From: Filipe Manana Date: Wed, 15 Apr 2026 11:52:16 +0000 (+0100) Subject: btrfs: reduce size of struct btrfs_block_group X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=a8d1f0b5392473102f08d6f91f4dcdfaf9e29b0f;p=thirdparty%2Flinux.git btrfs: reduce size of struct btrfs_block_group We currently have several holes in the structure: struct btrfs_block_group { struct btrfs_fs_info * fs_info; /* 0 8 */ struct btrfs_inode * inode; /* 8 8 */ spinlock_t lock __attribute__((__aligned__(4))); /* 16 4 */ /* XXX 4 bytes hole, try to pack */ u64 start; /* 24 8 */ u64 length; /* 32 8 */ u64 pinned; /* 40 8 */ u64 reserved; /* 48 8 */ u64 used; /* 56 8 */ /* --- cacheline 1 boundary (64 bytes) --- */ u64 delalloc_bytes; /* 64 8 */ u64 bytes_super; /* 72 8 */ u64 flags; /* 80 8 */ u64 cache_generation; /* 88 8 */ u64 global_root_id; /* 96 8 */ u64 remap_bytes; /* 104 8 */ u32 identity_remap_count; /* 112 4 */ /* XXX 4 bytes hole, try to pack */ u64 last_used; /* 120 8 */ /* --- cacheline 2 boundary (128 bytes) --- */ u64 last_remap_bytes; /* 128 8 */ u32 last_identity_remap_count; /* 136 4 */ /* XXX 4 bytes hole, try to pack */ u64 last_flags; /* 144 8 */ u32 bitmap_high_thresh; /* 152 4 */ u32 bitmap_low_thresh; /* 156 4 */ struct rw_semaphore data_rwsem __attribute__((__aligned__(8))); /* 160 40 */ /* --- cacheline 3 boundary (192 bytes) was 8 bytes ago --- */ long unsigned int full_stripe_len; /* 200 8 */ long unsigned int runtime_flags; /* 208 8 */ unsigned int ro; /* 216 4 */ int disk_cache_state; /* 220 4 */ int cached; /* 224 4 */ /* XXX 4 bytes hole, try to pack */ struct btrfs_caching_control * caching_ctl; /* 232 8 */ struct btrfs_space_info * space_info; /* 240 8 */ struct btrfs_free_space_ctl * free_space_ctl; /* 248 8 */ /* --- cacheline 4 boundary (256 bytes) --- */ struct rb_node cache_node __attribute__((__aligned__(8))); /* 256 24 */ struct list_head list; /* 280 16 */ refcount_t refs __attribute__((__aligned__(4))); /* 296 4 */ /* XXX 4 bytes hole, try to pack */ struct list_head cluster_list; /* 304 16 */ /* --- cacheline 5 boundary (320 bytes) --- */ struct list_head bg_list; /* 320 16 */ struct list_head ro_list; /* 336 16 */ atomic_t frozen __attribute__((__aligned__(4))); /* 352 4 */ /* XXX 4 bytes hole, try to pack */ struct list_head discard_list; /* 360 16 */ int discard_index; /* 376 4 */ /* XXX 4 bytes hole, try to pack */ /* --- cacheline 6 boundary (384 bytes) --- */ u64 discard_eligible_time; /* 384 8 */ u64 discard_cursor; /* 392 8 */ enum btrfs_discard_state discard_state; /* 400 4 */ /* XXX 4 bytes hole, try to pack */ struct list_head dirty_list; /* 408 16 */ struct list_head io_list; /* 424 16 */ struct btrfs_io_ctl io_ctl; /* 440 72 */ /* --- cacheline 8 boundary (512 bytes) --- */ atomic_t reservations __attribute__((__aligned__(4))); /* 512 4 */ atomic_t nocow_writers __attribute__((__aligned__(4))); /* 516 4 */ struct mutex free_space_lock __attribute__((__aligned__(8))); /* 520 32 */ bool using_free_space_bitmaps; /* 552 1 */ bool using_free_space_bitmaps_cached; /* 553 1 */ /* XXX 2 bytes hole, try to pack */ int swap_extents; /* 556 4 */ u64 alloc_offset; /* 560 8 */ u64 zone_unusable; /* 568 8 */ /* --- cacheline 9 boundary (576 bytes) --- */ u64 zone_capacity; /* 576 8 */ u64 meta_write_pointer; /* 584 8 */ struct btrfs_chunk_map * physical_map; /* 592 8 */ struct list_head active_bg_list; /* 600 16 */ struct work_struct zone_finish_work; /* 616 32 */ /* --- cacheline 10 boundary (640 bytes) was 8 bytes ago --- */ struct extent_buffer * last_eb; /* 648 8 */ enum btrfs_block_group_size_class size_class; /* 656 4 */ /* XXX 4 bytes hole, try to pack */ u64 reclaim_mark; /* 664 8 */ /* size: 672, cachelines: 11, members: 61 */ /* sum members: 634, holes: 10, sum holes: 38 */ /* forced alignments: 8 */ /* last cacheline: 32 bytes */ } __attribute__((__aligned__(8))); Reorder some fields to eliminate the holes while keeping closely related or frequently accessed fields together. After the reordering the size of the structure is reduced down to 632 bytes and the number of cache lines decreases from 11 to 10. We can still only pack 6 block groups per 4K page but on a 64K page system we will now be able to pack 103 block groups instead of 97. The new structure layout, on a release kernel, is the following: struct btrfs_block_group { struct btrfs_fs_info * fs_info; /* 0 8 */ struct btrfs_inode * inode; /* 8 8 */ spinlock_t lock __attribute__((__aligned__(4))); /* 16 4 */ unsigned int ro; /* 20 4 */ u64 start; /* 24 8 */ u64 length; /* 32 8 */ u64 pinned; /* 40 8 */ u64 reserved; /* 48 8 */ u64 used; /* 56 8 */ /* --- cacheline 1 boundary (64 bytes) --- */ u64 delalloc_bytes; /* 64 8 */ u64 bytes_super; /* 72 8 */ u64 flags; /* 80 8 */ u64 cache_generation; /* 88 8 */ u64 global_root_id; /* 96 8 */ u64 remap_bytes; /* 104 8 */ u32 identity_remap_count; /* 112 4 */ u32 last_identity_remap_count; /* 116 4 */ u64 last_used; /* 120 8 */ /* --- cacheline 2 boundary (128 bytes) --- */ u64 last_remap_bytes; /* 128 8 */ u64 last_flags; /* 136 8 */ u32 bitmap_high_thresh; /* 144 4 */ u32 bitmap_low_thresh; /* 148 4 */ struct rw_semaphore data_rwsem __attribute__((__aligned__(8))); /* 152 40 */ /* --- cacheline 3 boundary (192 bytes) --- */ long unsigned int full_stripe_len; /* 192 8 */ long unsigned int runtime_flags; /* 200 8 */ int disk_cache_state; /* 208 4 */ int cached; /* 212 4 */ struct btrfs_caching_control * caching_ctl; /* 216 8 */ struct btrfs_space_info * space_info; /* 224 8 */ struct btrfs_free_space_ctl * free_space_ctl; /* 232 8 */ struct rb_node cache_node __attribute__((__aligned__(8))); /* 240 24 */ /* --- cacheline 4 boundary (256 bytes) was 8 bytes ago --- */ struct list_head list; /* 264 16 */ refcount_t refs __attribute__((__aligned__(4))); /* 280 4 */ atomic_t frozen __attribute__((__aligned__(4))); /* 284 4 */ struct list_head cluster_list; /* 288 16 */ struct list_head bg_list; /* 304 16 */ /* --- cacheline 5 boundary (320 bytes) --- */ struct list_head ro_list; /* 320 16 */ struct list_head discard_list; /* 336 16 */ int discard_index; /* 352 4 */ enum btrfs_discard_state discard_state; /* 356 4 */ u64 discard_eligible_time; /* 360 8 */ u64 discard_cursor; /* 368 8 */ struct list_head dirty_list; /* 376 16 */ /* --- cacheline 6 boundary (384 bytes) was 8 bytes ago --- */ struct list_head io_list; /* 392 16 */ struct btrfs_io_ctl io_ctl; /* 408 72 */ /* --- cacheline 7 boundary (448 bytes) was 32 bytes ago --- */ atomic_t reservations __attribute__((__aligned__(4))); /* 480 4 */ atomic_t nocow_writers __attribute__((__aligned__(4))); /* 484 4 */ struct mutex free_space_lock __attribute__((__aligned__(8))); /* 488 32 */ /* --- cacheline 8 boundary (512 bytes) was 8 bytes ago --- */ bool using_free_space_bitmaps; /* 520 1 */ bool using_free_space_bitmaps_cached; /* 521 1 */ /* XXX 2 bytes hole, try to pack */ /* Bitfield combined with previous fields */ static enum btrfs_block_group_size_class size_class; /* 0: 0 0 */ int swap_extents; /* 524 4 */ u64 alloc_offset; /* 528 8 */ u64 zone_unusable; /* 536 8 */ u64 zone_capacity; /* 544 8 */ u64 meta_write_pointer; /* 552 8 */ struct btrfs_chunk_map * physical_map; /* 560 8 */ struct list_head active_bg_list; /* 568 16 */ /* --- cacheline 9 boundary (576 bytes) was 8 bytes