diff options
Diffstat (limited to 'drivers/md')
| -rw-r--r-- | drivers/md/bcache/bset.c | 63 |
1 files changed, 63 insertions, 0 deletions
diff --git a/drivers/md/bcache/bset.c b/drivers/md/bcache/bset.c index f3403b45bc28..596c93b44e9b 100644 --- a/drivers/md/bcache/bset.c +++ b/drivers/md/bcache/bset.c @@ -366,6 +366,10 @@ EXPORT_SYMBOL(bch_btree_keys_init); /* Binary tree stuff for auxiliary search trees */ +/* + * return array index next to j when does in-order traverse + * of a binary tree which is stored in a linear array + */ static unsigned inorder_next(unsigned j, unsigned size) { if (j * 2 + 1 < size) { @@ -379,6 +383,10 @@ static unsigned inorder_next(unsigned j, unsigned size) return j; } +/* + * return array index previous to j when does in-order traverse + * of a binary tree which is stored in a linear array + */ static unsigned inorder_prev(unsigned j, unsigned size) { if (j * 2 < size) { @@ -421,6 +429,10 @@ static unsigned __to_inorder(unsigned j, unsigned size, unsigned extra) return j; } +/* + * Return the cacheline index in bset_tree->data, where j is index + * from a linear array which stores the auxiliar binary tree + */ static unsigned to_inorder(unsigned j, struct bset_tree *t) { return __to_inorder(j, t->size, t->extra); @@ -441,6 +453,10 @@ static unsigned __inorder_to_tree(unsigned j, unsigned size, unsigned extra) return j; } +/* + * Return an index from a linear array which stores the auxiliar binary + * tree, j is the cacheline index of t->data. + */ static unsigned inorder_to_tree(unsigned j, struct bset_tree *t) { return __inorder_to_tree(j, t->size, t->extra); @@ -546,6 +562,20 @@ static inline uint64_t shrd128(uint64_t high, uint64_t low, uint8_t shift) return low; } +/* + * Calculate mantissa value for struct bkey_float. + * If most significant bit of f->exponent is not set, then + * - f->exponent >> 6 is 0 + * - p[0] points to bkey->low + * - p[-1] borrows bits from KEY_INODE() of bkey->high + * if most isgnificant bits of f->exponent is set, then + * - f->exponent >> 6 is 1 + * - p[0] points to bits from KEY_INODE() of bkey->high + * - p[-1] points to other bits from KEY_INODE() of + * bkey->high too. + * See make_bfloat() to check when most significant bit of f->exponent + * is set or not. + */ static inline unsigned bfloat_mantissa(const struct bkey *k, struct bkey_float *f) { @@ -570,6 +600,16 @@ static void make_bfloat(struct bset_tree *t, unsigned j) BUG_ON(m < l || m > r); BUG_ON(bkey_next(p) != m); + /* + * If l and r have different KEY_INODE values (different backing + * device), f->exponent records how many least significant bits + * are different in KEY_INODE values and sets most significant + * bits to 1 (by +64). + * If l and r have same KEY_INODE value, f->exponent records + * how many different bits in least significant bits of bkey->low. + * See bfloat_mantiss() how the most significant bit of + * f->exponent is used to calculate bfloat mantissa value. + */ if (KEY_INODE(l) != KEY_INODE(r)) f->exponent = fls64(KEY_INODE(r) ^ KEY_INODE(l)) + 64; else @@ -633,6 +673,15 @@ void bch_bset_init_next(struct btree_keys *b, struct bset *i, uint64_t magic) } EXPORT_SYMBOL(bch_bset_init_next); +/* + * Build auxiliary binary tree 'struct bset_tree *t', this tree is used to + * accelerate bkey search in a btree node (pointed by bset_tree->data in + * memory). After search in the auxiliar tree by calling bset_search_tree(), + * a struct bset_search_iter is returned which indicates range [l, r] from + * bset_tree->data where the searching bkey might be inside. Then a followed + * linear comparison does the exact search, see __bch_bset_search() for how + * the auxiliary tree is used. + */ void bch_bset_build_written_tree(struct btree_keys *b) { struct bset_tree *t = bset_tree_last(b); @@ -898,6 +947,17 @@ static struct bset_search_iter bset_search_tree(struct bset_tree *t, unsigned inorder, j, n = 1; do { + /* + * A bit trick here. + * If p < t->size, (int)(p - t->size) is a minus value and + * the most significant bit is set, right shifting 31 bits + * gets 1. If p >= t->size, the most significant bit is + * not set, right shifting 31 bits gets 0. + * So the following 2 lines equals to + * if (p >= t->size) + * p = 0; + * but a branch instruction is avoided. + */ unsigned p = n << 4; p &= ((int) (p - t->size)) >> 31; @@ -907,6 +967,9 @@ static struct bset_search_iter bset_search_tree(struct bset_tree *t, f = &t->tree[j]; /* + * Similar bit trick, use subtract operation to avoid a branch + * instruction. + * * n = (f->mantissa > bfloat_mantissa()) * ? j * 2 * : j * 2 + 1; |