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net: Remove ndo_xmit_flush netdev operation, use signalling instead.

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As reported by Jesper Dangaard Brouer, for high packet rates the
overhead of having another indirect call in the TX path is
non-trivial.

There is the indirect call itself, and then there is all of the
reloading of the state to refetch the tail pointer value and
then write the device register.

Move to a more passive scheme, which requires very light modifications
to the device drivers.

The signal is a new skb->xmit_more value, if it is non-zero it means
that more SKBs are pending to be transmitted on the same queue as the
current SKB.  And therefore, the driver may elide the tail pointer
update.

Right now skb->xmit_more is always zero.

Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
David S. Miller
2014-08-25 15:51:53 -07:00
parent 44a52ffd64
commit 0b725a2ca6
4 changed files with 19 additions and 56 deletions
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12
drivers/net/virtio_net.c
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@@ -953,17 +953,12 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
}
}
if (!skb->xmit_more)
virtqueue_kick(sq->vq);
return NETDEV_TX_OK;
}
static void xmit_flush(struct net_device *dev, u16 qnum)
{
struct virtnet_info *vi = netdev_priv(dev);
struct send_queue *sq = &vi->sq[qnum];
virtqueue_kick(sq->vq);
}
/*
* Send command via the control virtqueue and check status. Commands
* supported by the hypervisor, as indicated by feature bits, should
@@ -1393,7 +1388,6 @@ static const struct net_device_ops virtnet_netdev = {
.ndo_open = virtnet_open,
.ndo_stop = virtnet_close,
.ndo_start_xmit = start_xmit,
.ndo_xmit_flush = xmit_flush,
.ndo_validate_addr = eth_validate_addr,
.ndo_set_mac_address = virtnet_set_mac_address,
.ndo_set_rx_mode = virtnet_set_rx_mode,