netdev - Re: [PATCH net] virtio_net: Account for tx bytes and packets on sending xdp

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Date:   Wed, 6 Feb 2019 14:48:14 +0100
From:   Jesper Dangaard Brouer <brouer@...hat.com>
To:     Saeed Mahameed <saeedm@...lanox.com>
Cc:     "dsahern@...il.com" <dsahern@...il.com>,
        "thoiland@...hat.com" <thoiland@...hat.com>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "borkmann@...earbox.net" <borkmann@...earbox.net>,
        Tariq Toukan <tariqt@...lanox.com>,
        "john.fastabend@...il.com" <john.fastabend@...il.com>,
        "mst@...hat.com" <mst@...hat.com>,
        "jakub.kicinski@...ronome.com" <jakub.kicinski@...ronome.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "jasowang@...hat.com" <jasowang@...hat.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "makita.toshiaki@....ntt.co.jp" <makita.toshiaki@....ntt.co.jp>,
        brouer@...hat.com
Subject: Re: [PATCH net] virtio_net: Account for tx bytes and packets on
 sending xdp_frames

On Wed, 6 Feb 2019 00:06:33 +0000
Saeed Mahameed <saeedm@...lanox.com> wrote:

> 3) Unrelated, In non XDP case, if skb allocation fails or driver fails
> to pass the skb up to the stack for somereason, should the driver
> increase rx packets ? IMHO the answer should be yes if we want to have
> similar behavior between XDP and non XDP cases.

I don't think "skb allocation fails" should increase rx packets
counter.  The difference is that these events are outside sysadm/users
control, and is an error detected inside the driver.  The XDP program
takes a policy choice to XDP_DROP a packet, which can be accounted
inside the XDP prog (as the samples show) or as we also discuss via a
more generic XDP-action counters.

That said, I took at quick look at driver code, and it seems this
behavior differs per driver... ixgbe and mlx5 does not count "skb
allocation fails" as RX-ed packets, while mlx4 seems to count them.


> But this could result in netdev->stats.rx_packets +
> netdev->stats.rx_dropped to be more than the actual rx-ed packets, is
> this acceptable ?

This is one reasons I think this is wrong.

--Jesper