netdev - Re: [PATCH bpf] xsk: mark napi

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Date:   Wed, 29 Jun 2022 14:45:11 +0200
From:   Björn Töpel <bjorn.topel@...il.com>
To:     Maciej Fijalkowski <maciej.fijalkowski@...el.com>
Cc:     bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Netdev <netdev@...r.kernel.org>,
        "Karlsson, Magnus" <magnus.karlsson@...el.com>
Subject: Re: [PATCH bpf] xsk: mark napi_id on sendmsg()

On Wed, 29 Jun 2022 at 12:58, Maciej Fijalkowski
<maciej.fijalkowski@...el.com> wrote:
>
> When application runs in zero copy busy poll mode and does not receive a
> single packet but only sends them, it is currently impossible to get
> into napi_busy_loop() as napi_id is only marked on Rx side in
> xsk_rcv_check(). In there, napi_id is being taken from xdp_rxq_info
> carried by xdp_buff. From Tx perspective, we do not have access to it.
> What we have handy is the xsk pool.

The fact that the napi_id is not set unless set from the ingress side
is actually "by design". It's CONFIG_NET_RX_BUSY_POLL after all. I
followed the semantics of the regular busy-polling sockets. So, I
wouldn't say it's a fix! The busy-polling in sendmsg is really just
about "driving the RX busy-polling from another socket syscall".

That being said, I definitely see that this is useful for AF_XDP
sockets, but keep in mind that it sort of changes the behavior from
regular sockets. And we'll get different behavior for
copy-mode/zero-copy mode.

TL;DR, I think it's a good addition. One small nit below:

> +                       __sk_mark_napi_id_once(sk, xs->pool->heads[0].xdp.rxq->napi_id);

Please hide this hideous pointer chasing in something neater:
xsk_pool_get_napi_id() or something.

Björn