Re: [PATCH net-next 2/2] udp: allow forwarding of plain (non-fraglisted) UDP GRO packets

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

On Fri, Jan 22, 2021 at 6:25 AM Alexander Lobakin <alobakin@pm.me> wrote:
>
> From: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
> Date: Thu, 21 Jan 2021 21:47:47 -0500
>
> > On Mon, Jan 18, 2021 at 2:33 PM Alexander Lobakin <alobakin@pm.me> wrote:
> > >
> > > Commit 9fd1ff5d2ac7 ("udp: Support UDP fraglist GRO/GSO.") actually
> > > not only added a support for fraglisted UDP GRO, but also tweaked
> > > some logics the way that non-fraglisted UDP GRO started to work for
> > > forwarding too.
> > > Commit 2e4ef10f5850 ("net: add GSO UDP L4 and GSO fraglists to the
> > > list of software-backed types") added GSO UDP L4 to the list of
> > > software GSO to allow virtual netdevs to forward them as is up to
> > > the real drivers.
> > >
> > > Tests showed that currently forwarding and NATing of plain UDP GRO
> > > packets are performed fully correctly, regardless if the target
> > > netdevice has a support for hardware/driver GSO UDP L4 or not.
> > > Plain UDP GRO forwarding even shows better performance than fraglisted
> > > UDP GRO in some cases due to not wasting one skbuff_head per every
> > > segment.
> >
> > That is surprising. The choice for fraglist based forwarding was made
> > on the assumption that it is cheaper if software segmentation is needed.
> >
> > Do you have a more specific definition of the relevant cases?
>
> "Classic" UDP GRO shows better performance when forwarding to a NIC
> that supports GSO UDP L4 (i.e. no software segmentation occurs), like
> the one that I test kernel on.
> I don't have much info about performance without UDP GSO offload
> as I usually test NAT, and fralisted UDP GRO currently fails on
> this [0].
>
> > There currently is no option to enable GRO for forwarding, without
> > fraglist if to a device with h/w udp segmentation offload. This would
> > add that option too.
>
> Yes, that's exactly what I want. I want to maximize UDP
> forwarding/NATing performance when NIC is capable of UDP GSO offload,
> as I said above, non-fraglisted UDP GRO is better for that case.

That makes sense. Better to make explicit that that is the case
targeted here, rather than "some cases".

> > Though under admin control, which may make it a rarely exercised option.
> > Assuming most hosts to have single or homogeneous NICs, the OS should
> > be able to choose the preferred option in most cases (e.g.,: use fraglist
> > unless all devices support h/w gro).
>
> I though about some sort of auto-selection, but at the moment of
> receiving we can't know which interface this skb will be forwarded
> to.
> Also, as Paolo Abeni said in a comment to v2, UDP GRO may cause
> sensible delays, which may be inacceptable in some environments.
> That's why we have to use a sockopt and netdev features to explicitly
> enable UDP GRO.

I'm suspect that such fine-grained toggles end up broadly unused.

Agreed that it is not always possible to predict the destination NIC,
but that is why I suggested a very low bar that I believe captures the
majority of installed systems: where all NICs support the feature.
Anyway, that can always be added later -- as long as having this flag
off is not interpreted as demanding fraglist on forwarding.

> Regarding all this, I introduced NETIF_F_UDP_GRO to have the
> following chose:
>  - both NETIF_F_UDP_GRO and NETIF_F_GRO_FRAGLIST is off - no UDP GRO;
>  - NETIF_F_UDP_GRO is on, NETIF_F_GRO_FRAGLIST is off - classic GRO;
>  - both NETIF_F_UDP_GRO and NETIF_F_GRO_FRAGLIST is on - fraglisted
>    UDP GRO.
>
> > > Add the last element and allow to form plain UDP GRO packets if
> > > there is no socket -> we are on forwarding path, and the new
> > > NETIF_F_GRO_UDP is enabled on a receiving netdevice.
> > > Note that fraglisted UDP GRO now also depends on this feature, as
> >
> > That may cause a regression for applications that currently enable
> > that device feature.
>
> Thought about this one too. Not sure if it would be better to leave
> it as it is for now or how it's done in this series. The problem
> that we may have in future is that in some day we may get fraglisted
> TCP GRO, and then NETIF_F_GRO_FRAGLIST will affect both TCP and UDP,
> which is not desirable as for me. So I decided to guard this possible
> case.
>
> > > NETIF_F_GRO_FRAGLIST isn't tied to any particular L4 protocol.

