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

From: Alexander Lobakin
Date: Mon Jan 18 2021 - 09:46:30 EST

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.

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",

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