Re: [PATCH v5 net-next 01/36] net: Introduce direct data placement tcp offload

[Eric Dumazet <edumazet@google.com>]

On Thu, Jul 22, 2021 at 2:18 PM Boris Pismenny <borispismenny@gmail.com> wrote:
>
> On 22/07/2021 14:26, Eric Dumazet wrote:
> > On Thu, Jul 22, 2021 at 1:04 PM Boris Pismenny <borisp@nvidia.com> wrote:
> >>
> >> From: Boris Pismenny <borisp@mellanox.com>
> >>
> >>
> > ...
> >
> >>  };
> >>
> >>  const char
> >> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> >> index e6ca5a1f3b59..4a7160bba09b 100644
> >> --- a/net/ipv4/tcp_input.c
> >> +++ b/net/ipv4/tcp_input.c
> >> @@ -5149,6 +5149,9 @@ tcp_collapse(struct sock *sk, struct sk_buff_head *list, struct rb_root *root,
> >>                 memcpy(nskb->cb, skb->cb, sizeof(skb->cb));
> >>  #ifdef CONFIG_TLS_DEVICE
> >>                 nskb->decrypted = skb->decrypted;
> >> +#endif
> >> +#ifdef CONFIG_ULP_DDP
> >> +               nskb->ddp_crc = skb->ddp_crc;
> >
> > Probably you do not want to attempt any collapse if skb->ddp_crc is
> > set right there.
> >
>
> Right.
>
> >>  #endif
> >>                 TCP_SKB_CB(nskb)->seq = TCP_SKB_CB(nskb)->end_seq = start;
> >>                 if (list)
> >> @@ -5182,6 +5185,11 @@ tcp_collapse(struct sock *sk, struct sk_buff_head *list, struct rb_root *root,
> >>  #ifdef CONFIG_TLS_DEVICE
> >>                                 if (skb->decrypted != nskb->decrypted)
> >>                                         goto end;
> >> +#endif
> >> +#ifdef CONFIG_ULP_DDP
> >> +
> >> +                               if (skb->ddp_crc != nskb->ddp_crc)
> >
> > This checks only the second, third, and remaining skbs.
> >
>
> Right, as we handle the head skb above. Could you clarify?

I was simply saying you missed the first skb.

>
> >> +                                       goto end;
> >>  #endif
> >>                         }
> >>                 }
> >
> >
> > tcp_collapse() is copying data from small skbs to pack it to bigger
> > skb (one page of payload), in case
> > of memory emergency/pressure (socket queues are full)
> >
> > If your changes are trying to avoid 'needless'  copies, maybe you
> > should reconsider and let the emergency packing be done.
> >
> > If the copy is not _possible_, you should rephrase your changelog to
> > clearly state the kernel _cannot_ access this memory in any way.
> >
>
> The issue is that skb_condense also gets called on many skbs in
> tcp_add_backlog and it will identify skbs that went through DDP as ideal
> for packing, even though they are not small and packing is
> counter-productive as data already resides in its destination.
>
> As mentioned above, it is possible to copy, but it is counter-productive
> in this case. If there was a real need to access this memory, then it is
> allowed.

Standard GRO packets from high perf drivers have no room in their
skb->head (ie skb_tailroom() should be 0)

If you have a driver using GRO and who pulled some payload in
skb->head, it is already too late for DDP.

So I think you are trying to add code in TCP that should not be
needed. Perhaps mlx5 driver is doing something it should not ?
(If this is ' copybreak'  this has been documented as being
suboptimal, transports have better strategies)

Secondly, tcp_collapse() should absolutely not be called under regular
workloads.

Trying to optimize this last-resort thing is a lost cause:
If an application is dumb enough to send small packets back-to-back,
it should be fixed (sender side has this thing called autocork, for
applications that do not know about MSG_MORE or TC_CORK.)

(tcp_collapse is a severe source of latencies)