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Date:   Tue, 20 Jun 2023 23:16:59 +0200
From:   Lorenzo Bianconi <lorenzo@kernel.org>
To:     Alexander Duyck <alexander.duyck@gmail.com>
Cc:     Jesper Dangaard Brouer <jbrouer@redhat.com>, brouer@redhat.com,
        Yunsheng Lin <linyunsheng@huawei.com>, davem@davemloft.net,
        kuba@kernel.org, pabeni@redhat.com, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        Jesper Dangaard Brouer <hawk@kernel.org>,
        Ilias Apalodimas <ilias.apalodimas@linaro.org>,
        Eric Dumazet <edumazet@google.com>,
        Maryam Tahhan <mtahhan@redhat.com>, bpf <bpf@vger.kernel.org>
Subject: Re: [PATCH net-next v3 3/4] page_pool: introduce page_pool_alloc()
 API
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[...]

> > I did some experiments using page_frag_cache/page_frag_alloc() instead =
of
> > page_pools in a simple environment I used to test XDP for veth driver.
> > In particular, I allocate a new buffer in veth_convert_skb_to_xdp_buff(=
) from
> > the page_frag_cache in order to copy the full skb in the new one, actua=
lly
> > "linearizing" the packet (since we know the original skb length).
> > I run an iperf TCP connection over a veth pair where the
> > remote device runs the xdp_rxq_info sample (available in the kernel sou=
rce
> > tree, with action XDP_PASS):
> >
> > TCP clietn -- v0 =3D=3D=3D v1 (xdp_rxq_info) -- TCP server
> >
> > net-next (page_pool):
> > - MTU 1500B: ~  7.5 Gbps
> > - MTU 8000B: ~ 15.3 Gbps
> >
> > net-next + page_frag_alloc:
> > - MTU 1500B: ~  8.4 Gbps
> > - MTU 8000B: ~ 14.7 Gbps
> >
> > It seems there is no a clear "win" situation here (at least in this env=
ironment
> > and we this simple approach). Moreover:
>=20
> For the 1500B packets it is a win, but for 8000B it looks like there
> is a regression. Any idea what is causing it?

nope, I have not looked into it yet.

>=20
> > - can the linearization introduce any issue whenever we perform XDP_RED=
IRECT
> >   into a destination device?
>=20
> It shouldn't. If it does it would probably point to an issue w/ the
> destination driver rather than an issue with the code doing this.

ack, fine.

>=20
> > - can the page_frag_cache introduce more memory fragmentation (IIRC we =
were
> >   experiencing this issue in mt76 before switching to page_pools).
>=20
> I think it largely depends on where the packets are ending up. I know
> this is the approach we are using for sockets, see
> skb_page_frag_refill(). If nothing else, if you took a similar
> approach to it you might be able to bypass the need for the
> page_frag_cache itself, although you would likely still end up
> allocating similar structures.

ack.

Regards,
Lorenzo

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