From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Michael S. Tsirkin" Subject: [PATCH net-next] virtio: document queue state logic Date: Thu, 2 Apr 2015 13:05:47 +0200 Message-ID: <20150402125743-mutt-send-email-mst@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Rusty Russell , David Miller , virtualization@lists.linux-foundation.org, netdev@vger.kernel.org Return-path: Content-Disposition: inline List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: virtualization-bounces@lists.linux-foundation.org Errors-To: virtualization-bounces@lists.linux-foundation.org List-Id: netdev.vger.kernel.org commit d631b94e7a15277858ec5f88d674d93080506999 virtio: change comment in transmit started clarifying the logic behind queue state management, but introduced an inaccuracy: TX_BUSY does not cause a BUG message. Clean this up some more, explaining the tradeoffs in detail. Signed-off-by: Michael S. Tsirkin --- diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c index a829930..63c7810 100644 --- a/drivers/net/virtio_net.c +++ b/drivers/net/virtio_net.c @@ -939,11 +939,15 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev) skb_orphan(skb); nf_reset(skb); - /* It is better to stop queue if running out of space - * instead of forcing queuing layer to requeue the skb - * by returning TX_BUSY (and cause a BUG message). - * Since most packets only take 1 or 2 ring slots - * this means 16 slots are typically wasted. + /* If running out of space, stop queue to avoid getting packets that we + * are then unable to transmit. + * An alternative would be to force queuing layer to requeue the skb by + * returning NETDEV_TX_BUSY. However, NETDEV_TX_BUSY should not be + * returned in a normal path of operation: it means that driver is not + * maintaining the TX queue stop/start state properly, and causes + * the stack to do a non-trivial amount of useless work. + * Since most packets only take 1 or 2 ring slots, stopping the queue + * early means 16 slots are typically wasted. */ if (sq->vq->num_free < 2+MAX_SKB_FRAGS) { netif_stop_subqueue(dev, qnum);