netdev - Re: [Patch net-next v2] tcp: add a tracepoint for tcp_retransmit

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Date:   Fri, 13 Oct 2017 10:33:40 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Hannes Frederic Sowa <hannes@...essinduktion.org>,
        Brendan Gregg <brendan.d.gregg@...il.com>,
        Neal Cardwell <ncardwell@...gle.com>
Subject: Re: [Patch net-next v2] tcp: add a tracepoint for tcp_retransmit_skb()

On Thu, Oct 12, 2017 at 4:18 PM, Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
> On Thu, Oct 12, 2017 at 03:48:07PM -0700, Cong Wang wrote:
>> We need a real-time notification for tcp retransmission
>> for monitoring.
>>
>> Of course we could use ftrace to dynamically instrument this
>> kernel function too, however we can't retrieve the connection
>> information at the same time, for example perf-tools [1] reads
>> /proc/net/tcp for socket details, which is slow when we have
>> a lots of connections.
>>
>> Therefore, this patch adds a tracepoint for tcp_retransmit_skb()
>> and exposes src/dst IP addresses and ports of the connection.
>> This also makes it easier to integrate into perf.
>>
>> Note, I expose both IPv4 and IPv6 addresses at the same time:
>> for a IPv4 socket, v4 mapped address is used as IPv6 addresses,
>> for a IPv6 socket, LOOPBACK4_IPV6 is already filled by kernel.
>> Also, add sk and skb pointers as they are useful for BPF.
>>
>> 1. https://github.com/brendangregg/perf-tools/blob/master/net/tcpretrans
>>
>> Cc: Eric Dumazet <edumazet@...gle.com>
>> Cc: Alexei Starovoitov <alexei.starovoitov@...il.com>
>> Cc: Hannes Frederic Sowa <hannes@...essinduktion.org>
>> Cc: Brendan Gregg <brendan.d.gregg@...il.com>
>> Cc: Neal Cardwell <ncardwell@...gle.com>
>> Signed-off-by: Cong Wang <xiyou.wangcong@...il.com>
>> ---
>>  include/trace/events/tcp.h | 68 ++++++++++++++++++++++++++++++++++++++++++++++
>>  net/core/net-traces.c      |  1 +
>>  net/ipv4/tcp_output.c      |  3 ++
>>  3 files changed, 72 insertions(+)
>>  create mode 100644 include/trace/events/tcp.h
>>
>> diff --git a/include/trace/events/tcp.h b/include/trace/events/tcp.h
>> new file mode 100644
>> index 000000000000..749f93c542ab
>> --- /dev/null
>> +++ b/include/trace/events/tcp.h
>> @@ -0,0 +1,68 @@
>> +#undef TRACE_SYSTEM
>> +#define TRACE_SYSTEM tcp
>> +
>> +#if !defined(_TRACE_TCP_H) || defined(TRACE_HEADER_MULTI_READ)
>> +#define _TRACE_TCP_H
>> +
>> +#include <linux/ipv6.h>
>> +#include <linux/tcp.h>
>> +#include <linux/tracepoint.h>
>> +#include <net/ipv6.h>
>> +
>> +TRACE_EVENT(tcp_retransmit_skb,
>> +
>> +     TP_PROTO(struct sock *sk, struct sk_buff *skb, int segs),
>> +
>> +     TP_ARGS(sk, skb, segs),
>> +
>> +     TP_STRUCT__entry(
>> +             __field(void *, skbaddr)
>> +             __field(void *, skaddr)
>> +             __field(__u16, sport)
>> +             __field(__u16, dport)
>> +             __array(__u8, saddr, 4)
>> +             __array(__u8, daddr, 4)
>> +             __array(__u8, saddr_v6, 16)
>> +             __array(__u8, daddr_v6, 16)
>> +     ),
> ...
>>       if (likely(!err)) {
>>               TCP_SKB_CB(skb)->sacked |= TCPCB_EVER_RETRANS;
>> +             trace_tcp_retransmit_skb(sk, skb, segs);
>
> looks great to me, but why 'segs' is there?
> It's unused.

Ah, I copy-n-paste the tcp_retransmit_skb() prototype...
I will remove it.

Thanks.