Re: [PATCH 6/7] blk-mq: Rerun hardware queues after having called .put_budget()

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On Thu, Nov 30, 2017 at 04:08:47PM -0800, Bart Van Assche wrote:
> During request dispatch, after a scheduler or per-CPU queue has
> been examined, .put_budget() is called if the examined queue is
> empty. Since a new request may be queued concurrently with the
> .put_budget() call, a request queue needs to be rerun after each
> .put_budget() call.

If a request is queued concurrently from another path, it can be run
from that path, so don't need to rerun in __blk_mq_run_hw_queue().

> 
> Fixes: commit 1f460b63d4b3 ("blk-mq: don't restart queue when .get_budget returns BLK_STS_RESOURCE")
> Signed-off-by: Bart Van Assche <bart.vanassche@xxxxxxx>
> Cc: Ming Lei <ming.lei@xxxxxxxxxx>
> Cc: Omar Sandoval <osandov@xxxxxx>
> Cc: Christoph Hellwig <hch@xxxxxx>
> Cc: Hannes Reinecke <hare@xxxxxxx>
> Cc: Johannes Thumshirn <jthumshirn@xxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx>
> ---
>  block/blk-mq-sched.c | 39 ++++++++++++++++++++-------------------
>  block/blk-mq-sched.h |  2 +-
>  block/blk-mq.c       | 17 ++++++++++++-----
>  3 files changed, 33 insertions(+), 25 deletions(-)
> 
> diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
> index 398545d94521..3a935081a2d3 100644
> --- a/block/blk-mq-sched.c
> +++ b/block/blk-mq-sched.c
> @@ -82,12 +82,8 @@ static bool blk_mq_sched_restart_hctx(struct blk_mq_hw_ctx *hctx)
>  	return blk_mq_run_hw_queue(hctx, true);
>  }
>  
> -/*
> - * 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.
> - */
> -static void blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx)
> +/* returns true if hctx needs to be run again */
> +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;
> @@ -106,7 +102,7 @@ static void blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx)
>  		rq = e->type->ops.mq.dispatch_request(hctx);
>  		if (!rq) {
>  			blk_mq_put_dispatch_budget(hctx);
> -			break;
> +			return true;
>  		}
>  
>  		/*
> @@ -116,6 +112,8 @@ 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 false;
>  }
>  
>  static struct blk_mq_ctx *blk_mq_next_ctx(struct blk_mq_hw_ctx *hctx,
> @@ -129,16 +127,13 @@ static struct blk_mq_ctx *blk_mq_next_ctx(struct blk_mq_hw_ctx *hctx,
>  	return hctx->ctxs[idx];
>  }
>  
> -/*
> - * 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.
> - */
> -static void blk_mq_do_dispatch_ctx(struct blk_mq_hw_ctx *hctx)
> +/* returns true if hctx needs to be run again */
> +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;
> @@ -152,6 +147,7 @@ static void blk_mq_do_dispatch_ctx(struct blk_mq_hw_ctx *hctx)
>  		rq = blk_mq_dequeue_from_ctx(hctx, ctx);
>  		if (!rq) {
>  			blk_mq_put_dispatch_budget(hctx);
> +			ret = true;
>  			break;
>  		}
>  
> @@ -168,19 +164,22 @@ 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;
>  }
>  
>  /* return true if hw queue need to be run again */
> -void blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx)
> +bool 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.mq.dispatch_request;
>  	LIST_HEAD(rq_list);
> +	bool run_queue = false;
>  
>  	/* RCU or SRCU read lock is needed before checking quiesced flag */
>  	if (unlikely(blk_mq_hctx_stopped(hctx) || blk_queue_quiesced(q)))
> -		return;
> +		return false;
>  
>  	hctx->run++;
>  
> @@ -212,12 +211,12 @@ void blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx)
>  	if (!list_empty(&rq_list)) {
>  		if (blk_mq_dispatch_rq_list(q, &rq_list, false)) {
>  			if (has_sched_dispatch)
> -				blk_mq_do_dispatch_sched(hctx);
