Re: [PATCH bpf-next] xsk: build skb by page

From: Magnus Karlsson
Date: Mon Jan 18 2021 - 10:20:08 EST

On Mon, Jan 18, 2021 at 3:47 PM Alexander Lobakin <alobakin@xxxxx> wrote:
>
> From: Alexander Lobakin <alobakin@xxxxx>
> Date: Mon, 18 Jan 2021 13:00:17 +0000
>
> > From: Yunsheng Lin <linyunsheng@xxxxxxxxxx>
> > Date: Mon, 18 Jan 2021 20:40:52 +0800
> >
> >> On 2021/1/16 10:44, Xuan Zhuo wrote:
> >>> This patch is used to construct skb based on page to save memory copy
> >>> overhead.
> >>>
> >>> This has one problem:
> >>>
> >>> We construct the skb by fill the data page as a frag into the skb. In
> >>> this way, the linear space is empty, and the header information is also
> >>> in the frag, not in the linear space, which is not allowed for some
> >>> network cards. For example, Mellanox Technologies MT27710 Family
> >>> [ConnectX-4 Lx] will get the following error message:
> >>>
> >>> mlx5_core 0000:3b:00.1 eth1: Error cqe on cqn 0x817, ci 0x8, qn 0x1dbb, opcode 0xd, syndrome 0x1, vendor syndrome 0x68
> >>> 00000000: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> >>> 00000010: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> >>> 00000020: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> >>> 00000030: 00 00 00 00 60 10 68 01 0a 00 1d bb 00 0f 9f d2
> >>> WQE DUMP: WQ size 1024 WQ cur size 0, WQE index 0xf, len: 64
> >>> 00000000: 00 00 0f 0a 00 1d bb 03 00 00 00 08 00 00 00 00
> >>> 00000010: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> >>> 00000020: 00 00 00 2b 00 08 00 00 00 00 00 05 9e e3 08 00
> >>> 00000030: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> >>> mlx5_core 0000:3b:00.1 eth1: ERR CQE on SQ: 0x1dbb
> >>>
> >>> I also tried to use build_skb to construct skb, but because of the
> >>> existence of skb_shinfo, it must be behind the linear space, so this
> >>> method is not working. We can't put skb_shinfo on desc->addr, it will be
> >>> exposed to users, this is not safe.
> >>>
> >>> Finally, I added a feature NETIF_F_SKB_NO_LINEAR to identify whether the
> >>
> >> Does it make sense to use ETHTOOL_TX_COPYBREAK tunable in ethtool to
> >> configure if the data is copied or not?
> >
> > As far as I can grep, only mlx4 supports this, and it has a different
> > meaning in that driver.
> > So I guess a new netdev_feature would be a better solution.
> >
> >>> network card supports the header information of the packet in the frag
> >>> and not in the linear space.
> >>>
> >>> ---------------- Performance Testing ------------
> >>>
> >>> The test environment is Aliyun ECS server.
> >>> Test cmd:
> >>> ```
> >>> xdpsock -i eth0 -t -S -s <msg size>
> >>> ```
> >>>
> >>> Test result data:
> >>>
> >>> size 64 512 1024 1500
> >>> copy 1916747 1775988 1600203 1440054
> >>> page 1974058 1953655 1945463 1904478
> >>> percent 3.0% 10.0% 21.58% 32.3%
