 ++--
    > fs/ext4/mballoc.c | 22 ++++++++++++----------
    > 3 files changed, 28 insertions(+), 22 deletions(-)
    >
    > diff --git a/fs/ext4/dir.c b/fs/ext4/dir.c
    > index e8b365000d73..011df6bba13c 100644
    > --- a/fs/ext4/dir.c
    > +++ b/fs/ext4/dir.c
    > @@ -391,18 +391,18 @@ struct fname {
    > * This functoin implements a non-recursive way of freeing all of the
    > * nodes in the red-black tree.
    > */
    > -static void free_rb_tree_fname(struct rb_root *root)
    > +static void free_rb_tree_fname(struct rb_root_cached *root)
    > {
    > struct fname *fname, *next;
    >
    > - rbtree_postorder_for_each_entry_safe(fname, next, root, rb_hash)
    > + rbtree_postorder_for_each_entry_safe(fname, next, &root->rb_root, rb_hash)
    > while (fname) {
    > struct fname *old = fname;
    > fname = fname->next;
    > kfree(old);
    > }
    >
    > - *root = RB_ROOT;
    > + *root = RB_ROOT_CACHED;
    > }
    >
    >
    > @@ -441,9 +441,10 @@ int ext4_htree_store_dirent(struct file *dir_file, __u32 hash,
    > struct fname *fname, *new_fn;
    > struct dir_private_info *info;
    > int len;
    > + bool leftmost = true;
    >
    > info = dir_file->private_data;
    > - p = &info->root.rb_node;
    > + p = &info->root.rb_root.rb_node;
    >
    > /* Create and allocate the fname structure */
    > len = sizeof(struct fname) + ent_name->len + 1;
    > @@ -475,16 +476,19 @@ int ext4_htree_store_dirent(struct file *dir_file, __u32 hash,
    >
    > if (new_fn->hash < fname->hash)
    > p = &(*p)->rb_left;
    > - else if (new_fn->hash > fname->hash)
    > + else if (new_fn->hash > fname->hash) {
    > p = &(*p)->rb_right;
    > - else if (new_fn->minor_hash < fname->minor_hash)
    > + leftmost = false;
    > + } else if (new_fn->minor_hash < fname->minor_hash)
    > p = &(*p)->rb_left;
    > - else /* if (new_fn->minor_hash > fname->minor_hash) */
    > + else { /* if (new_fn->minor_hash > fname->minor_hash) */
    > p = &(*p)->rb_right;
    > + leftmost = false;
    > + }
    > }
    >
    > rb_link_node(&new_fn->rb_hash, parent, p);
    > - rb_insert_color(&new_fn->rb_hash, &info->root);
    > + rb_insert_color_cached(&new_fn->rb_hash, &info->root, leftmost);
    > return 0;
    > }
    >
    > @@ -558,7 +562,7 @@ static int ext4_dx_readdir(struct file *file, struct dir_context *ctx)
    > info->extra_fname = NULL;
    > goto next_node;
    > } else if (!info->curr_node)
    > - info->curr_node = rb_first(&info->root);
    > + info->curr_node = rb_first_cached(&info->root);
    >
    > while (1) {
    > /*
    > @@ -580,7 +584,7 @@ static int ext4_dx_readdir(struct file *file, struct dir_context *ctx)
    > ctx->pos = ext4_get_htree_eof(file);
    > break;
    > }
    > - info->curr_node = rb_first(&info->root);
    > + info->curr_node = rb_first_cached(&info->root);
    > }
    >
    > fname = rb_entry(info->curr_node, struct fname, rb_hash);
    > diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
    > index 9ebde0cd632e..0242c6801600 100644
    > --- a/fs/ext4/ext4.h
    > +++ b/fs/ext4/ext4.h
    > @@ -2116,7 +2116,7 @@ static inline bool ext4_is_quota_file(struct inode *inode)
    > * readdir operations in hash tree order.
