[PATCH net-next v5 1/2] net: mana: Batch ringing RX queue doorbell on receiving packets

[longli@linuxonhyperv.com]

From: Long Li <longli@microsoft.com>

It's inefficient to ring the doorbell page every time a WQE is posted to
the received queue. Excessive MMIO writes result in CPU spending more
time waiting on LOCK instructions (atomic operations), resulting in
poor scaling performance.

Move the code for ringing doorbell page to where after we have posted all
WQEs to the receive queue during a callback from napi_poll().

With this change, tests showed an improvement from 120G/s to 160G/s on a
200G physical link, with 16 or 32 hardware queues.

Tests showed no regression in network latency benchmarks on single
connection.

Fixes: ca9c54d2d6a5 ("net: mana: Add a driver for Microsoft Azure Network Adapter (MANA)")
Reviewed-by: Haiyang Zhang <haiyangz@microsoft.com>
Reviewed-by: Dexuan Cui <decui@microsoft.com>
Signed-off-by: Long Li <longli@microsoft.com>
---
Change log:
v2:
Check for comp_read > 0 as it might be negative on completion error.
Set rq.wqe_cnt to 0 according to BNIC spec.

v3:
Add details in the commit on the reason of performance increase and test numbers.
Add details in the commit on why rq.wqe_cnt should be set to 0 according to hardware spec.
Add "Reviewed-by" from Haiyang and Dexuan.

v4:
Split the original patch into two: one for batching doorbell, one for setting the correct wqe count

v5:
drop Cc: stable and use net-next

 drivers/net/ethernet/microsoft/mana/mana_en.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/drivers/net/ethernet/microsoft/mana/mana_en.c b/drivers/net/ethernet/microsoft/mana/mana_en.c
index cd4d5ceb9f2d..1d8abe63fcb8 100644
--- a/drivers/net/ethernet/microsoft/mana/mana_en.c
+++ b/drivers/net/ethernet/microsoft/mana/mana_en.c
@@ -1383,8 +1383,8 @@ static void mana_post_pkt_rxq(struct mana_rxq *rxq)
 
 	recv_buf_oob = &rxq->rx_oobs[curr_index];
 
-	err = mana_gd_post_and_ring(rxq->gdma_rq, &recv_buf_oob->wqe_req,
-				    &recv_buf_oob->wqe_inf);
+	err = mana_gd_post_work_request(rxq->gdma_rq, &recv_buf_oob->wqe_req,
+					&recv_buf_oob->wqe_inf);
 	if (WARN_ON_ONCE(err))
 		return;
 
@@ -1654,6 +1654,12 @@ static void mana_poll_rx_cq(struct mana_cq *cq)
 		mana_process_rx_cqe(rxq, cq, &comp[i]);
 	}
 
+	if (comp_read > 0) {
+		struct gdma_context *gc = rxq->gdma_rq->gdma_dev->gdma_context;
+
+		mana_gd_wq_ring_doorbell(gc, rxq->gdma_rq);
+	}
+
 	if (rxq->xdp_flush)
 		xdp_do_flush();
 }
-- 
2.34.1