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

[Yafang Shao]

On Mon, Jul 30, 2018 at 10:27 AM, Eric Dumazet <edumazet@xxxxxxxxxx> wrote:
> On Sun, Jul 29, 2018 at 7:06 PM Yafang Shao <laoar.shao@xxxxxxxxx> wrote:
>>
>> 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.
>
> Yes, we _drop_ a packet.
>
> That is pretty clear that the payload is dropped, and that the sender
> will have to _retransmit_.
>
>>
>> I am not mean to disagree with you. I am just confused and  want to
>> make it clear.
>
>
> Collapsing is :
>
> For (a bunch of packets)
>    Try (to compress them in order to reduce memory overhead)
>
> No drop of payload happens here. Sender wont have to retransmit.

OK.
Thanks for your patient.

Should we put NET_INC_STATS(sock_net(sk),  mib_idx) into the funtion
tcp_drop() ?
Then we could easily relate the sk_drops with the SNMP counters.

Something like that,

    static void tcp_drop(struct sock *sk, struct sk_buff *skb, int mib_idx)
    {
        int segs = max_t(u16, 1, skb_shinfo(skb)->gso_segs);

        atomic_add(segs, &sk->sk_drops);
        NET_ADD_STATS(sock_net(sk), mib_idx, segs);
        __kfree_skb(skb);
    }


Thanks
Yafang