Subject: Re: [PATCH net-next 2/2] tuntap: XDP transmission

From: Jesper Dangaard Brouer <[email-masked]>
Date: Fri, 29 Dec 2017 13:32:54 +0100

On Fri, 29 Dec 2017 18:00:04 +0800
Jason Wang <[email-masked]> wrote:

> This patch implements XDP transmission for TAP. Since we can't create
> new queues for TAP during XDP set, exist ptr_ring was reused for
> queuing XDP buffers. To differ xdp_buff from sk_buff, TUN_XDP_FLAG
> (0x1ULL) was encoded into lowest bit of xpd_buff pointer during
> ptr_ring_produce, and was decoded during consuming. XDP metadata was
> stored in the headroom of the packet which should work in most of
> cases since driver usually reserve enough headroom. Very minor changes
> were done for vhost_net: it just need to peek the length depends on
> the type of pointer.
> 
> Tests was done on two Intel E5-2630 2.40GHz machines connected back to
> back through two 82599ES. Traffic were generated through MoonGen and
> testpmd(rxonly) in guest reports 2.97Mpps when xdp_redirect_map is
> doing redirection from ixgbe to TAP.

IMHO a performance measurement without something to compare against is
useless.  What was the performance before?

> Cc: Jesper Dangaard Brouer <[email-masked]>
> Signed-off-by: Jason Wang <[email-masked]>
> ---
>  drivers/net/tun.c      | 205 ++++++++++++++++++++++++++++++++++++++++---------
>  drivers/vhost/net.c    |  13 +++-
>  include/linux/if_tun.h |  17 ++++
>  3 files changed, 197 insertions(+), 38 deletions(-)
> 
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index 2c89efe..be6d993 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -240,6 +240,24 @@ struct tun_struct {
>  	struct tun_steering_prog __rcu *steering_prog;
>  };
>  
> +bool tun_is_xdp_buff(void *ptr)
> +{
> +	return (unsigned long)ptr & TUN_XDP_FLAG;
> +}
> +EXPORT_SYMBOL(tun_is_xdp_buff);
> +
> +void *tun_xdp_to_ptr(void *ptr)
> +{
> +	return (void *)((unsigned long)ptr | TUN_XDP_FLAG);
> +}
> +EXPORT_SYMBOL(tun_xdp_to_ptr);
> +
> +void *tun_ptr_to_xdp(void *ptr)
> +{
> +	return (void *)((unsigned long)ptr & ~TUN_XDP_FLAG);
> +}
> +EXPORT_SYMBOL(tun_ptr_to_xdp);
> +
>  static int tun_napi_receive(struct napi_struct *napi, int budget)
>  {
>  	struct tun_file *tfile = container_of(napi, struct tun_file, napi);
> @@ -630,12 +648,25 @@ static struct tun_struct *tun_enable_queue(struct tun_file *tfile)
>  	return tun;
>  }
>  
> +static void tun_ptr_free(void *ptr)
> +{
> +	if (!ptr)
> +		return;
> +	if (tun_is_xdp_buff(ptr)) {
> +		struct xdp_buff *xdp = tun_ptr_to_xdp(ptr);
> +
> +		put_page(virt_to_head_page(xdp->data));

(Yet another XDP-free call point, I need to convert to use an accessor
later to transition driver into an xdp_buff return API)

