netdev - Re: Get amount of fast retransmissions from TCP info

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Message-ID: <CADVnQyn=6wYbVP0m3mepGU23LcEn_BK_TKoSNxVf=TUz9Q+f8g@mail.gmail.com>
Date:   Mon, 24 Apr 2017 17:00:25 -0400
From:   Neal Cardwell <ncardwell@...gle.com>
To:     Lars Erik Storbukås <storbukas.dev@...il.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Netdev <netdev@...r.kernel.org>
Subject: Re: Get amount of fast retransmissions from TCP info

"

On Mon, Apr 24, 2017 at 4:20 PM, Lars Erik Storbukås
<storbukas.dev@...il.com> wrote:
> 2017-04-24 21:42 GMT+02:00 Neal Cardwell <ncardwell@...gle.com>:
>> On Mon, Apr 24, 2017 at 3:11 PM, Lars Erik Storbukås
>> <storbukas.dev@...il.com> wrote:
>>> I'm trying to get amount of congestion events in TCP caused by
>>> DUPACK's (fast retransmissions), and can't seem to find any variable
>>> in the TCP info struct which hold that value. There are three
>>> variables in the TCP info struct that seem to hold similar congestion
>>> values: __u8 tcpi_retransmits;__u32 tcpi_retrans; __u32
>>> tcpi_total_retrans;
>>>
>>> Does anyone have any pointers on how to find this value in the TCP code?
>>>
>>> Please CC me personally if answering this question. Any help is
>>> greatly appreciated.
>>
>> [I'm cc-ing the netdev list.]
>>
>> Do you need this per-socket? On a per-socket basis, I do not think
>> there are separate totals for fast retransmits and timeout
>> retransmits.
>>
>> If a global number is good enough, then you can get that number from
>> the global network statistics. In "nstat" output they look like:
>>
>>   TcpExtTCPFastRetrans = packets sent in fast retransmit / fast recovery
>>
>>   TcpExtTCPSlowStartRetrans = packets sent in timeout recovery
>>
>> It sounds like TcpExtTCPFastRetrans is what you are after.
>>
>> Hope that helps,
>> neal
>
> Thanks for your answer Neal.
>
> Yes, I need this information per-socket. What would be the most
> appropriate place to update this value?

Is this for a custom kernel you are building? Or are you proposing
this for upstream?

IMHO the best place to add this for your custom kernel would be in
_tcp_retransmit_skb() around the spot with the comment "Update global
and local TCP statistics". Something like:

  /* Update global and local TCP statistics. */
...
  tp->total_retrans += segs;
  if (icsk->icsk_ca_state == TCP_CA_Loss)
    tp->slow_retrans += segs;
  else
    tp->fast_retrans += segs;

> If none of the variables (mentioned above) contain any value in
> regards to fast retransmits, what does the different values represent?

tcpi_retransmits: consecutive retransmits of lowest-sequence outstanding packet

tcpi_retrans: retransmitted packets estimated to be in-flight in the network now

tcpi_total_retrans: total number of retransmitted packets over the
life of the connection

Can you sketch out why you need to have separate counts for fast
retransmits and timeout/slow-start retransmits?

neal