> +				run_queue = blk_mq_do_dispatch_sched(hctx);
>  			else
> -				blk_mq_do_dispatch_ctx(hctx);
> +				run_queue = blk_mq_do_dispatch_ctx(hctx);
>  		}
>  	} else if (has_sched_dispatch) {
> -		blk_mq_do_dispatch_sched(hctx);
> +		run_queue = blk_mq_do_dispatch_sched(hctx);
>  	} else if (q->mq_ops->get_budget) {
>  		/*
>  		 * If we need to get budget before queuing request, we
> @@ -227,11 +226,13 @@ void blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx)
>  		 * TODO: get more budgets, and dequeue more requests in
>  		 * one time.
>  		 */
> -		blk_mq_do_dispatch_ctx(hctx);
> +		run_queue = blk_mq_do_dispatch_ctx(hctx);
>  	} else {
>  		blk_mq_flush_busy_ctxs(hctx, &rq_list);
>  		blk_mq_dispatch_rq_list(q, &rq_list, false);
>  	}
> +
> +	return run_queue;
>  }
>  
>  bool blk_mq_sched_try_merge(struct request_queue *q, struct bio *bio,
> diff --git a/block/blk-mq-sched.h b/block/blk-mq-sched.h
> index ba1d1418a96d..1ccfb8027cfc 100644
> --- a/block/blk-mq-sched.h
> +++ b/block/blk-mq-sched.h
> @@ -23,7 +23,7 @@ void blk_mq_sched_insert_requests(struct request_queue *q,
>  				  struct blk_mq_ctx *ctx,
>  				  struct list_head *list, bool run_queue_async);
>  
> -void blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx);
> +bool blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx);
>  
>  int blk_mq_init_sched(struct request_queue *q, struct elevator_type *e);
>  void blk_mq_exit_sched(struct request_queue *q, struct elevator_queue *e);
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 3e0ce940377f..b4225f606737 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -1079,7 +1079,7 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list,
>  {
>  	struct blk_mq_hw_ctx *hctx;
>  	struct request *rq, *nxt;
> -	bool no_tag = false;
> +	bool restart = false, no_tag = false;
>  	int errors, queued;
>  
>  	if (list_empty(list))
> @@ -1105,8 +1105,10 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list,
>  			 * we'll re-run it below.
>  			 */
>  			if (!blk_mq_mark_tag_wait(&hctx, rq)) {
> -				if (got_budget)
> +				if (got_budget) {
>  					blk_mq_put_dispatch_budget(hctx);
> +					restart = true;
> +				}
>  				/*
>  				 * For non-shared tags, the RESTART check
>  				 * will suffice.
> @@ -1193,7 +1195,8 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list,
>  		 *   returning BLK_STS_RESOURCE. Two exceptions are scsi-mq
>  		 *   and dm-rq.
>  		 */
> -		if (!blk_mq_sched_needs_restart(hctx) ||
> +		if (restart ||
> +		    !blk_mq_sched_needs_restart(hctx) ||
>  		    (no_tag && list_empty_careful(&hctx->dispatch_wait.entry)))
>  			blk_mq_run_hw_queue(hctx, true);
>  	}
> @@ -1204,6 +1207,7 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list,
>  static void __blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx)
>  {
>  	int srcu_idx;
> +	bool run_queue;
>  
>  	/*
>  	 * We should be running this queue from one of the CPUs that
> @@ -1220,15 +1224,18 @@ static void __blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx)
>  
>  	if (!(hctx->flags & BLK_MQ_F_BLOCKING)) {
>  		rcu_read_lock();
> -		blk_mq_sched_dispatch_requests(hctx);
> +		run_queue = blk_mq_sched_dispatch_requests(hctx);
>  		rcu_read_unlock();
>  	} else {
>  		might_sleep();
>  
>  		srcu_idx = srcu_read_lock(hctx->queue_rq_srcu);
> -		blk_mq_sched_dispatch_requests(hctx);
> +		run_queue = blk_mq_sched_dispatch_requests(hctx);
>  		srcu_read_unlock(hctx->queue_rq_srcu, srcu_idx);
>  	}
> +
> +	if (run_queue)
> +		blk_mq_sched_restart(hctx);
>  }
>  
>  /*
> -- 
> 2.15.0
> 

-- 
Ming



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