As its name implies. I think it makes more sense to see it as an
explicit request to use fraglist for any protocol that supports it.

> > >
> > > Signed-off-by: Alexander Lobakin <alobakin@pm.me>
> > > ---
> > >  net/ipv4/udp_offload.c | 16 +++++++++++-----
> > >  1 file changed, 11 insertions(+), 5 deletions(-)
> > >
> > > diff --git a/net/ipv4/udp_offload.c b/net/ipv4/udp_offload.c
> > > index ff39e94781bf..781a035de5a9 100644
> > > --- a/net/ipv4/udp_offload.c
> > > +++ b/net/ipv4/udp_offload.c
> > > @@ -454,13 +454,19 @@ struct sk_buff *udp_gro_receive(struct list_head *head, struct sk_buff *skb,
> > >         struct sk_buff *p;
> > >         struct udphdr *uh2;
> > >         unsigned int off = skb_gro_offset(skb);
> > > -       int flush = 1;
> > > +       int flist = 0, flush = 1;
> > > +       bool gro_by_feat = false;
> >
> > What is this variable shorthand for? By feature? Perhaps
> > gro_forwarding is more descriptive.
>
> Yes, I chose "by feature" because fraglisted GRO also starts to
> work for local traffic if enabled, so "gro_forwarding" would be
> inaccurate naming.
>
> > >
> > > -       NAPI_GRO_CB(skb)->is_flist = 0;
> > > -       if (skb->dev->features & NETIF_F_GRO_FRAGLIST)
> > > -               NAPI_GRO_CB(skb)->is_flist = sk ? !udp_sk(sk)->gro_enabled: 1;
>
> I mean this. is_flist gets enabled if socket GRO option is disabled.
>
> > > +       if (skb->dev->features & NETIF_F_GRO_UDP) {
> > > +               if (skb->dev->features & NETIF_F_GRO_FRAGLIST)
> > > +                       flist = !sk || !udp_sk(sk)->gro_enabled;
> > >
> > > -       if ((sk && udp_sk(sk)->gro_enabled) || NAPI_GRO_CB(skb)->is_flist) {
> >
> > I would almost rename NETIF_F_GRO_FRAGLIST to NETIF_F_UDP_GRO_FWD.
> > Then this could be a !NETIF_F_UDP_GRO_FWD_FRAGLIST toggle on top of
> > that. If it wasn't for this fraglist option also enabling UDP GRO to
> > local sockets if set.
> >
> > That is, if the performance difference is significant enough to
> > require supporting both types of forwarding, under admin control.
> >
> > Perhaps the simplest alternative is to add the new feature without
> > making fraglist dependent on it:
> >
> >   if ((sk && udp_sk(sk)->gro_enabled) ||
> >       (skb->dev->features & NETIF_F_GRO_FRAGLIST) ||
> >       (!sk && skb->dev->features & NETIF_F_GRO_UDP_FWD))
>
> Yep, this will be the exact code if we end up with that
> NETIF_F_GRO_FRAGLIST should not depends on new netdev feature.
> But again, I wanted to protect TCP GRO if fraglisted TCP GRO will
> ever land the kernel. May be it's too much for the feature that
> currently doesn't exists even as a draft or plan, not sure.

If a protocol lands an fraglist implementation, I think the
expectation is that it will respond to the fraglist bit. I don't
understand why to preemptively block this. Then we would need yet
another feature bit for "forward <protocol> with fraglist".

>
> So, I'd stick to this variant (NETIF_F_UDP_GRO_FWD for plain,
> NETIF_F_GRO_FRAGLIST without changes for fraglisted) if preferred.
>
> > > +               gro_by_feat = !sk || flist;
> > > +       }
> > > +
> > > +       NAPI_GRO_CB(skb)->is_flist = flist;
> > > +
> > > +       if (gro_by_feat || (sk && udp_sk(sk)->gro_enabled)) {
> > >                 pp = call_gro_receive(udp_gro_receive_segment, head, skb);
> > >                 return pp;
> > >         }
> > > --
> > > 2.30.0
>
> [0] https://lore.kernel.org/netdev/1611235479-39399-1-git-send-email-dseok.yi@samsung.com
>
> Thanks,
> Al
>