Re: [PATCH RFC v7 00/12] blk-mq/scsi: Provide hostwide shared tags for SCSI HBAs

[Hannes Reinecke <hare@suse.de>]

On 6/17/20 1:26 PM, Kashyap Desai wrote:
>>
>> ->queued is increased only and not decreased just for debug purpose so
>> ->far, so
>> it can't be relied for this purpose.
> 
> Thanks. I overlooked that that it is only incremental counter.
> 
>>
>> One approach is to add one similar counter, and maintain it by
> scheduler's
>> insert/dispatch callback.
> 
> I tried below  and I see performance is on expected range.
> 
> diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
> index fdcc2c1..ea201d0 100644
> --- a/block/blk-mq-sched.c
> +++ b/block/blk-mq-sched.c
> @@ -485,6 +485,7 @@ void blk_mq_sched_insert_request(struct request *rq,
> bool at_head,
> 
>                  list_add(&rq->queuelist, &list);
>                  e->type->ops.insert_requests(hctx, &list, at_head);
> +               atomic_inc(&hctx->elevator_queued);
>          } else {
>                  spin_lock(&ctx->lock);
>                  __blk_mq_insert_request(hctx, rq, at_head);
> @@ -511,8 +512,10 @@ void blk_mq_sched_insert_requests(struct
> blk_mq_hw_ctx *hctx,
>          percpu_ref_get(&q->q_usage_counter);
> 
>          e = hctx->queue->elevator;
> -       if (e && e->type->ops.insert_requests)
> +       if (e && e->type->ops.insert_requests) {
>                  e->type->ops.insert_requests(hctx, list, false);
> +               atomic_inc(&hctx->elevator_queued);
> +       }
>          else {
>                  /*
>                   * try to issue requests directly if the hw queue isn't
> diff --git a/block/blk-mq-sched.h b/block/blk-mq-sched.h
> index 126021f..946b47a 100644
> --- a/block/blk-mq-sched.h
> +++ b/block/blk-mq-sched.h
> @@ -74,6 +74,13 @@ static inline bool blk_mq_sched_has_work(struct
> blk_mq_hw_ctx *hctx)
>   {
>          struct elevator_queue *e = hctx->queue->elevator;
> 
> +       /* If current hctx has not queued any request, there is no need to
> run.
> +        * blk_mq_run_hw_queue() on hctx which has queued IO will handle
> +        * running specific hctx.
> +        */
> +       if (!atomic_read(&hctx->elevator_queued))
> +               return false;
> +
>          if (e && e->type->ops.has_work)
>                  return e->type->ops.has_work(hctx);
> 
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index f73a2f9..48f1824 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -517,8 +517,10 @@ void blk_mq_free_request(struct request *rq)
>          struct blk_mq_hw_ctx *hctx = rq->mq_hctx;
> 
>          if (rq->rq_flags & RQF_ELVPRIV) {
> -               if (e && e->type->ops.finish_request)
> +               if (e && e->type->ops.finish_request) {
>                          e->type->ops.finish_request(rq);
> +                       atomic_dec(&hctx->elevator_queued);
> +               }
>                  if (rq->elv.icq) {
>                          put_io_context(rq->elv.icq->ioc);
>                          rq->elv.icq = NULL;
> @@ -2571,6 +2573,7 @@ blk_mq_alloc_hctx(struct request_queue *q, struct
> blk_mq_tag_set *set,
>                  goto free_hctx;
> 
>          atomic_set(&hctx->nr_active, 0);
> +       atomic_set(&hctx->elevator_queued, 0);
>          if (node == NUMA_NO_NODE)
>                  node = set->numa_node;
>          hctx->numa_node = node;
> diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
> index 66711c7..ea1ddb1 100644
> --- a/include/linux/blk-mq.h
> +++ b/include/linux/blk-mq.h
> @@ -139,6 +139,10 @@ struct blk_mq_hw_ctx {
>           * shared across request queues.
>           */
>          atomic_t                nr_active;
> +       /**
> +        * @elevator_queued: Number of queued requests on hctx.
> +        */
> +       atomic_t                elevator_queued;
> 
>          /** @cpuhp_online: List to store request if CPU is going to die */
>          struct hlist_node       cpuhp_online;
> 
> 
> 
Would it make sense to move it into the elevator itself?
It's a bit odd having a value 'elevator_queued' with no direct reference 
to the elevator...

Cheers,

Hannes
-- 
Dr. Hannes Reinecke            Teamlead Storage & Networking
hare@suse.de                               +49 911 74053 688
SUSE Software Solutions GmbH, Maxfeldstr. 5, 90409 Nürnberg
HRB 36809 (AG Nürnberg), Geschäftsführer: Felix Imendörffer