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

Anuj gupta
2022-09-28T13:39:20Z

On Tue, Sep 27, 2022 at 7:19 AM Jens Axboe <axboe@kernel.dk> wrote:
>
> 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
>

A large chunk of this improvement in passthrough performance is coming by
enabling request caching. On my setup, the performance improves from
2.34 to 2.54 MIOPS. I have tested this using the t/io_uring utility (in fio) and
I am using an Intel Optane Gen2 device.

Tested-by: Anuj Gupta <anuj20.g@samsung.com>

--
Anuj Gupta