> +	} else {
> +		__skb_array_destroy_skb(ptr);
> +	}
> +}
> +
>  static void tun_queue_purge(struct tun_file *tfile)
>  {
> -	struct sk_buff *skb;
> +	void *ptr;
>  
> -	while ((skb = ptr_ring_consume(&tfile->tx_ring)) != NULL)
> -		kfree_skb(skb);
> +	while ((ptr = ptr_ring_consume(&tfile->tx_ring)) != NULL)
> +		tun_ptr_free(ptr);
>  
>  	skb_queue_purge(&tfile->sk.sk_write_queue);
>  	skb_queue_purge(&tfile->sk.sk_error_queue);
> @@ -688,8 +719,7 @@ static void __tun_detach(struct tun_file *tfile, bool clean)
>  				unregister_netdevice(tun->dev);
>  		}
>  		if (tun)
> -			ptr_ring_cleanup(&tfile->tx_ring,
> -					 __skb_array_destroy_skb);
> +			ptr_ring_cleanup(&tfile->tx_ring, tun_ptr_free);
>  		sock_put(&tfile->sk);
>  	}
>  }
> @@ -1201,6 +1231,54 @@ static const struct net_device_ops tun_netdev_ops = {
>  	.ndo_get_stats64	= tun_net_get_stats64,
>  };
>  
> +static int tun_xdp_xmit(struct net_device *dev, struct xdp_buff *xdp)
> +{
> +	struct tun_struct *tun = netdev_priv(dev);
> +	struct xdp_buff *buff = xdp->data_hard_start;
> +	int headroom = xdp->data - xdp->data_hard_start;
> +	struct tun_file *tfile;
> +	u32 numqueues;
> +	int ret = 0;
> +
> +	/* Assure headroom is available and buff is properly aligned */
> +	if (unlikely(headroom < sizeof(*xdp) || tun_is_xdp_buff(xdp)))
> +		return -ENOSPC;
> +
> +	*buff = *xdp;
> +
> +	rcu_read_lock();
> +
> +	numqueues = READ_ONCE(tun->numqueues);
> +	if (!numqueues) {
> +		ret = -ENOSPC;
> +		goto out;
> +	}
> +	tfile = rcu_dereference(tun->tfiles[smp_processor_id() %
> +					    numqueues]);

Several concurrent CPUs can get the same 'tfile'.

> +	/* Encode the XDP flag into lowest bit for consumer to differ
> +	 * XDP buffer from sk_buff.
> +	 */
> +	if (ptr_ring_produce(&tfile->tx_ring, tun_xdp_to_ptr(buff))) {
> +		this_cpu_inc(tun->pcpu_stats->tx_dropped);
> +		ret = -ENOSPC;
> +	}

The ptr_ring_produce() will take a lock per packet, limiting the
performance.  (Again a case where I would have liked a bulk API for
ndo_xdp_xmit()).

