block: improve batched tag allocation

Add a blk_mq_get_tags() helper, which uses the new sbitmap API for
allocating a batch of tags all at once. This both simplifies the block
code for batched allocation, and it is also more efficient than just
doing repeated calls into __sbitmap_queue_get().

This reduces the sbitmap overhead in peak runs from ~3% to ~1% and
yields a performanc increase from 6.6M IOPS to 6.8M IOPS for a single
CPU core.

Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
index 72a2724..c43b972 100644 (file)
--- a/block/blk-mq-tag.c
+++ b/block/blk-mq-tag.c
@@ -86,6 +86,21 @@ static int __blk_mq_get_tag(struct blk_mq_alloc_data *data,
                return __sbitmap_queue_get(bt);
 }
 
+unsigned long blk_mq_get_tags(struct blk_mq_alloc_data *data, int nr_tags,
+                             unsigned int *offset)
+{
+       struct blk_mq_tags *tags = blk_mq_tags_from_data(data);
+       struct sbitmap_queue *bt = &tags->bitmap_tags;
+       unsigned long ret;
+
+       if (data->shallow_depth ||data->flags & BLK_MQ_REQ_RESERVED ||
+           data->hctx->flags & BLK_MQ_F_TAG_QUEUE_SHARED)
+               return 0;
+       ret = __sbitmap_queue_get_batch(bt, nr_tags, offset);
+       *offset += tags->nr_reserved_tags;
+       return ret;
+}
+
 unsigned int blk_mq_get_tag(struct blk_mq_alloc_data *data)
 {
        struct blk_mq_tags *tags = blk_mq_tags_from_data(data);

diff --git a/block/blk-mq-tag.h b/block/blk-mq-tag.h
index d8ce89f..71c2f7d 100644 (file)
--- a/block/blk-mq-tag.h
+++ b/block/blk-mq-tag.h
@@ -38,6 +38,8 @@ extern int blk_mq_init_bitmaps(struct sbitmap_queue *bitmap_tags,
                               int node, int alloc_policy);
 
 extern unsigned int blk_mq_get_tag(struct blk_mq_alloc_data *data);
+unsigned long blk_mq_get_tags(struct blk_mq_alloc_data *data, int nr_tags,
+                             unsigned int *offset);
 extern void blk_mq_put_tag(struct blk_mq_tags *tags, struct blk_mq_ctx *ctx,
                           unsigned int tag);
 extern int blk_mq_tag_update_depth(struct blk_mq_hw_ctx *hctx,

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 568773d..97b9118 100644 (file)
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -354,6 +354,38 @@ static struct request *blk_mq_rq_ctx_init(struct blk_mq_alloc_data *data,
        return rq;
 }
 
+static inline struct request *
+__blk_mq_alloc_requests_batch(struct blk_mq_alloc_data *data,
+               u64 alloc_time_ns)
+{
+       unsigned int tag, tag_offset;
+       struct request *rq;
+       unsigned long tags;
+       int i, nr = 0;
+
+       tags = blk_mq_get_tags(data, data->nr_tags, &tag_offset);
+       if (unlikely(!tags))
+               return NULL;
+
+       for (i = 0; tags; i++) {
+               if (!(tags & (1UL << i)))
+                       continue;
+               tag = tag_offset + i;
+               tags &= ~(1UL << i);
+               rq = blk_mq_rq_ctx_init(data, tag, alloc_time_ns);
+               rq->rq_next = *data->cached_rq;
+               *data->cached_rq = rq;
+       }
+       data->nr_tags -= nr;
+
+       if (!data->cached_rq)
+               return NULL;
+
+       rq = *data->cached_rq;
+       *data->cached_rq = rq->rq_next;
+       return rq;
+}
+
 static struct request *__blk_mq_alloc_requests(struct blk_mq_alloc_data *data)
 {
        struct request_queue *q = data->q;
@@ -389,42 +421,35 @@ retry:
                blk_mq_tag_busy(data->hctx);
 
        /*
+        * Try batched alloc if we want more than 1 tag.
+        */
+       if (data->nr_tags > 1) {
+               rq = __blk_mq_alloc_requests_batch(data, alloc_time_ns);
+               if (rq)
+                       return rq;
+               data->nr_tags = 1;
+       }
+
+       /*
         * Waiting allocations only fail because of an inactive hctx.  In that
         * case just retry the hctx assignment and tag allocation as CPU hotplug
         * should have migrated us to an online CPU by now.
         */
-       do {
-               tag = blk_mq_get_tag(data);
-               if (tag == BLK_MQ_NO_TAG) {
-                       if (data->flags & BLK_MQ_REQ_NOWAIT)
-                               break;
-                       /*
-                        * Give up the CPU and sleep for a random short time to
-                        * ensure that thread using a realtime scheduling class
-                        * are migrated off the CPU, and thus off the hctx that
-                        * is going away.
-                        */
-                       msleep(3);
-                       goto retry;
-               }
-
-               rq = blk_mq_rq_ctx_init(data, tag, alloc_time_ns);
-               if (!--data->nr_tags || e ||
-                   (data->hctx->flags & BLK_MQ_F_TAG_QUEUE_SHARED))
-                       return rq;
-
-               /* link into the cached list */
-               rq->rq_next = *data->cached_rq;
-               *data->cached_rq = rq;
-               data->flags |= BLK_MQ_REQ_NOWAIT;
-       } while (1);
-
-       if (!data->cached_rq)
-               return NULL;
+       tag = blk_mq_get_tag(data);
+       if (tag == BLK_MQ_NO_TAG) {
+               if (data->flags & BLK_MQ_REQ_NOWAIT)
+                       return NULL;
+               /*
+                * Give up the CPU and sleep for a random short time to
+                * ensure that thread using a realtime scheduling class
+                * are migrated off the CPU, and thus off the hctx that
+                * is going away.
+                */
+               msleep(3);
+               goto retry;
+       }
 
-       rq = *data->cached_rq;
-       *data->cached_rq = rq->rq_next;
-       return rq;
+       return blk_mq_rq_ctx_init(data, tag, alloc_time_ns);
 }
 
 struct request *blk_mq_alloc_request(struct request_queue *q, unsigned int op,