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Date: Thu, 18 Feb 2021 10:18:56 +0800
From: Jason Wang <jasowang@...hat.com>
To: "Michael S. Tsirkin" <mst@...hat.com>,
Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
Cc: netdev@...r.kernel.org, "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 2021/2/11 5:40 上午, Michael S. Tsirkin wrote:
> 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.
I think this is acceptable on the device that changing the number of
queues is not easy. For virtio-net, it probably requires a lot of
changes in the management.
Another example is TUN which use TX lock for XDP.
Thanks
>
>> ---
>> 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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