    > */
    > struct dir_private_info {
    > - struct rb_root root;
    > + struct rb_root_cached root;
    > struct rb_node *curr_node;
    > struct fname *extra_fname;
    > loff_t last_pos;
    > @@ -2883,7 +2883,7 @@ int ext4_update_disksize_before_punch(struct inode *inode, loff_t offset,
    >
    > struct ext4_group_info {
    > unsigned long bb_state;
    > - struct rb_root bb_free_root;
    > + struct rb_root_cached bb_free_root;
    > ext4_grpblk_t bb_first_free; /* first free block */
    > ext4_grpblk_t bb_free; /* total free blocks */
    > ext4_grpblk_t bb_fragments; /* nr of freespace fragments */
    > diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
    > index 581e357e8406..df36f9123a99 100644
    > --- a/fs/ext4/mballoc.c
    > +++ b/fs/ext4/mballoc.c
    > @@ -2462,7 +2462,7 @@ int ext4_mb_add_groupinfo(struct super_block *sb, ext4_group_t group,
    >
    > INIT_LIST_HEAD(&meta_group_info[i]->bb_prealloc_list);
    > init_rwsem(&meta_group_info[i]->alloc_sem);
    > - meta_group_info[i]->bb_free_root = RB_ROOT;
    > + meta_group_info[i]->bb_free_root = RB_ROOT_CACHED;
    > meta_group_info[i]->bb_largest_free_order = -1; /* uninit */
    >
    > #ifdef DOUBLE_CHECK
    > @@ -2824,7 +2824,7 @@ static void ext4_free_data_in_buddy(struct super_block *sb,
    > count2++;
    > ext4_lock_group(sb, entry->efd_group);
    > /* Take it out of per group rb tree */
    > - rb_erase(&entry->efd_node, &(db->bb_free_root));
    > + rb_erase_cached(&entry->efd_node, &(db->bb_free_root));
    > mb_free_blocks(NULL, &e4b, entry->efd_start_cluster, entry->efd_count);
    >
    > /*
    > @@ -2836,7 +2836,7 @@ static void ext4_free_data_in_buddy(struct super_block *sb,
    > if (!test_opt(sb, DISCARD))
    > EXT4_MB_GRP_CLEAR_TRIMMED(db);
    >
    > - if (!db->bb_free_root.rb_node) {
    > + if (!db->bb_free_root.rb_root.rb_node) {
    > /* No more items in the per group rb tree
    > * balance refcounts from ext4_mb_free_metadata()
    > */
    > @@ -3519,7 +3519,7 @@ static void ext4_mb_generate_from_freelist(struct super_block *sb, void *bitmap,
    > struct ext4_free_data *entry;
    >
    > grp = ext4_get_group_info(sb, group);
    > - n = rb_first(&(grp->bb_free_root));
    > + n = rb_first_cached(&(grp->bb_free_root));
    >
    > while (n) {
    > entry = rb_entry(n, struct ext4_free_data, efd_node);
    > @@ -4624,7 +4624,7 @@ ext4_fsblk_t ext4_mb_new_blocks(handle_t *handle,
    > static void ext4_try_merge_freed_extent(struct ext4_sb_info *sbi,
    > struct ext4_free_data *entry,
    > struct ext4_free_data *new_entry,
    > - struct rb_root *entry_rb_root)
    > + struct rb_root_cached *entry_rb_root)
    > {
    > if ((entry->efd_tid != new_entry->efd_tid) ||
    > (entry->efd_group != new_entry->efd_group))
    > @@ -4641,7 +4641,7 @@ static void ext4_try_merge_freed_extent(struct ext4_sb_info *sbi,
    > spin_lock(&sbi->s_md_lock);
    > list_del(&entry->efd_list);
    > spin_unlock(&sbi->s_md_lock);
    > - rb_erase(&entry->efd_node, entry_rb_root);
    > + rb_erase_cached(&entry->efd_node, entry_rb_root);
    > kmem_cache_free(ext4_free_data_cachep, entry);
    > }
    >
    > @@ -4656,8 +4656,9 @@ ext4_mb_free_metadata(handle_t *handle, struct ext4_buddy *e4b,
    > struct ext4_group_info *db = e4b->bd_info;
    > struct super_block *sb = e4b->bd_sb;
    > struct ext4_sb_info *sbi = EXT4_SB(sb);
    > - struct rb_node **n = &db->bb_free_root.rb_node, *node;
    > + struct rb_node **n = &db->bb_free_root.rb_root.rb_node, *node;
    > struct rb_node *parent = NULL, *new_node;
    > + bool leftmost = true;
    >
    > BUG_ON(!ext4_handle_valid(handle));
    > BUG_ON(e4b->bd_bitmap_page == NULL);
    > @@ -4680,9 +4681,10 @@ ext4_mb_free_metadata(handle_t *handle, struct ext4_buddy *e4b,
    > entry = rb_entry(parent, struct ext4_free_data, efd_node);
    > if (cluster < entry->efd_start_cluster)
    > n = &(*n)->rb_left;
    > - else if (cluster >= (entry->efd_start_cluster + entry->efd_count))
    > + else if (cluster >= (entry->efd_start_cluster + entry->efd_count)) {
    > n = &(*n)->rb_right;
    > - else {
    > + leftmost = false;
    > + } else {
    > ext4_grp_locked_error(sb, group, 0,
    > ext4_group_first_block_no(sb, group) +
    > EXT4_C2B(sbi, cluster),
    > @@ -4692,7 +4694,7 @@ ext4_mb_free_metadata(handle_t *handle, struct ext4_buddy *e4b,
    > }
    >
    > rb_link_node(new_node, parent, n);
    > - rb_insert_color(new_node, &db->bb_free_root);
    > + rb_insert_color_cached(new_node, &db->bb_free_root, leftmost);
    >
    > /* Now try to see the extent can be merged to left and right */
    > node = rb_prev(new_node);
    > --
    > 2.12.0
    >
    --
    Jan Kara <jack@suse.com>
    SUSE Labs, CR

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