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-rw-r--r--block/bfq-iosched.c77
1 files changed, 77 insertions, 0 deletions
diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index f352b1677143..a7ab0cb50733 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -417,6 +417,82 @@ static struct request *bfq_choose_req(struct bfq_data *bfqd,
}
}
+/*
+ * See the comments on bfq_limit_depth for the purpose of
+ * the depths set in the function.
+ */
+static void bfq_update_depths(struct bfq_data *bfqd, struct sbitmap_queue *bt)
+{
+ bfqd->sb_shift = bt->sb.shift;
+
+ /*
+ * In-word depths if no bfq_queue is being weight-raised:
+ * leaving 25% of tags only for sync reads.
+ *
+ * In next formulas, right-shift the value
+ * (1U<<bfqd->sb_shift), instead of computing directly
+ * (1U<<(bfqd->sb_shift - something)), to be robust against
+ * any possible value of bfqd->sb_shift, without having to
+ * limit 'something'.
+ */
+ /* no more than 50% of tags for async I/O */
+ bfqd->word_depths[0][0] = max((1U<<bfqd->sb_shift)>>1, 1U);
+ /*
+ * no more than 75% of tags for sync writes (25% extra tags
+ * w.r.t. async I/O, to prevent async I/O from starving sync
+ * writes)
+ */
+ bfqd->word_depths[0][1] = max(((1U<<bfqd->sb_shift) * 3)>>2, 1U);
+
+ /*
+ * In-word depths in case some bfq_queue is being weight-
+ * raised: leaving ~63% of tags for sync reads. This is the
+ * highest percentage for which, in our tests, application
+ * start-up times didn't suffer from any regression due to tag
+ * shortage.
+ */
+ /* no more than ~18% of tags for async I/O */
+ bfqd->word_depths[1][0] = max(((1U<<bfqd->sb_shift) * 3)>>4, 1U);
+ /* no more than ~37% of tags for sync writes (~20% extra tags) */
+ bfqd->word_depths[1][1] = max(((1U<<bfqd->sb_shift) * 6)>>4, 1U);
+}
+
+/*
+ * Async I/O can easily starve sync I/O (both sync reads and sync
+ * writes), by consuming all tags. Similarly, storms of sync writes,
+ * such as those that sync(2) may trigger, can starve sync reads.
+ * Limit depths of async I/O and sync writes so as to counter both
+ * problems.
+ */
+static void bfq_limit_depth(unsigned int op, struct blk_mq_alloc_data *data)
+{
+ struct blk_mq_tags *tags = blk_mq_tags_from_data(data);
+ struct bfq_data *bfqd = data->q->elevator->elevator_data;
+ struct sbitmap_queue *bt;
+
+ if (op_is_sync(op) && !op_is_write(op))
+ return;
+
+ if (data->flags & BLK_MQ_REQ_RESERVED) {
+ if (unlikely(!tags->nr_reserved_tags)) {
+ WARN_ON_ONCE(1);
+ return;
+ }
+ bt = &tags->breserved_tags;
+ } else
+ bt = &tags->bitmap_tags;
+
+ if (unlikely(bfqd->sb_shift != bt->sb.shift))
+ bfq_update_depths(bfqd, bt);
+
+ data->shallow_depth =
+ bfqd->word_depths[!!bfqd->wr_busy_queues][op_is_sync(op)];
+
+ bfq_log(bfqd, "[%s] wr_busy %d sync %d depth %u",
+ __func__, bfqd->wr_busy_queues, op_is_sync(op),
+ data->shallow_depth);
+}
+
static struct bfq_queue *
bfq_rq_pos_tree_lookup(struct bfq_data *bfqd, struct rb_root *root,
sector_t sector, struct rb_node **ret_parent,
@@ -5285,6 +5361,7 @@ static struct elv_fs_entry bfq_attrs[] = {
static struct elevator_type iosched_bfq_mq = {
.ops.mq = {
+ .limit_depth = bfq_limit_depth,
.prepare_request = bfq_prepare_request,
.finish_request = bfq_finish_request,
.exit_icq = bfq_exit_icq,