sbitmap maintains a set of words that we use to set and clear bits, with each bit representing a tag for blk-mq. Even though we spread the bits out and maintain a hint cache, one particular bit allocated will end up being cleared in the exact same spot.
This introduces batched clearing of bits. Instead of clearing a given bit, the same bit is set in a cleared/free mask instead. If we fail allocating a bit from a given word, then we check the free mask, and batch move those cleared bits at that time. This trades 64 atomic bitops for 2 cmpxchg(). In a threaded poll test case, half the overhead of getting and clearing tags is removed with this change. On another poll test case with a single thread, performance is unchanged. Signed-off-by: Jens Axboe <[email protected]> --- include/linux/sbitmap.h | 31 +++++++++++++--- lib/sbitmap.c | 80 +++++++++++++++++++++++++++++++++++++---- 2 files changed, 100 insertions(+), 11 deletions(-) diff --git a/include/linux/sbitmap.h b/include/linux/sbitmap.h index 804a50983ec5..07f117ee19dc 100644 --- a/include/linux/sbitmap.h +++ b/include/linux/sbitmap.h @@ -30,14 +30,24 @@ struct seq_file; */ struct sbitmap_word { /** - * @word: The bitmap word itself. + * @depth: Number of bits being used in @word/@cleared */ - unsigned long word; + unsigned long depth; /** - * @depth: Number of bits being used in @word. + * @word: word holding free bits */ - unsigned long depth; + unsigned long word ____cacheline_aligned_in_smp; + + /** + * @cleared: word holding cleared bits + */ + unsigned long cleared ____cacheline_aligned_in_smp; + + /** + * @swap_lock: Held while swapping word <-> cleared + */ + spinlock_t swap_lock; } ____cacheline_aligned_in_smp; /** @@ -310,6 +320,19 @@ static inline void sbitmap_clear_bit(struct sbitmap *sb, unsigned int bitnr) clear_bit(SB_NR_TO_BIT(sb, bitnr), __sbitmap_word(sb, bitnr)); } +/* + * This one is special, since it doesn't actually clear the bit, rather it + * sets the corresponding bit in the ->cleared mask instead. Paired with + * the caller doing sbitmap_batch_clear() if a given index is full, which + * will clear the previously freed entries in the corresponding ->word. + */ +static inline void sbitmap_deferred_clear_bit(struct sbitmap *sb, unsigned int bitnr) +{ + unsigned long *addr = &sb->map[SB_NR_TO_INDEX(sb, bitnr)].cleared; + + set_bit(SB_NR_TO_BIT(sb, bitnr), addr); +} + static inline void sbitmap_clear_bit_unlock(struct sbitmap *sb, unsigned int bitnr) { diff --git a/lib/sbitmap.c b/lib/sbitmap.c index 45cab6bbc1c7..f6a9553617bd 100644 --- a/lib/sbitmap.c +++ b/lib/sbitmap.c @@ -59,6 +59,7 @@ int sbitmap_init_node(struct sbitmap *sb, unsigned int depth, int shift, for (i = 0; i < sb->map_nr; i++) { sb->map[i].depth = min(depth, bits_per_word); depth -= sb->map[i].depth; + spin_lock_init(&sb->map[i].swap_lock); } return 0; } @@ -111,6 +112,57 @@ static int __sbitmap_get_word(unsigned long *word, unsigned long depth, return nr; } +/* + * See if we have deferred clears that we can batch move + */ +static inline bool sbitmap_deferred_clear(struct sbitmap *sb, int index) +{ + unsigned long mask, val; + bool ret = false; + + spin_lock(&sb->map[index].swap_lock); + + if (!sb->map[index].cleared) + goto out_unlock; + + /* + * First get a stable cleared mask, setting the old mask to 0. + */ + do { + mask = sb->map[index].cleared; + } while (cmpxchg(&sb->map[index].cleared, mask, 0) != mask); + + /* + * Now clear the masked bits in our free word + */ + do { + val = sb->map[index].word; + } while (cmpxchg(&sb->map[index].word, val, val & ~mask) != val); + + ret = true; +out_unlock: + spin_unlock(&sb->map[index].swap_lock); + return ret; +} + +static int sbitmap_find_bit_in_index(struct sbitmap *sb, int index, + unsigned int alloc_hint, bool round_robin) +{ + int nr; + + do { + nr = __sbitmap_get_word(&sb->map[index].word, + sb->map[index].depth, alloc_hint, + !round_robin); + if (nr != -1) + break; + if (!sbitmap_deferred_clear(sb, index)) + break; + } while (1); + + return nr; +} + int sbitmap_get(struct sbitmap *sb, unsigned int alloc_hint, bool round_robin) { unsigned int i, index; @@ -129,9 +181,8 @@ int sbitmap_get(struct sbitmap *sb, unsigned int alloc_hint, bool round_robin) alloc_hint = 0; for (i = 0; i < sb->map_nr; i++) { - nr = __sbitmap_get_word(&sb->map[index].word, - sb->map[index].depth, alloc_hint, - !round_robin); + nr = sbitmap_find_bit_in_index(sb, index, alloc_hint, + round_robin); if (nr != -1) { nr += index << sb->shift; break; @@ -206,23 +257,37 @@ bool sbitmap_any_bit_clear(const struct sbitmap *sb) } EXPORT_SYMBOL_GPL(sbitmap_any_bit_clear); -unsigned int sbitmap_weight(const struct sbitmap *sb) +static unsigned int __sbitmap_weight(const struct sbitmap *sb, bool set) { unsigned int i, weight = 0; for (i = 0; i < sb->map_nr; i++) { const struct sbitmap_word *word = &sb->map[i]; - weight += bitmap_weight(&word->word, word->depth); + if (set) + weight += bitmap_weight(&word->word, word->depth); + else + weight += bitmap_weight(&word->cleared, word->depth); } return weight; } + +unsigned int sbitmap_weight(const struct sbitmap *sb) +{ + return __sbitmap_weight(sb, true); +} EXPORT_SYMBOL_GPL(sbitmap_weight); +static unsigned int sbitmap_cleared(const struct sbitmap *sb) +{ + return __sbitmap_weight(sb, false); +} + void sbitmap_show(struct sbitmap *sb, struct seq_file *m) { seq_printf(m, "depth=%u\n", sb->depth); - seq_printf(m, "busy=%u\n", sbitmap_weight(sb)); + seq_printf(m, "busy=%u\n", sbitmap_weight(sb) - sbitmap_cleared(sb)); + seq_printf(m, "cleared=%u\n", sbitmap_cleared(sb)); seq_printf(m, "bits_per_word=%u\n", 1U << sb->shift); seq_printf(m, "map_nr=%u\n", sb->map_nr); } @@ -514,7 +579,8 @@ EXPORT_SYMBOL_GPL(sbitmap_queue_wake_up); void sbitmap_queue_clear(struct sbitmap_queue *sbq, unsigned int nr, unsigned int cpu) { - sbitmap_clear_bit_unlock(&sbq->sb, nr); + sbitmap_deferred_clear_bit(&sbq->sb, nr); + /* * Pairs with the memory barrier in set_current_state() to ensure the * proper ordering of clear_bit_unlock()/waitqueue_active() in the waker -- 2.17.1