> +
> +out:
> +	rcu_read_unlock();
> +	return ret;
> +}
> +
> +static void tun_xdp_flush(struct net_device *dev)
> +{
> +	struct tun_struct *tun = netdev_priv(dev);
> +	struct tun_file *tfile = tun->tfiles[0];
> +
> +	/* Notify and wake up reader process */
> +	if (tfile->flags & TUN_FASYNC)
> +		kill_fasync(&tfile->fasync, SIGIO, POLL_IN);
> +	tfile->socket.sk->sk_data_ready(tfile->socket.sk);
> +}
> +
>  static const struct net_device_ops tap_netdev_ops = {
>  	.ndo_uninit		= tun_net_uninit,
>  	.ndo_open		= tun_net_open,
> @@ -1218,6 +1296,8 @@ static const struct net_device_ops tap_netdev_ops = {
>  	.ndo_set_rx_headroom	= tun_set_headroom,
>  	.ndo_get_stats64	= tun_net_get_stats64,
>  	.ndo_bpf		= tun_xdp,
> +	.ndo_xdp_xmit		= tun_xdp_xmit,
> +	.ndo_xdp_flush		= tun_xdp_flush,
>  };
>  
>  static void tun_flow_init(struct tun_struct *tun)
> @@ -1841,6 +1921,40 @@ static ssize_t tun_chr_write_iter(struct kiocb *iocb, struct iov_iter *from)
>  	return result;
>  }
>  
> +static ssize_t tun_put_user_xdp(struct tun_struct *tun,
> +				struct tun_file *tfile,
> +				struct xdp_buff *xdp,
> +				struct iov_iter *iter)
> +{
> +	int vnet_hdr_sz = 0;
> +	size_t size = xdp->data_end - xdp->data;
> +	struct tun_pcpu_stats *stats;
> +	size_t ret;
> +
> +	if (tun->flags & IFF_VNET_HDR) {
> +		struct virtio_net_hdr gso = { 0 };
> +
> +		vnet_hdr_sz = READ_ONCE(tun->vnet_hdr_sz);
> +		if (unlikely(iov_iter_count(iter) < vnet_hdr_sz))
> +			return -EINVAL;
> +		if (unlikely(copy_to_iter(&gso, sizeof(gso), iter) !=
> +			     sizeof(gso)))
> +			return -EFAULT;
> +		iov_iter_advance(iter, vnet_hdr_sz - sizeof(gso));
> +	}
> +
> +	ret = copy_to_iter(xdp->data, size, iter) + vnet_hdr_sz;
> +
> +	stats = get_cpu_ptr(tun->pcpu_stats);
> +	u64_stats_update_begin(&stats->syncp);
> +	stats->tx_packets++;
> +	stats->tx_bytes += ret;
> +	u64_stats_update_end(&stats->syncp);
> +	put_cpu_ptr(tun->pcpu_stats);
> +
> +	return ret;
> +}
> +
>  /* Put packet to the user space buffer */
>  static ssize_t tun_put_user(struct tun_struct *tun,
>  			    struct tun_file *tfile,
[...]
>  			error = -ERESTARTSYS;
> @@ -1977,36 +2090,42 @@ static struct sk_buff *tun_ring_recv(struct tun_file *tfile, int noblock,
>  
>  out:
>  	*err = error;
> -	return skb;
> +	return ptr;
>  }
>  
>  static ssize_t tun_do_read(struct tun_struct *tun, struct tun_file *tfile,
>  			   struct iov_iter *to,
> -			   int noblock, struct sk_buff *skb)
> +			   int noblock, void *ptr)
>  {
>  	ssize_t ret;
>  	int err;
>  
>  	tun_debug(KERN_INFO, tun, "tun_do_read\n");
>  
> -	if (!iov_iter_count(to)) {
> -		if (skb)
> -			kfree_skb(skb);
> -		return 0;
> -	}
> +	if (!iov_iter_count(to))
> +		tun_ptr_free(ptr);
>  
> -	if (!skb) {
> +	if (!ptr) {
>  		/* Read frames from ring */
> -		skb = tun_ring_recv(tfile, noblock, &err);
> -		if (!skb)
> +		ptr = tun_ring_recv(tfile, noblock, &err);
> +		if (!ptr)
>  			return err;
>  	}
>  
> -	ret = tun_put_user(tun, tfile, skb, to);
> -	if (unlikely(ret < 0))
> -		kfree_skb(skb);
> -	else
> -		consume_skb(skb);
> +	if (tun_is_xdp_buff(ptr)) {
> +		struct xdp_buff *xdp = tun_ptr_to_xdp(ptr);
> +
> +		ret = tun_put_user_xdp(tun, tfile, xdp, to);
> +		put_page(virt_to_head_page(xdp->data));

This again tie us info a page-refcnt based XDP scheme.

> +	} else {
> +		struct sk_buff *skb = ptr;
> +
> +		ret = tun_put_user(tun, tfile, skb, to);
> +		if (unlikely(ret < 0))
> +			kfree_skb(skb);
> +		else
> +			consume_skb(skb);
> +	}
>  
>  	return ret;
>  }
[...]

> @@ -3191,8 +3323,7 @@ struct socket *tun_get_socket(struct file *file)
>  	struct tun_file *tfile;
>  	if (file->f_op != &tun_fops)
>  		return ERR_PTR(-EINVAL);
> -	tfile = file->private_data;
> -	if (!tfile)
> +	tfile = file->private_data;	if (!tfile)

This change looks like a typo...

>  		return ERR_PTR(-EBADFD);
>  	return &tfile->socket;
>  }

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer