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From:   Yafang Shao <laoar.shao@gmail.com>
Date:   Wed, 18 Apr 2018 05:54:55 +0800
Message-ID: <CALOAHbASY7-rJ8p-OciOqY9u90sKB7qP1hasiHsRaQ-g39nTqA@mail.gmail.com>
Subject: Re: [PATCH v2 net-next] net: introduce a new tracepoint for tcp_rcv_space_adjust
To:     Song Liu <songliubraving@fb.com>
Cc:     "eric.dumazet@gmail.com" <eric.dumazet@gmail.com>,
        "davem@davemloft.net" <davem@davemloft.net>,
        "kuznet@ms2.inr.ac.ru" <kuznet@ms2.inr.ac.ru>,
        "yoshfuji@linux-ipv6.org" <yoshfuji@linux-ipv6.org>,
        "netdev@vger.kernel.org" <netdev@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
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On Wed, Apr 18, 2018 at 1:38 AM, Song Liu <songliubraving@fb.com> wrote:
>
>
>> On Apr 17, 2018, at 9:36 AM, Yafang Shao <laoar.shao@gmail.com> wrote:
>>
>> tcp_rcv_space_adjust is called every time data is copied to user space,
>> introducing a tcp tracepoint for which could show us when the packet is
>> copied to user.
>> This could help us figure out whether there's latency in user process.
>>
>> When a tcp packet arrives, tcp_rcv_established() will be called and with
>> the existed tracepoint tcp_probe we could get the time when this packet
>> arrives.
>> Then this packet will be copied to user, and tcp_rcv_space_adjust will
>> be called and with this new introduced tracepoint we could get the time
>> when this packet is copied to user.
>>
>>        arrives time : user process time    => latency caused by user
>>        tcp_probe      tcp_rcv_space_adjust
>>
>> Hence in the printk message, sk_cookie is printed as a key to relate
>> tcp_rcv_space_adjust with tcp_probe.
>>
>> Maybe we could export sockfd in this new tracepoint as well, then we
>> could relate this new tracepoint with epoll/read/recv* tracepoints, and
>> finally that could show us the whole lifespan of this packet. But we
>> could also implement that with pid as these functions are executed in
>> process context.
>>
>> Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
>>
>> ---
>> v1 -> v2: use sk_cookie as key suggested by Eric.
>> ---
>> include/trace/events/tcp.h | 33 +++++++++++++++++++++++++++------
>> net/ipv4/tcp_input.c       |  2 ++
>> 2 files changed, 29 insertions(+), 6 deletions(-)
>>
>> diff --git a/include/trace/events/tcp.h b/include/trace/events/tcp.h
>> index 3dd6802..814f754 100644
>> --- a/include/trace/events/tcp.h
>> +++ b/include/trace/events/tcp.h
>> @@ -10,6 +10,7 @@
>> #include <linux/tracepoint.h>
>> #include <net/ipv6.h>
>> #include <net/tcp.h>
>> +#include <linux/sock_diag.h>
>>
>> #define TP_STORE_V4MAPPED(__entry, saddr, daddr)              \
>>       do {                                                    \
>> @@ -125,6 +126,7 @@
>>               __array(__u8, daddr, 4)
>>               __array(__u8, saddr_v6, 16)
>>               __array(__u8, daddr_v6, 16)
>> +             __field(__u64, sock_cookie)
>>       ),
>>
>>       TP_fast_assign(
>> @@ -144,12 +146,24 @@
>>
>>               TP_STORE_ADDRS(__entry, inet->inet_saddr, inet->inet_daddr,
>>                              sk->sk_v6_rcv_saddr, sk->sk_v6_daddr);
>> +
>> +             /*
>> +              * sk_cookie is used to identify a socket, with which we could
>> +              * relate this tracepoint with other tracepoints,
>> +              * i.e. tcp_probe.
>> +              * If we needn't this relation, then sk_cookie is useless;
>> +              * if we need this relation, then tcp_probe is already set,
>> +              * and sk_cookie is already set in tcp_probe, so we could get
>> +              * the value directly.
