netdev - Re: [PATCH bpf] xdp: Fix spurious packet loss in generic XDP TX path

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Message-ID: <CANn89i+FZ7t6F6tA8iFMjAzGmKkK=A+kdFpsm6ioygg5DnwT8g@mail.gmail.com>
Date:   Fri, 1 Jul 2022 11:57:51 +0200
From:   Eric Dumazet <edumazet@...gle.com>
To:     Johan Almbladh <johan.almbladh@...finetworks.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        John Fastabend <john.fastabend@...il.com>, song@...nel.org,
        martin.lau@...ux.dev, Yonghong Song <yhs@...com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>, haoluo@...gle.com,
        jolsa@...nel.org, bpf <bpf@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH bpf] xdp: Fix spurious packet loss in generic XDP TX path

On Fri, Jul 1, 2022 at 11:43 AM Johan Almbladh
<johan.almbladh@...finetworks.com> wrote:
>
> The byte queue limits (BQL) mechanism is intended to move queuing from
> the driver to the network stack in order to reduce latency caused by
> excessive queuing in hardware. However, when transmitting or redirecting
> a packet with XDP, the qdisc layer is bypassed and there are no
> additional queues. Since netif_xmit_stopped() also takes BQL limits into
> account, but without having any alternative queuing, packets are
> silently dropped.
>
> This patch modifies the drop condition to only consider cases when the
> driver itself cannot accept any more packets. This is analogous to the
> condition in __dev_direct_xmit(). Dropped packets are also counted on
> the device.

This means XDP packets are able to starve other packets going through a qdisc,
DDOS attacks will be more effective.

in-driver-XDP use dedicated TX queues, so they do not have this
starvation issue.

This should be mentioned somewhere I guess.

>
> Signed-off-by: Johan Almbladh <johan.almbladh@...finetworks.com>
> ---
>  net/core/dev.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 8e6f22961206..41b5d7ac5ec5 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -4875,10 +4875,12 @@ void generic_xdp_tx(struct sk_buff *skb, struct bpf_prog *xdp_prog)
>         txq = netdev_core_pick_tx(dev, skb, NULL);
>         cpu = smp_processor_id();
>         HARD_TX_LOCK(dev, txq, cpu);
> -       if (!netif_xmit_stopped(txq)) {
> +       if (!netif_xmit_frozen_or_drv_stopped(txq)) {
>                 rc = netdev_start_xmit(skb, dev, txq, 0);
>                 if (dev_xmit_complete(rc))
>                         free_skb = false;
> +       } else {
> +               dev_core_stats_tx_dropped_inc(dev);
>         }
>         HARD_TX_UNLOCK(dev, txq);
>         if (free_skb) {
> --
> 2.30.2
>