Re: [PATCH 1/4] blk-mq: introduce BLK_MQ_F_SCHED_USE_HW_TAG

Omar Sandoval
2017-05-03T16:22:02Z

On Fri, Apr 28, 2017 at 11:15:36PM +0800, Ming Lei wrote:
> When blk-mq I/O scheduler is used, we need two tags for
> submitting one request. One is called scheduler tag for
> allocating request and scheduling I/O, another one is called
> driver tag, which is used for dispatching IO to hardware/driver.
> This way introduces one extra per-queue allocation for both tags
> and request pool, and may not be as efficient as case of none
> scheduler.
>
> Also currently we put a default per-hctx limit on schedulable
> requests, and this limit may be a bottleneck for some devices,
> especialy when these devices have a quite big tag space.
>
> This patch introduces BLK_MQ_F_SCHED_USE_HW_TAG so that we can
> allow to use hardware/driver tags directly for IO scheduling if
> devices's hardware tag space is big enough. Then we can avoid
> the extra resource allocation and make IO submission more
> efficient.
>
> Signed-off-by: Ming Lei <ming.lei@redhat.com>
> ---
> block/blk-mq-sched.c | 10 +++++++++-
> block/blk-mq.c | 35 +++++++++++++++++++++++++++++------
> include/linux/blk-mq.h | 1 +
> 3 files changed, 39 insertions(+), 7 deletions(-)
>
> diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
> index 27c67465f856..45a675f07b8b 100644
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 0168b27469cb..e530bc54f0d9 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -263,9 +263,19 @@ struct request *__blk_mq_alloc_request(struct blk_mq_alloc_data *data,
> rq->rq_flags = RQF_MQ_INFLIGHT;
> atomic_inc(&data->hctx->nr_active);
> }
> - rq->tag = tag;
> - rq->internal_tag = -1;
> - data->hctx->tags->rqs[rq->tag] = rq;
> + data->hctx->tags->rqs[tag] = rq;
> +
> + /*
> + * If we use hw tag for scheduling, postpone setting
> + * rq->tag in blk_mq_get_driver_tag().
> + */
> + if (data->hctx->flags & BLK_MQ_F_SCHED_USE_HW_TAG) {
> + rq->tag = -1;
> + rq->internal_tag = tag;
> + } else {
> + rq->tag = tag;
> + rq->internal_tag = -1;
> + }

I'm guessing you did it this way because we currently check rq->tag to
decided whether this is a flush that needs to be bypassed? Makes sense,
but I'm adding it to my list of reasons why the flush stuff sucks.

> @@ -893,9 +909,15 @@ bool blk_mq_get_driver_tag(struct request *rq, struct blk_mq_hw_ctx **hctx,
> static void __blk_mq_put_driver_tag(struct blk_mq_hw_ctx *hctx,
> struct request *rq)
> {
> - blk_mq_put_tag(hctx, hctx->tags, rq->mq_ctx, rq->tag);
> + unsigned tag = rq->tag;

The pickiest of all nits, but we mostly spell out `unsigned int` in this
file, it'd be nice to stay consistent.