Re: [PATCH] block: Limit number of items taken from the I/O scheduler in one go

[Ming Lei]

On Wed, Feb 05, 2020 at 11:57:06AM -0800, Salman Qazi wrote:
> Flushes bypass the I/O scheduler and get added to hctx->dispatch
> in blk_mq_sched_bypass_insert.  This can happen while a kworker is running
> hctx->run_work work item and is past the point in
> blk_mq_sched_dispatch_requests where hctx->dispatch is checked.
> 
> The blk_mq_do_dispatch_sched call is not guaranteed to end in bounded time,
> because the I/O scheduler can feed an arbitrary number of commands.
> 
> Since we have only one hctx->run_work, the commands waiting in
> hctx->dispatch will wait an arbitrary length of time for run_work to be
> rerun.
> 
> A similar phenomenon exists with dispatches from the software queue.
> 
> The solution is to poll hctx->dispatch in blk_mq_do_dispatch_sched and
> blk_mq_do_dispatch_ctx and return from the run_work handler and let it
> rerun.
> 
> Signed-off-by: Salman Qazi <sqazi@xxxxxxxxxx>
> ---
>  block/blk-mq-sched.c | 37 +++++++++++++++++++++++++++++++------
>  1 file changed, 31 insertions(+), 6 deletions(-)
> 
> diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
> index ca22afd47b3d..52249fddeb66 100644
> --- a/block/blk-mq-sched.c
> +++ b/block/blk-mq-sched.c
> @@ -84,12 +84,16 @@ void blk_mq_sched_restart(struct blk_mq_hw_ctx *hctx)
>   * Only SCSI implements .get_budget and .put_budget, and SCSI restarts
>   * its queue by itself in its completion handler, so we don't need to
>   * restart queue if .get_budget() returns BLK_STS_NO_RESOURCE.
> + *
> + * Returns true if hctx->dispatch was found non-empty and
> + * run_work has to be run again.
>   */
> -static void blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx)
> +static bool blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx)
>  {
>  	struct request_queue *q = hctx->queue;
>  	struct elevator_queue *e = q->elevator;
>  	LIST_HEAD(rq_list);
> +	bool ret = false;
>  
>  	do {
>  		struct request *rq;
> @@ -97,6 +101,11 @@ static void blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx)
>  		if (e->type->ops.has_work && !e->type->ops.has_work(hctx))
>  			break;
>  
> +		if (!list_empty_careful(&hctx->dispatch)) {
> +			ret = true;
> +			break;
> +		}
> +
>  		if (!blk_mq_get_dispatch_budget(hctx))
>  			break;
>  
> @@ -113,6 +122,8 @@ static void blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx)
>  		 */
>  		list_add(&rq->queuelist, &rq_list);
>  	} while (blk_mq_dispatch_rq_list(q, &rq_list, true));
> +
> +	return ret;
>  }
>  
>  static struct blk_mq_ctx *blk_mq_next_ctx(struct blk_mq_hw_ctx *hctx,
> @@ -130,16 +141,25 @@ static struct blk_mq_ctx *blk_mq_next_ctx(struct blk_mq_hw_ctx *hctx,
>   * Only SCSI implements .get_budget and .put_budget, and SCSI restarts
>   * its queue by itself in its completion handler, so we don't need to
>   * restart queue if .get_budget() returns BLK_STS_NO_RESOURCE.
> + *
> + * Returns true if hctx->dispatch was found non-empty and
> + * run_work has to be run again.
>   */
> -static void blk_mq_do_dispatch_ctx(struct blk_mq_hw_ctx *hctx)
> +static bool blk_mq_do_dispatch_ctx(struct blk_mq_hw_ctx *hctx)
>  {
>  	struct request_queue *q = hctx->queue;
>  	LIST_HEAD(rq_list);
>  	struct blk_mq_ctx *ctx = READ_ONCE(hctx->dispatch_from);
> +	bool ret = false;
>  
>  	do {
>  		struct request *rq;
>  
> +		if (!list_empty_careful(&hctx->dispatch)) {
> +			ret = true;
> +			break;
> +		}
> +
>  		if (!sbitmap_any_bit_set(&hctx->ctx_map))
>  			break;
>  
> @@ -165,6 +185,7 @@ static void blk_mq_do_dispatch_ctx(struct blk_mq_hw_ctx *hctx)
>  	} while (blk_mq_dispatch_rq_list(q, &rq_list, true));
>  
>  	WRITE_ONCE(hctx->dispatch_from, ctx);
> +	return ret;
>  }
>  
>  void blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx)
> @@ -172,6 +193,7 @@ void blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx)
>  	struct request_queue *q = hctx->queue;
>  	struct elevator_queue *e = q->elevator;
>  	const bool has_sched_dispatch = e && e->type->ops.dispatch_request;
> +	bool run_again = false;
>  	LIST_HEAD(rq_list);
>  
>  	/* RCU or SRCU read lock is needed before checking quiesced flag */
> @@ -208,19 +230,22 @@ void blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx)
>  		blk_mq_sched_mark_restart_hctx(hctx);
>  		if (blk_mq_dispatch_rq_list(q, &rq_list, false)) {
>  			if (has_sched_dispatch)
> -				blk_mq_do_dispatch_sched(hctx);
> +				run_again = blk_mq_do_dispatch_sched(hctx);
>  			else
> -				blk_mq_do_dispatch_ctx(hctx);
> +				run_again = blk_mq_do_dispatch_ctx(hctx);
>  		}
>  	} else if (has_sched_dispatch) {
> -		blk_mq_do_dispatch_sched(hctx);
> +		run_again = blk_mq_do_dispatch_sched(hctx);
>  	} else if (hctx->dispatch_busy) {
>  		/* dequeue request one by one from sw queue if queue is busy */
> -		blk_mq_do_dispatch_ctx(hctx);
> +		run_again = blk_mq_do_dispatch_ctx(hctx);
>  	} else {
>  		blk_mq_flush_busy_ctxs(hctx, &rq_list);
>  		blk_mq_dispatch_rq_list(q, &rq_list, false);
>  	}
> +
> +	if (run_again)
> +		blk_mq_run_hw_queue(hctx, true);

One improvement may be to run again locally in this function by
limited times(such as 1) first, then switch to blk_mq_run_hw_queue()
if run again is still needed.

This way may save one async run hw queue.

Thanks,
Ming