From ea86ea2cdced20057da4d2c32965c1219c238197 Mon Sep 17 00:00:00 2001 From: Jens Axboe Date: Fri, 30 Nov 2018 13:18:06 -0700 Subject: sbitmap: ammortize cost of clearing bits 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. Reviewed-by: Omar Sandoval Signed-off-by: Jens Axboe --- include/linux/sbitmap.h | 33 +++++++++++++++++++++++++++------ 1 file changed, 27 insertions(+), 6 deletions(-) (limited to 'include/linux/sbitmap.h') diff --git a/include/linux/sbitmap.h b/include/linux/sbitmap.h index 804a50983ec5..81359d45751e 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) { @@ -321,8 +344,6 @@ static inline int sbitmap_test_bit(struct sbitmap *sb, unsigned int bitnr) return test_bit(SB_NR_TO_BIT(sb, bitnr), __sbitmap_word(sb, bitnr)); } -unsigned int sbitmap_weight(const struct sbitmap *sb); - /** * sbitmap_show() - Dump &struct sbitmap information to a &struct seq_file. * @sb: Bitmap to show. -- cgit v1.2.3