block: reorder bio::__bi_remaining for better packing

Simple reordering of __bi_remaining can reduce bio size by 8 bytes that are now wasted on padding (measured on x86_64): struct bio { struct bio * bi_next; /* 0 8 */ struct gendisk * bi_disk; /* 8 8 */ unsigned int bi_opf; /* 16 4 */ short unsigned int bi_flags; /* 20 2 */ short unsigned int bi_ioprio; /* 22 2 */ short unsigned int bi_write_hint; /* 24 2 */ blk_status_t bi_status; /* 26 1 */ u8 bi_partno; /* 27 1 */ /* XXX 4 bytes hole, try to pack */ struct bvec_iter bi_iter; /* 32 24 */ /* XXX last struct has 4 bytes of padding */ atomic_t __bi_remaining; /* 56 4 */ /* XXX 4 bytes hole, try to pack */ [...] /* size: 104, cachelines: 2, members: 19 */ /* sum members: 96, holes: 2, sum holes: 8 */ /* paddings: 1, sum paddings: 4 */ /* last cacheline: 40 bytes */ }; Now becomes: struct bio { struct bio * bi_next; /* 0 8 */ struct gendisk * bi_disk; /* 8 8 */ unsigned int bi_opf; /* 16 4 */ short unsigned int bi_flags; /* 20 2 */ short unsigned int bi_ioprio; /* 22 2 */ short unsigned int bi_write_hint; /* 24 2 */ blk_status_t bi_status; /* 26 1 */ u8 bi_partno; /* 27 1 */ atomic_t __bi_remaining; /* 28 4 */ struct bvec_iter bi_iter; /* 32 24 */ /* XXX last struct has 4 bytes of padding */ [...] /* size: 96, cachelines: 2, members: 19 */ /* paddings: 1, sum paddings: 4 */ /* last cacheline: 32 bytes */ };

DSDavid Sterba committed 7 years ago