On Sat, 2014-10-11 at 15:16 +0800, Jason Wang wrote:
> We free transmitted packets in ndo_start_xmit() in the past to get better
> performance in the past. One side effect is that skb_orphan() needs to be
> called in ndo_start_xmit() which makes sk_wmem_alloc not accurate in
> fact. For TCP protocol, this means several optimization could not work well
> such as TCP small queue and auto corking. This can lead extra low
> throughput of small packets stream.
>
> Thanks to the urgent descriptor support. This patch tries to solve this
> issue by enable the tx interrupt selectively for stream packets. This means
> we don't need to orphan TCP stream packets in ndo_start_xmit() but enable
> tx interrupt for those packets. After we get tx interrupt, a tx napi was
> scheduled to free those packets.
>
> With this method, sk_wmem_alloc of TCP socket were more accurate than in
> the past which let TCP can batch more through TSQ and auto corking.
>
> Signed-off-by: Jason Wang <jasow...@redhat.com>
> ---
> drivers/net/virtio_net.c | 164
> ++++++++++++++++++++++++++++++++++++-----------
> 1 file changed, 128 insertions(+), 36 deletions(-)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 5810841..b450fc4 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -72,6 +72,8 @@ struct send_queue {
>
> /* Name of the send queue: output.$index */
> char name[40];
> +
> + struct napi_struct napi;
> };
>
> /* Internal representation of a receive virtqueue */
> @@ -217,15 +219,40 @@ static struct page *get_a_page(struct receive_queue
> *rq, gfp_t gfp_mask)
> return p;
> }
>
> +static int free_old_xmit_skbs(struct send_queue *sq, int budget)
> +{
> + struct sk_buff *skb;
> + unsigned int len;
> + struct virtnet_info *vi = sq->vq->vdev->priv;
> + struct virtnet_stats *stats = this_cpu_ptr(vi->stats);
> + int sent = 0;
> +
> + while (sent < budget &&
> + (skb = virtqueue_get_buf(sq->vq, &len)) != NULL) {
> + pr_debug("Sent skb %p\n", skb);
> +
> + u64_stats_update_begin(&stats->tx_syncp);
> + stats->tx_bytes += skb->len;
> + stats->tx_packets++;
> + u64_stats_update_end(&stats->tx_syncp);
> +
> + dev_kfree_skb_any(skb);
> + sent++;
> + }
> +
You could accumulate skb->len in a totlen var, and perform a single
u64_stats_update_begin(&stats->tx_syncp);
stats->tx_bytes += totlen;
stats->tx_packets += sent;
u64_stats_update_end(&stats->tx_syncp);
after the loop.
> + return sent;
> +}
> +
...
> +
> +static bool virtnet_skb_needs_intr(struct sk_buff *skb)
> +{
> + union {
> + unsigned char *network;
> + struct iphdr *ipv4;
> + struct ipv6hdr *ipv6;
> + } hdr;
> + struct tcphdr *th = tcp_hdr(skb);
> + u16 payload_len;
> +
> + hdr.network = skb_network_header(skb);
> +
> + /* Only IPv4/IPv6 with TCP is supported */
Oh well, yet another packet flow dissector :)
If most packets were caught by your implementation, you could use it
for fast patj and fallback to skb_flow_dissect() for encapsulated
traffic.
struct flow_keys keys;
if (!skb_flow_dissect(skb, &keys))
return false;
if (keys.ip_proto != IPPROTO_TCP)
return false;
then check __skb_get_poff() how to get th, and check if there is some
payload...
> + if ((skb->protocol == htons(ETH_P_IP)) &&
> + hdr.ipv4->protocol == IPPROTO_TCP) {
> + payload_len = ntohs(hdr.ipv4->tot_len) - hdr.ipv4->ihl * 4 -
> + th->doff * 4;
> + } else if ((skb->protocol == htons(ETH_P_IPV6) ||
> + hdr.ipv6->nexthdr == IPPROTO_TCP)) {
> + payload_len = ntohs(hdr.ipv6->payload_len) - th->doff * 4;
> + } else {
> + return false;
> + }
> +
> + /* We don't want to dealy packet with PUSH bit and pure ACK packet */
> + if (!th->psh && payload_len)
> + return true;
> +
> + return false;
> }
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