> >>>
> >>> Signed-off-by: Xuan Zhuo <xuanzhuo@xxxxxxxxxxxxxxxxx>
> >>> Reviewed-by: Dust Li <dust.li@xxxxxxxxxxxxxxxxx>
> >>> ---
> >>> drivers/net/virtio_net.c | 2 +-
> >>> include/linux/netdev_features.h | 5 +-
> >>> net/ethtool/common.c | 1 +
> >>> net/xdp/xsk.c | 108 +++++++++++++++++++++++++++++++++-------
> >>> 4 files changed, 97 insertions(+), 19 deletions(-)
> >>>
> >>> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> >>> index 4ecccb8..841a331 100644
> >>> --- a/drivers/net/virtio_net.c
> >>> +++ b/drivers/net/virtio_net.c
> >>> @@ -2985,7 +2985,7 @@ static int virtnet_probe(struct virtio_device *vdev)
> >>> /* Set up network device as normal. */
> >>> dev->priv_flags |= IFF_UNICAST_FLT | IFF_LIVE_ADDR_CHANGE;
> >>> dev->netdev_ops = &virtnet_netdev;
> >>> - dev->features = NETIF_F_HIGHDMA;
> >>> + dev->features = NETIF_F_HIGHDMA | NETIF_F_SKB_NO_LINEAR;
> >>>
> >>> dev->ethtool_ops = &virtnet_ethtool_ops;
> >>> SET_NETDEV_DEV(dev, &vdev->dev);
> >>> diff --git a/include/linux/netdev_features.h b/include/linux/netdev_features.h
> >>> index 934de56..8dd28e2 100644
> >>> --- a/include/linux/netdev_features.h
> >>> +++ b/include/linux/netdev_features.h
> >>> @@ -85,9 +85,11 @@ enum {
> >>>
> >>> NETIF_F_HW_MACSEC_BIT, /* Offload MACsec operations */
> >>>
> >>> + NETIF_F_SKB_NO_LINEAR_BIT, /* Allow skb linear is empty */
> >>> +
> >>> /*
> >>> * Add your fresh new feature above and remember to update
> >>> - * netdev_features_strings[] in net/core/ethtool.c and maybe
> >>> + * netdev_features_strings[] in net/ethtool/common.c and maybe
> >>> * some feature mask #defines below. Please also describe it
> >>> * in Documentation/networking/netdev-features.rst.
> >>> */
> >>> @@ -157,6 +159,7 @@ enum {
> >>> #define NETIF_F_GRO_FRAGLIST __NETIF_F(GRO_FRAGLIST)
> >>> #define NETIF_F_GSO_FRAGLIST __NETIF_F(GSO_FRAGLIST)
> >>> #define NETIF_F_HW_MACSEC __NETIF_F(HW_MACSEC)
> >>> +#define NETIF_F_SKB_NO_LINEAR __NETIF_F(SKB_NO_LINEAR)
> >>>
> >>> /* Finds the next feature with the highest number of the range of start till 0.
> >>> */
> >>> diff --git a/net/ethtool/common.c b/net/ethtool/common.c
> >>> index 24036e3..2f3d309 100644
> >>> --- a/net/ethtool/common.c
> >>> +++ b/net/ethtool/common.c
> >>> @@ -68,6 +68,7 @@
> >>> [NETIF_F_HW_TLS_RX_BIT] = "tls-hw-rx-offload",
> >>> [NETIF_F_GRO_FRAGLIST_BIT] = "rx-gro-list",
> >>> [NETIF_F_HW_MACSEC_BIT] = "macsec-hw-offload",
> >>> + [NETIF_F_SKB_NO_LINEAR_BIT] = "skb-no-linear",
> >
> > I completely forgot to add that you'd better to mention in both
> > enumeration/feature and its Ethtool string that the feature applies
> > to Tx path.
> > Smth like:
> >
> > NETIF_F_SKB_TX_NO_LINEAR{,_BIT}, "skb-tx-no-linear"
> > or
> > NETIF_F_TX_SKB_NO_LINEAR{,_BIT}, "tx-skb-no-linear"
> >
> > Otherwise, it may be confusing for users and developers.

