Re: [PATCH net-next 1/2] tcp: call tcp_drop() in tcp collapse

[Yafang Shao]

On Sun, Jul 29, 2018 at 12:28 AM, Eric Dumazet <edumazet@xxxxxxxxxx> wrote:
> On Sat, Jul 28, 2018 at 12:43 AM Yafang Shao <laoar.shao@xxxxxxxxx> wrote:
>>
>> On Sat, Jul 28, 2018 at 11:38 AM, Eric Dumazet <edumazet@xxxxxxxxxx> wrote:
>> > On Fri, Jul 27, 2018 at 8:35 PM Yafang Shao <laoar.shao@xxxxxxxxx> wrote:
>> >
>> >> So what about LINUX_MIB_TCPOFOMERGE ?
>> >> Regarding LINUX_MIB_TCPOFOMERGE,  a skb is already covered by another
>> >> skb, is that dropping the packet or simply lowering the memory
>> >> overhead ?
>> >
>> > What do you think ?
>> >
>> > If you receive two times the same payload, don't you have to drop one
>> > of the duplicate ?
>> >
>> > There is a a big difference between the two cases.
>>
>> If the drop caused some data lost (which may then cause retransmition
>> or something), then this is a really DROP.
>> While if the drop won't cause any data lost, meaning it is a
>> non-harmful behavior, I think it should not be defined as DROP.
>> This is my suggestion anyway.
>
> Sigh.
>
> We count drops, not because they are ' bad or something went wrong'.
>
> If TCP stack receives twice the same sequence (same payload), we
> _drop_ one of the duplicate, so we account for this event.
>
> When ' collapsing'  we reorganize our own storage, not because we have
> to drop a payload,
> but for some memory pressure reason.

Thanks for you clarification.
So what about LINUX_MIB_TCPOFODROP ?

        if (unlikely(tcp_try_rmem_schedule(sk, skb, skb->truesize))) {
                NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPOFODROP);
                tcp_drop(sk, skb);
                return;
        }


It is also because of our own memory pressure, but we call tcp_drop() here.

I am not mean to disagree with you. I am just confused and  want to
make it clear.

> We have specific SNMP counters to account for these, we do not want to
> pretend a packet was ' dropped' since it was not.
>
> If we have to _drop_ some packets, it is called Pruning, and we do
> properly account for these drops.

Agreed.


Thanks
Yafang