>> +              */
>> +             __entry->sock_cookie = atomic64_read(&sk->sk_cookie);
>>       ),
>>
>> -     TP_printk("sport=%hu dport=%hu saddr=%pI4 daddr=%pI4 saddrv6=%pI6c daddrv6=%pI6c",
>> +     TP_printk("sport=%hu dport=%hu saddr=%pI4 daddr=%pI4 saddrv6=%pI6c daddrv6=%pI6c sock_cookie=%llu",
>>                 __entry->sport, __entry->dport,
>>                 __entry->saddr, __entry->daddr,
>> -               __entry->saddr_v6, __entry->daddr_v6)
>> +               __entry->saddr_v6, __entry->daddr_v6,
>> +               __entry->sock_cookie)
>> );
>>
>> DEFINE_EVENT(tcp_event_sk, tcp_receive_reset,
>> @@ -166,6 +180,13 @@
>>       TP_ARGS(sk)
>> );
>>
>> +DEFINE_EVENT(tcp_event_sk, tcp_rcv_space_adjust,
>> +
>> +     TP_PROTO(const struct sock *sk),
>> +
>> +     TP_ARGS(sk)
>> +);
>> +
>> TRACE_EVENT(tcp_retransmit_synack,
>>
>>       TP_PROTO(const struct sock *sk, const struct request_sock *req),
>> @@ -232,6 +253,7 @@
>>               __field(__u32, snd_wnd)
>>               __field(__u32, srtt)
>>               __field(__u32, rcv_wnd)
>> +             __field(__u64, sock_cookie)
>>       ),
>>
>>       TP_fast_assign(
>> @@ -256,15 +278,14 @@
>>               __entry->rcv_wnd = tp->rcv_wnd;
>>               __entry->ssthresh = tcp_current_ssthresh(sk);
>>               __entry->srtt = tp->srtt_us >> 3;
>> +             __entry->sock_cookie = sock_gen_cookie(sk);
>>       ),
>>
>> -     TP_printk("src=%pISpc dest=%pISpc mark=%#x length=%d snd_nxt=%#x "
>> -               "snd_una=%#x snd_cwnd=%u ssthresh=%u snd_wnd=%u srtt=%u "
>> -               "rcv_wnd=%u",
>> +     TP_printk("src=%pISpc dest=%pISpc mark=%#x length=%d snd_nxt=%#x snd_una=%#x snd_cwnd=%u ssthresh=%u snd_wnd=%u srtt=%u rcv_wnd=%u sock_cookie=%llu",
>>                 __entry->saddr, __entry->daddr, __entry->mark,
>>                 __entry->length, __entry->snd_nxt, __entry->snd_una,
>>                 __entry->snd_cwnd, __entry->ssthresh, __entry->snd_wnd,
>> -               __entry->srtt, __entry->rcv_wnd)
>> +               __entry->srtt, __entry->rcv_wnd, __entry->sock_cookie)
>> );
>>
>> #endif /* _TRACE_TCP_H */
>> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
>> index f93687f..43ad468 100644
>> --- a/net/ipv4/tcp_input.c
>> +++ b/net/ipv4/tcp_input.c
>> @@ -582,6 +582,8 @@ void tcp_rcv_space_adjust(struct sock *sk)
>>       u32 copied;
>>       int time;
>>
>> +     trace_tcp_rcv_space_adjust(sk);
>> +
>>       tcp_mstamp_refresh(tp);
>>       time = tcp_stamp_us_delta(tp->tcp_mstamp, tp->rcvq_space.time);
>>       if (time < (tp->rcv_rtt_est.rtt_us >> 3) || tp->rcv_rtt_est.rtt_us == 0)
>> --
>> 1.8.3.1
>>
>
> If I understand this correctly, you can get all the information you need with
> a kprobe on tcp_rcv_space_adjust(). Why is it necessary to introduce a new
> tracepoint?
>

Tracepoint is less expensive and more cnovinient, that is the same
reason why tcp_probe.c was removed and tcp_probe tracepoint was
introduced.

Thanks
Yafang