Re: [RESEND RFC PATCH 2/8] block: Allow sending a batch of requests from the scheduler to hardware

[Ming Lei]

On Mon, Mar 16, 2020 at 06:01:19PM +0800, Baolin Wang wrote:
> As we know, some SD/MMC host controllers can support packed request,
> that means we can package several requests to host controller at one
> time to improve performence. So the hardware driver expects the blk-mq
> can dispatch a batch of requests at one time, and driver can use bd.last
> to indicate if it is the last request in the batch to help to combine
> requests as much as possible.
> 
> Thus we should add batch requests setting from the block driver to tell
> the scheduler how many requests can be dispatched in a batch, as well
> as changing the scheduler to dispatch more than one request if setting
> the maximum batch requests number.
> 

I feel this batch dispatch style is more complicated, and some other
drivers(virtio blk/scsi) still may get benefit if we can pass real 'last' flag in
.queue_rq().

So what about the following way by extending .commit_rqs() to this usage?
And you can do whatever batch processing in .commit_rqs() which will be
guaranteed to be called if BLK_MQ_F_FORCE_COMMIT_RQS is set by driver.

diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
index 856356b1619e..cd2bbe56f83f 100644
--- a/block/blk-mq-sched.c
+++ b/block/blk-mq-sched.c
@@ -85,11 +85,12 @@ void blk_mq_sched_restart(struct blk_mq_hw_ctx *hctx)
  * 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)
+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;
@@ -112,7 +113,10 @@ static void blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx)
 		 * in blk_mq_dispatch_rq_list().
 		 */
 		list_add(&rq->queuelist, &rq_list);
-	} while (blk_mq_dispatch_rq_list(q, &rq_list, true));
+		ret = blk_mq_dispatch_rq_list(q, &rq_list, true);
+	} while (ret);
+
+	return ret;
 }
 
 static struct blk_mq_ctx *blk_mq_next_ctx(struct blk_mq_hw_ctx *hctx,
@@ -131,11 +135,12 @@ static struct blk_mq_ctx *blk_mq_next_ctx(struct blk_mq_hw_ctx *hctx,
  * 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)
+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;
@@ -161,10 +166,12 @@ static void blk_mq_do_dispatch_ctx(struct blk_mq_hw_ctx *hctx)
 
 		/* round robin for fair dispatch */
 		ctx = blk_mq_next_ctx(hctx, rq->mq_ctx);
-
-	} while (blk_mq_dispatch_rq_list(q, &rq_list, true));
+		ret = blk_mq_dispatch_rq_list(q, &rq_list, true);
+	} while (ret);
 
 	WRITE_ONCE(hctx->dispatch_from, ctx);
+
+	return ret;
 }
 
 void blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx)
@@ -173,6 +180,7 @@ void blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx)
 	struct elevator_queue *e = q->elevator;
 	const bool has_sched_dispatch = e && e->type->ops.dispatch_request;
 	LIST_HEAD(rq_list);
+	bool dispatch_ret;
 
 	/* RCU or SRCU read lock is needed before checking quiesced flag */
 	if (unlikely(blk_mq_hctx_stopped(hctx) || blk_queue_quiesced(q)))
@@ -206,20 +214,26 @@ void blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx)
 	 */
 	if (!list_empty(&rq_list)) {
 		blk_mq_sched_mark_restart_hctx(hctx);
-		if (blk_mq_dispatch_rq_list(q, &rq_list, false)) {
+		dispatch_ret = blk_mq_dispatch_rq_list(q, &rq_list, false);
+		if (dispatch_ret) {
 			if (has_sched_dispatch)
-				blk_mq_do_dispatch_sched(hctx);
+				dispatch_ret = blk_mq_do_dispatch_sched(hctx);
 			else
-				blk_mq_do_dispatch_ctx(hctx);
+				dispatch_ret = blk_mq_do_dispatch_ctx(hctx);
 		}
 	} else if (has_sched_dispatch) {
-		blk_mq_do_dispatch_sched(hctx);
+		dispatch_ret = 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);
+		dispatch_ret = blk_mq_do_dispatch_ctx(hctx);
 	} else {
 		blk_mq_flush_busy_ctxs(hctx, &rq_list);
-		blk_mq_dispatch_rq_list(q, &rq_list, false);
+		dispatch_ret = blk_mq_dispatch_rq_list(q, &rq_list, false);
+	}
+
+	if (dispatch_ret) {
+		if (hctx->flags & BLK_MQ_F_FORCE_COMMIT_RQS)
+			hctx->queue->mq_ops->commit_rqs(hctx);
 	}
 }
 
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 87c6699f35ae..9b46f5d6c7fd 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1238,11 +1238,15 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list,
 		 * Flag last if we have no more requests, or if we have more
 		 * but can't assign a driver tag to it.
 		 */
-		if (list_empty(list))
-			bd.last = true;
-		else {
-			nxt = list_first_entry(list, struct request, queuelist);
-			bd.last = !blk_mq_get_driver_tag(nxt);
+		if (!(hctx->flags & BLK_MQ_F_FORCE_COMMIT_RQS)) {
+			if (list_empty(list))
+				bd.last = true;
+			else {
+				nxt = list_first_entry(list, struct request, queuelist);
+				bd.last = !blk_mq_get_driver_tag(nxt);
+			}
+		} else {
+			bd.last = false;
 		}
 
 		ret = q->mq_ops->queue_rq(hctx, &bd);
diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
index 07fa767bff86..c0ef6990b698 100644
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -394,6 +394,7 @@ enum {
 	BLK_MQ_F_SHOULD_MERGE	= 1 << 0,
 	BLK_MQ_F_TAG_SHARED	= 1 << 1,
 	BLK_MQ_F_NO_MANAGED_IRQ	= 1 << 2,
+	BLK_MQ_F_FORCE_COMMIT_RQS = 1 << 3,
 	BLK_MQ_F_BLOCKING	= 1 << 5,
 	BLK_MQ_F_NO_SCHED	= 1 << 6,
 	BLK_MQ_F_ALLOC_POLICY_START_BIT = 8,

Thanks, 
Ming