Re: [PATCH netdev] virtio-net: support XDP_TX when not more queues

[Jason Wang <jasowang@redhat.com>]

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@linux.alibaba.com>
>> Reviewed-by: Dust Li <dust.li@linux.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