ago --- */ struct work_struct zone_finish_work; /* 584 32 */ struct extent_buffer * last_eb; /* 616 8 */ u64 reclaim_mark; /* 624 8 */ /* size: 632, cachelines: 10, members: 60, static members: 1 */ /* sum members: 630, holes: 1, sum holes: 2 */ /* sum bitfield members: 8 bits (1 bytes) */ /* forced alignments: 8 */ /* last cacheline: 56 bytes */ } __attribute__((__aligned__(8))); Signed-off-by: Filipe Manana Reviewed-by: David Sterba Signed-off-by: David Sterba --- diff --git a/fs/btrfs/block-group.h b/fs/btrfs/block-group.h index b414f4268d2d3..60a3b1c0a8abd 100644 --- a/fs/btrfs/block-group.h +++ b/fs/btrfs/block-group.h @@ -122,6 +122,7 @@ struct btrfs_block_group { struct btrfs_fs_info *fs_info; struct btrfs_inode *inode; spinlock_t lock; + unsigned int ro; u64 start; u64 length; u64 pinned; @@ -134,7 +135,8 @@ struct btrfs_block_group { u64 global_root_id; u64 remap_bytes; u32 identity_remap_count; - + /* The last commited identity_remap_count value of this block group. */ + u32 last_identity_remap_count; /* * The last committed used bytes of this block group, if the above @used * is still the same as @last_used, we don't need to update block @@ -143,8 +145,6 @@ struct btrfs_block_group { u64 last_used; /* The last committed remap_bytes value of this block group. */ u64 last_remap_bytes; - /* The last commited identity_remap_count value of this block group. */ - u32 last_identity_remap_count; /* The last committed flags value for this block group. */ u64 last_flags; @@ -171,8 +171,6 @@ struct btrfs_block_group { unsigned long full_stripe_len; unsigned long runtime_flags; - unsigned int ro; - int disk_cache_state; /* Cache tracking stuff */ @@ -192,6 +190,16 @@ struct btrfs_block_group { refcount_t refs; + /* + * When non-zero it means the block group's logical address and its + * device extents can not be reused for future block group allocations + * until the counter goes down to 0. This is to prevent them from being + * reused while some task is still using the block group after it was + * deleted - we want to make sure they can only be reused for new block + * groups after that task is done with the deleted block group. + */ + atomic_t frozen; + /* * List of struct btrfs_free_clusters for this block group. * Today it will only have one thing on it, but that may change @@ -211,22 +219,12 @@ struct btrfs_block_group { /* For read-only block groups */ struct list_head ro_list; - /* - * When non-zero it means the block group's logical address and its - * device extents can not be reused for future block group allocations - * until the counter goes down to 0. This is to prevent them from being - * reused while some task is still using the block group after it was - * deleted - we want to make sure they can only be reused for new block - * groups after that task is done with the deleted block group. - */ - atomic_t frozen; - /* For discard operations */ struct list_head discard_list; int discard_index; + enum btrfs_discard_state discard_state; u64 discard_eligible_time; u64 discard_cursor; - enum btrfs_discard_state discard_state; /* For dirty block groups */ struct list_head dirty_list; @@ -263,6 +261,8 @@ struct btrfs_block_group { /* Protected by @free_space_lock. */ bool using_free_space_bitmaps_cached; + enum btrfs_block_group_size_class size_class:8; + /* * Number of extents in this block group used for swap files. * All accesses protected by the spinlock 'lock'. @@ -281,7 +281,6 @@ struct btrfs_block_group { struct list_head active_bg_list; struct work_struct zone_finish_work; struct extent_buffer *last_eb; - enum btrfs_block_group_size_class size_class; u64 reclaim_mark; };