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Date: Wed, 10 Feb 2021 16:40:41 -0500
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
Cc: netdev@...r.kernel.org, Jason Wang <jasowang@...hat.com>,
"David S. Miller" <davem@...emloft.net>,
Jakub Kicinski <kuba@...nel.org>,
Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>,
Jesper Dangaard Brouer <hawk@...nel.org>,
John Fastabend <john.fastabend@...il.com>,
virtualization@...ts.linux-foundation.org, bpf@...r.kernel.org,
dust.li@...ux.alibaba.com
Subject: Re: [PATCH netdev] virtio-net: support XDP_TX when not more queues
On Wed, Jan 13, 2021 at 04:08:57PM +0800, Xuan Zhuo wrote:
> The number of queues implemented by many virtio backends is limited,
> especially some machines have a large number of CPUs. In this case, it
> is often impossible to allocate a separate queue for XDP_TX.
>
> This patch allows XDP_TX to run by reuse the existing SQ with
> __netif_tx_lock() hold when there are not enough queues.
>
> Signed-off-by: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
> Reviewed-by: Dust Li <dust.li@...ux.alibaba.com>
I'd like to get some advice on whether this is ok from some
XDP experts - previously my understanding was that it is
preferable to disable XDP for such devices than
use locks on XDP fast path.
> ---
> drivers/net/virtio_net.c | 47 +++++++++++++++++++++++++++++++++++------------
> 1 file changed, 35 insertions(+), 12 deletions(-)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index ba8e637..7a3b2a7 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -195,6 +195,9 @@ struct virtnet_info {
> /* # of XDP queue pairs currently used by the driver */
> u16 xdp_queue_pairs;
>
> + /* xdp_queue_pairs may be 0, when xdp is already loaded. So add this. */
> + bool xdp_enabled;
> +
> /* I like... big packets and I cannot lie! */
> bool big_packets;
>
> @@ -481,14 +484,34 @@ static int __virtnet_xdp_xmit_one(struct virtnet_info *vi,
> return 0;
> }
>
> -static struct send_queue *virtnet_xdp_sq(struct virtnet_info *vi)
> +static struct send_queue *virtnet_get_xdp_sq(struct virtnet_info *vi)
> {
> unsigned int qp;
> + struct netdev_queue *txq;
> +
> + if (vi->curr_queue_pairs > nr_cpu_ids) {
> + qp = vi->curr_queue_pairs - vi->xdp_queue_pairs + smp_processor_id();
> + } else {
> + qp = smp_processor_id() % vi->curr_queue_pairs;
> + txq = netdev_get_tx_queue(vi->dev, qp);
> + __netif_tx_lock(txq, raw_smp_processor_id());
> + }
>
> - qp = vi->curr_queue_pairs - vi->xdp_queue_pairs + smp_processor_id();
> return &vi->sq[qp];
> }
>
> +static void virtnet_put_xdp_sq(struct virtnet_info *vi)
> +{
> + unsigned int qp;
> + struct netdev_queue *txq;
> +
> + if (vi->curr_queue_pairs <= nr_cpu_ids) {
> + qp = smp_processor_id() % vi->curr_queue_pairs;
> + txq = netdev_get_tx_queue(vi->dev, qp);
> + __netif_tx_unlock(txq);
> + }
> +}
> +
> static int virtnet_xdp_xmit(struct net_device *dev,
> int n, struct xdp_frame **frames, u32 flags)
> {
> @@ -512,7 +535,7 @@ static int virtnet_xdp_xmit(struct net_device *dev,
> if (!xdp_prog)
> return -ENXIO;
>
> - sq = virtnet_xdp_sq(vi);
> + sq = virtnet_get_xdp_sq(vi);
>
> if (unlikely(flags & ~XDP_XMIT_FLAGS_MASK)) {
> ret = -EINVAL;
> @@ -560,12 +583,13 @@ static int virtnet_xdp_xmit(struct net_device *dev,
> sq->stats.kicks += kicks;
> u64_stats_update_end(&sq->stats.syncp);
>
> + virtnet_put_xdp_sq(vi);
> return ret;
> }
>
> static unsigned int virtnet_get_headroom(struct virtnet_info *vi)
> {
> - return vi->xdp_queue_pairs ? VIRTIO_XDP_HEADROOM : 0;
> + return vi->xdp_enabled ? VIRTIO_XDP_HEADROOM : 0;
> }
>
> /* We copy the packet for XDP in the following cases:
> @@ -1457,12 +1481,13 @@ static int virtnet_poll(struct napi_struct *napi, int budget)
> xdp_do_flush();
>
> if (xdp_xmit & VIRTIO_XDP_TX) {
> - sq = virtnet_xdp_sq(vi);
> + sq = virtnet_get_xdp_sq(vi);
> if (virtqueue_kick_prepare(sq->vq) && virtqueue_notify(sq->vq)) {
> u64_stats_update_begin(&sq->stats.syncp);
> sq->stats.kicks++;
> u64_stats_update_end(&sq->stats.syncp);
> }
> + virtnet_put_xdp_sq(vi);
> }
>
> return received;
> @@ -2416,12 +2441,8 @@ static int virtnet_xdp_set(struct net_device *dev, struct bpf_prog *prog,
> xdp_qp = nr_cpu_ids;
>
> /* XDP requires extra queues for XDP_TX */
> - if (curr_qp + xdp_qp > vi->max_queue_pairs) {
> - NL_SET_ERR_MSG_MOD(extack, "Too few free TX rings available");
> - netdev_warn(dev, "request %i queues but max is %i\n",
> - curr_qp + xdp_qp, vi->max_queue_pairs);
> - return -ENOMEM;
> - }
> + if (curr_qp + xdp_qp > vi->max_queue_pairs)
> + xdp_qp = 0;
>
> old_prog = rtnl_dereference(vi->rq[0].xdp_prog);
> if (!prog && !old_prog)
> @@ -2453,12 +2474,14 @@ static int virtnet_xdp_set(struct net_device *dev, struct bpf_prog *prog,
> netif_set_real_num_rx_queues(dev, curr_qp + xdp_qp);
> vi->xdp_queue_pairs = xdp_qp;
>
> + vi->xdp_enabled = false;
> if (prog) {
> for (i = 0; i < vi->max_queue_pairs; i++) {
> rcu_assign_pointer(vi->rq[i].xdp_prog, prog);
> if (i == 0 && !old_prog)
> virtnet_clear_guest_offloads(vi);
> }
> + vi->xdp_enabled = true;
> }
>
> for (i = 0; i < vi->max_queue_pairs; i++) {
> @@ -2526,7 +2549,7 @@ static int virtnet_set_features(struct net_device *dev,
> int err;
>
> if ((dev->features ^ features) & NETIF_F_LRO) {
> - if (vi->xdp_queue_pairs)
> + if (vi->xdp_enabled)
> return -EBUSY;
>
> if (features & NETIF_F_LRO)
> --
> 1.8.3.1
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