Re: [PATCH 2/4] block: inline fast path of driver tag allocation

[Christoph Hellwig <hch@infradead.org>]

On Wed, Oct 13, 2021 at 10:49:35AM -0600, Jens Axboe wrote:
> If we don't use an IO scheduler or have shared tags, then we don't need
> to call into this external function at all. This saves ~2% for such
> a setup.

Hmm.  What happens if you just throw an inline tag onto
blk_mq_get_driver_tag?  All the high performance callers should be
in blk-mq.c anyway.  If that isn't enough maybe something like the
version below?

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 38e6651d8b94c..ba9af26d5209d 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1126,18 +1126,23 @@ static bool __blk_mq_get_driver_tag(struct request *rq)
 	return true;
 }
 
-bool blk_mq_get_driver_tag(struct request *rq)
+static void blk_mq_inc_active_requests(struct request *rq)
+{
+	if (!(rq->rq_flags & RQF_MQ_INFLIGHT)) {
+		rq->rq_flags |= RQF_MQ_INFLIGHT;
+		__blk_mq_inc_active_requests(rq->mq_hctx);
+	}
+}
+
+inline bool blk_mq_get_driver_tag(struct request *rq)
 {
 	struct blk_mq_hw_ctx *hctx = rq->mq_hctx;
 
 	if (rq->tag == BLK_MQ_NO_TAG && !__blk_mq_get_driver_tag(rq))
 		return false;
 
-	if ((hctx->flags & BLK_MQ_F_TAG_QUEUE_SHARED) &&
-			!(rq->rq_flags & RQF_MQ_INFLIGHT)) {
-		rq->rq_flags |= RQF_MQ_INFLIGHT;
-		__blk_mq_inc_active_requests(hctx);
-	}
+	if (hctx->flags & BLK_MQ_F_TAG_QUEUE_SHARED)
+		blk_mq_inc_active_requests(rq);
 	hctx->tags->rqs[rq->tag] = rq;
 	return true;
 }