bnxt_re/lib: Doorbell moderation in poll_cq

Lib needs to ring CQ doorbell to update the consumer
index. Reducing the number of DBs can help in the round trip
latency time measured by the benchmark tools. This patch aims
to reduce the CQ DBs in the data path. This patch ring the doorbell
only once the CQ is half occupied. So for a queue of size n,
instead of n doorbells, we will ring only 2 doorbells. This is
improving the latency by more than 100ns.
To avoid the queue full condition, create the CQs of double the
size of what user has requested.

Signed-off-by: Selvin Xavier <selvin.xavier@broadcom.com>

providers/bnxt_re/main.h

@@ -105,6 +105,8 @@ struct bnxt_re_cq {
 	uint32_t mem_handle;
 	void *toggle_map;
 	uint32_t toggle_size;
+	bool deffered_db_sup;
+	uint32_t hw_cqes;
 };
 
 struct bnxt_re_push_buffer {

providers/bnxt_re/verbs.c

@@ -319,6 +319,12 @@ struct ibv_cq *bnxt_re_create_cq(struct ibv_context *ibvctx, int ncqe,
 	if (!cq)
 		return NULL;
 
+	/* Enable deferred DB mode for CQ if the CQ is small */
+	if (ncqe * 2 < dev->max_cq_depth) {
+		cq->deffered_db_sup = true;
+		ncqe = 2 * ncqe;
+	}
+
 	cq->cqq.depth = bnxt_re_init_depth(ncqe + 1, cntx->comp_mask);
 	if (cq->cqq.depth > dev->max_cq_depth + 1)
 		cq->cqq.depth = dev->max_cq_depth + 1;
@@ -398,6 +404,16 @@ int bnxt_re_resize_cq(struct ibv_cq *ibvcq, int ncqe)
 	if (ncqe > dev->max_cq_depth)
 		return -EINVAL;
 
+	/* Check if we can be in defered DB mode with the
+	 * newer size of CQE.
+	 */
+	if (2 * ncqe > dev->max_cq_depth) {
+		cq->deffered_db_sup = false;
+	} else {
+		ncqe = 2 * ncqe;
+		cq->deffered_db_sup = true;
+	}
+
 	pthread_spin_lock(&cq->cqq.qlock);
 	cq->resize_cqq.depth = bnxt_re_init_depth(ncqe + 1, cntx->comp_mask);
 	if (cq->resize_cqq.depth > dev->max_cq_depth + 1)
@@ -754,6 +770,21 @@ static uint8_t bnxt_re_poll_term_cqe(struct bnxt_re_qp *qp, int *cnt)
 	return 0;
 }
 
+static inline void bnxt_re_check_and_ring_cq_db(struct bnxt_re_cq *cq,
+						int *hw_polled)
+{
+	/* Ring doorbell only if the CQ is at
+	 * least half when deferred db mode is active
+	 */
+	if (cq->deffered_db_sup) {
+		if (cq->hw_cqes < cq->cqq.depth / 2)
+			return;
+		*hw_polled = 0;
+		cq->hw_cqes = 0;
+	}
+	bnxt_re_ring_cq_db(cq);
+}
+
 static int bnxt_re_poll_one(struct bnxt_re_cq *cq, int nwc, struct ibv_wc *wc,
 			    uint32_t *resize)
 {
@@ -819,6 +850,7 @@ static int bnxt_re_poll_one(struct bnxt_re_cq *cq, int nwc, struct ibv_wc *wc,
 			goto skipp_real;
 
 		hw_polled++;
+		cq->hw_cqes++;
 		if (qp_handle) {
 			*qp_handle = 0x0ULL; /* mark cqe as read */
 			qp_handle = NULL;
@@ -832,10 +864,13 @@ static int bnxt_re_poll_one(struct bnxt_re_cq *cq, int nwc, struct ibv_wc *wc,
 			nwc--;
 			wc++;
 		}
+		/* Extra check required to avoid CQ full */
+		if (cq->deffered_db_sup)
+			bnxt_re_check_and_ring_cq_db(cq, &hw_polled);
 	}
 
 	if (likely(hw_polled))
-		bnxt_re_ring_cq_db(cq);
+		bnxt_re_check_and_ring_cq_db(cq, &hw_polled);
 
 	return dqed;
 }