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sbitmap: ammortize cost of clearing bits

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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 <osandov@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Jens Axboe
2018-11-30 13:18:06 -07:00
parent 531724abc3
commit ea86ea2cdc
2 changed files with 102 additions and 16 deletions
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37
include/linux/sbitmap.h
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@@ -30,14 +30,24 @@ struct seq_file;
*/
struct sbitmap_word {
/**
* @word: The bitmap word itself.
*/
unsigned long word;
/**
* @depth: Number of bits being used in @word.
* @depth: Number of bits being used in @word/@cleared
*/
unsigned long depth;
/**
* @word: word holding free bits
*/
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.