I prefer one of these names for the property as they clearly describe
a feature that the driver supports.

> OR, I think we may tight the feature with the new approach to build
> skbs by page as it makes no sense for anything else.
> So, if we define something like:
>
> NETIF_F_XSK_TX_GENERIC_ZC{,_BIT}, "xsk-tx-generic-zerocopy",

This one I misunderstood first. I thought: "this is not zerocopy", but
you are right it is. It is zero-copy implemented with skb:s. But in my
mind, the NO_LINEAR version that you suggested are clearer.

> then user can toggle your new XSK Tx path on/off via Ethtool for
> drivers that will support it (don't forget to add it to hw_features
> for virtio_net then).
>
> >>> };
> >>>
> >>> const char
> >>> diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
> >>> index 8037b04..94d17dc 100644
> >>> --- a/net/xdp/xsk.c
> >>> +++ b/net/xdp/xsk.c
> >>> @@ -430,6 +430,95 @@ static void xsk_destruct_skb(struct sk_buff *skb)
> >>> sock_wfree(skb);
> >>> }
> >>>
> >>> +static struct sk_buff *xsk_build_skb_zerocopy(struct xdp_sock *xs,
> >>> + struct xdp_desc *desc)
> >>> +{
> >>> + u32 len, offset, copy, copied;
> >>> + struct sk_buff *skb;
> >>> + struct page *page;
> >>> + char *buffer;
> >>> + int err, i;
> >>> + u64 addr;
> >>> +
> >>> + skb = sock_alloc_send_skb(&xs->sk, 0, 1, &err);
> >>> + if (unlikely(!skb))
> >>> + return NULL;
> >>> +
> >>> + addr = desc->addr;
> >>> + len = desc->len;
> >>> +
> >>> + buffer = xsk_buff_raw_get_data(xs->pool, addr);
> >>> + offset = offset_in_page(buffer);
> >>> + addr = buffer - (char *)xs->pool->addrs;
> >>> +
> >>> + for (copied = 0, i = 0; copied < len; ++i) {
> >>> + page = xs->pool->umem->pgs[addr >> PAGE_SHIFT];
> >>> +
> >>> + get_page(page);
> >>> +
> >>> + copy = min((u32)(PAGE_SIZE - offset), len - copied);
> >>> +
> >>> + skb_fill_page_desc(skb, i, page, offset, copy);
> >>> +
> >>> + copied += copy;
> >>> + addr += copy;
> >>> + offset = 0;
> >>> + }
> >>> +
> >>> + skb->len += len;
> >>> + skb->data_len += len;
> >>> + skb->truesize += len;
> >>> +
> >>> + refcount_add(len, &xs->sk.sk_wmem_alloc);
> >>> +
> >>> + return skb;
> >>> +}
> >>> +
> >>> +static struct sk_buff *xsk_build_skb(struct xdp_sock *xs,
> >>> + struct xdp_desc *desc, int *err)
> >>> +{
> >>> + struct sk_buff *skb;
> >>> +
> >>> + if (xs->dev->features & NETIF_F_SKB_NO_LINEAR) {
> >>> + skb = xsk_build_skb_zerocopy(xs, desc);
> >>> + if (unlikely(!skb)) {
> >>> + *err = -ENOMEM;
> >>> + return NULL;
> >>> + }
> >>> + } else {
> >>> + char *buffer;
> >>> + u64 addr;
> >>> + u32 len;
> >>> + int err;
> >>> +
> >>> + len = desc->len;
> >>> + skb = sock_alloc_send_skb(&xs->sk, len, 1, &err);
> >>> + if (unlikely(!skb)) {
> >>> + *err = -ENOMEM;
> >>> + return NULL;
> >>> + }
> >>> +
> >>> + skb_put(skb, len);
> >>> + addr = desc->addr;
> >>> + buffer = xsk_buff_raw_get_data(xs->pool, desc->addr);
> >>> + err = skb_store_bits(skb, 0, buffer, len);
> >>> +
> >>> + if (unlikely(err)) {
> >>> + kfree_skb(skb);
> >>> + *err = -EINVAL;
> >>> + return NULL;
> >>> + }
> >>> + }
> >>> +
> >>> + skb->dev = xs->dev;
> >>> + skb->priority = xs->sk.sk_priority;
> >>> + skb->mark = xs->sk.sk_mark;
> >>> + skb_shinfo(skb)->destructor_arg = (void *)(long)desc->addr;
> >>> + skb->destructor = xsk_destruct_skb;
> >>> +
> >>> + return skb;
> >>> +}
> >>> +
> >>> static int xsk_generic_xmit(struct sock *sk)
> >>> {
> >>> struct xdp_sock *xs = xdp_sk(sk);
> >>> @@ -446,43 +535,28 @@ static int xsk_generic_xmit(struct sock *sk)
> >>> goto out;
> >>>
> >>> while (xskq_cons_peek_desc(xs->tx, &desc, xs->pool)) {
> >>> - char *buffer;
> >>> - u64 addr;
> >>> - u32 len;
> >>> -
> >>> if (max_batch-- == 0) {
> >>> err = -EAGAIN;
> >>> goto out;
> >>> }
> >>>
> >>> - len = desc.len;
> >>> - skb = sock_alloc_send_skb(sk, len, 1, &err);
> >>> + skb = xsk_build_skb(xs, &desc, &err);
> >>> if (unlikely(!skb))
> >>> goto out;
> >>>
> >>> - skb_put(skb, len);
> >>> - addr = desc.addr;
> >>> - buffer = xsk_buff_raw_get_data(xs->pool, addr);
> >>> - err = skb_store_bits(skb, 0, buffer, len);
> >>> /* This is the backpressure mechanism for the Tx path.
> >>> * Reserve space in the completion queue and only proceed
> >>> * if there is space in it. This avoids having to implement
> >>> * any buffering in the Tx path.
> >>> */
> >>> spin_lock_irqsave(&xs->pool->cq_lock, flags);
> >>> - if (unlikely(err) || xskq_prod_reserve(xs->pool->cq)) {
> >>> + if (xskq_prod_reserve(xs->pool->cq)) {
> >>> spin_unlock_irqrestore(&xs->pool->cq_lock, flags);
> >>> kfree_skb(skb);
> >>> goto out;
> >>> }
> >>> spin_unlock_irqrestore(&xs->pool->cq_lock, flags);
> >>>
> >>> - skb->dev = xs->dev;
> >>> - skb->priority = sk->sk_priority;
> >>> - skb->mark = sk->sk_mark;
> >>> - skb_shinfo(skb)->destructor_arg = (void *)(long)desc.addr;
> >>> - skb->destructor = xsk_destruct_skb;
> >>> -
> >>> err = __dev_direct_xmit(skb, xs->queue_id);
> >>> if (err == NETDEV_TX_BUSY) {
> >>> /* Tell user-space to retry the send */
> >>>
> >
> > Al
>
> Al
>