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authorJens Axboe <axboe@kernel.dk>2018-11-30 13:18:06 -0700
committerJens Axboe <axboe@kernel.dk>2018-11-30 14:47:45 -0700
commitea86ea2cdced20057da4d2c32965c1219c238197 (patch)
tree08926009b00df1229668f131d41d2b467a78cc87 /lib/sbitmap.c
parent531724abc3bfb556c1dd68086cf9cb51f76464e3 (diff)
downloadlinux-ea86ea2cdced20057da4d2c32965c1219c238197.tar.bz2
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 <osandov@fb.com> Signed-off-by: Jens Axboe <axboe@kernel.dk>
Diffstat (limited to 'lib/sbitmap.c')
-rw-r--r--lib/sbitmap.c81
1 files changed, 73 insertions, 8 deletions
diff --git a/lib/sbitmap.c b/lib/sbitmap.c
index 45cab6bbc1c7..f99382e59314 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 +257,36 @@ 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;
}
-EXPORT_SYMBOL_GPL(sbitmap_weight);
+
+static unsigned int sbitmap_weight(const struct sbitmap *sb)
+{
+ return __sbitmap_weight(sb, true);
+}
+
+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 +578,8 @@ 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