netdev - [PATCH RFC v2 4/4] virtio_net: move txq wakeups under tx q lock

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Date:   Tue, 13 Apr 2021 01:47:54 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     linux-kernel@...r.kernel.org
Cc:     Jakub Kicinski <kuba@...nel.org>, Jason Wang <jasowang@...hat.com>,
        Wei Wang <weiwan@...gle.com>,
        David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
        Willem de Bruijn <willemb@...gle.com>,
        virtualization@...ts.linux-foundation.org
Subject: [PATCH RFC v2 4/4] virtio_net: move txq wakeups under tx q lock

We currently check num_free outside tx q lock
which is unsafe: new packets can arrive meanwhile
and there won't be space in the queue.
Thus a spurious queue wakeup causing overhead
and even packet drops.

Move the check under the lock to fix that.

Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
---
 drivers/net/virtio_net.c | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 460ccdbb840e..febaf55ec1f6 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -1431,11 +1431,12 @@ static void virtnet_poll_cleantx(struct receive_queue *rq)
 	if (__netif_tx_trylock(txq)) {
 		virtqueue_disable_cb(sq->vq);
 		free_old_xmit_skbs(sq, true);
+
+		if (sq->vq->num_free >= 2 + MAX_SKB_FRAGS)
+			netif_tx_wake_queue(txq);
+
 		__netif_tx_unlock(txq);
 	}
-
-	if (sq->vq->num_free >= 2 + MAX_SKB_FRAGS)
-		netif_tx_wake_queue(txq);
 }
 
 static int virtnet_poll(struct napi_struct *napi, int budget)
@@ -1519,6 +1520,9 @@ static int virtnet_poll_tx(struct napi_struct *napi, int budget)
 	virtqueue_disable_cb(sq->vq);
 	free_old_xmit_skbs(sq, true);
 
+	if (sq->vq->num_free >= 2 + MAX_SKB_FRAGS)
+		netif_tx_wake_queue(txq);
+
 	opaque = virtqueue_enable_cb_prepare(sq->vq);
 
 	done = napi_complete_done(napi, 0);
@@ -1539,9 +1543,6 @@ static int virtnet_poll_tx(struct napi_struct *napi, int budget)
 		}
 	}
 
-	if (sq->vq->num_free >= 2 + MAX_SKB_FRAGS)
-		netif_tx_wake_queue(txq);
-
 	return 0;
 }
 
-- 
MST