[PATCH 1/5] block: enable batched allocation for blk_mq_alloc_request()

Jens Axboe
2022-09-27T01:44:35Z

The filesystem IO path can take advantage of allocating batches of
requests, if the underlying submitter tells the block layer about it
through the blk_plug. For passthrough IO, the exported API is the
blk_mq_alloc_request() helper, and that one does not allow for
request caching.

Wire up request caching for blk_mq_alloc_request(), which is generally
done without having a bio available upfront.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
block/blk-mq.c | 80 ++++++++++++++++++++++++++++++++++++++++++++------
1 file changed, 71 insertions(+), 9 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index c11949d66163..d3a9f8b9c7ee 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -510,25 +510,87 @@ static struct request *__blk_mq_alloc_requests(struct blk_mq_alloc_data *data)
alloc_time_ns);
}

-struct request *blk_mq_alloc_request(struct request_queue *q, blk_opf_t opf,
- blk_mq_req_flags_t flags)
+static struct request *blk_mq_rq_cache_fill(struct request_queue *q,
+ struct blk_plug *plug,
+ blk_opf_t opf,
+ blk_mq_req_flags_t flags)
{
struct blk_mq_alloc_data data = {
.q = q,
.flags = flags,
.cmd_flags = opf,
- .nr_tags = 1,
+ .nr_tags = plug->nr_ios,
+ .cached_rq = &plug->cached_rq,
};
struct request *rq;
- int ret;

- ret = blk_queue_enter(q, flags);
- if (ret)
- return ERR_PTR(ret);
+ if (blk_queue_enter(q, flags))
+ return NULL;
+
+ plug->nr_ios = 1;

rq = __blk_mq_alloc_requests(&data);
- if (!rq)
- goto out_queue_exit;
+ if (unlikely(!rq))
+ blk_queue_exit(q);
+ return rq;
+}
+
+static struct request *blk_mq_alloc_cached_request(struct request_queue *q,
+ blk_opf_t opf,
+ blk_mq_req_flags_t flags)
+{
+ struct blk_plug *plug = current->plug;
+ struct request *rq;
+
+ if (!plug)
+ return NULL;
+ if (rq_list_empty(plug->cached_rq)) {
+ if (plug->nr_ios == 1)
+ return NULL;
+ rq = blk_mq_rq_cache_fill(q, plug, opf, flags);
+ if (rq)
+ goto got_it;
+ return NULL;
+ }
+ rq = rq_list_peek(&plug->cached_rq);
+ if (!rq || rq->q != q)
+ return NULL;
+
+ if (blk_mq_get_hctx_type(opf) != rq->mq_hctx->type)
+ return NULL;
+ if (op_is_flush(rq->cmd_flags) != op_is_flush(opf))
+ return NULL;
+
+ plug->cached_rq = rq_list_next(rq);
+got_it:
+ rq->cmd_flags = opf;
+ INIT_LIST_HEAD(&rq->queuelist);
+ return rq;
+}
+
+struct request *blk_mq_alloc_request(struct request_queue *q, blk_opf_t opf,
+ blk_mq_req_flags_t flags)
+{
+ struct request *rq;
+
+ rq = blk_mq_alloc_cached_request(q, opf, flags);
+ if (!rq) {
+ struct blk_mq_alloc_data data = {
+ .q = q,
+ .flags = flags,
+ .cmd_flags = opf,
+ .nr_tags = 1,
+ };
+ int ret;
+
+ ret = blk_queue_enter(q, flags);
+ if (ret)
+ return ERR_PTR(ret);
+
+ rq = __blk_mq_alloc_requests(&data);
+ if (!rq)
+ goto out_queue_exit;
+ }
rq->__data_len = 0;
rq->__sector = (sector_t) -1;
rq->bio = rq->biotail = NULL;
--
2.35.1