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

[Boris Pismenny <borispismenny@gmail.com>]

On 22/07/2021 16:10, Eric Dumazet wrote:
> 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.
> 

But, the first SKB got handled in the change above. The code here is the
same for TLS, if it is wrong, then we already have an issue here.

>>
>>>> +                                       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)
> 


Sorry. My response above was about skb_condense which I've confused with
tcp_collapse.

In tcp_collapse, we could allow the copy, but the problem is CRC, which
like TLS's skb->decrypted marks that the data passed the digest
validation in the NIC. If we allow collapsing SKBs with mixed marks, we
will need to force software copy+crc verification. As TCP collapse is
indeed rare and the offload is opportunistic in nature, we can make this
change and submit another version, but I'm confused; why was it OK for
TLS, while it is not OK for DDP+CRC?