linux-kernel - Re: [PATCH v2 net-next] net: introduce a new tracepoint for tcp_rcv_space

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Date:   Wed, 18 Apr 2018 05:53:28 +0800
From:   Yafang Shao <laoar.shao@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     David Miller <davem@...emloft.net>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        yoshfuji@...ux-ipv6.org, Song Liu <songliubraving@...com>,
        netdev@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 net-next] net: introduce a new tracepoint for tcp_rcv_space_adjust

On Wed, Apr 18, 2018 at 1:27 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>
>
> On 04/17/2018 09:36 AM, Yafang Shao 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
>
> Sorry, I could not parse these :/
>

Sorry for the poor expression. Will improve it.
I mean with these two tracepoint we could calculate the latency that
if the user process can't process this packet immdiately.

>>
>> 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@...il.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);
>
> Please scrap this comment and simply use the real thing.
>
>         _entry->sock_cookie = sock_gen_cookie(sk);
>
> We build generic events.
>
> Being able to filter many TCP events on one socket cookie will be useful
>
> If you worry about sock_gen_cookie(sk) being too expensive, then we can add an inline helper
> for the fast path (when sk_cookie has been already set)
>

Will improve.

>>       ),
>>
>> -     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",
>
>
> iproute2/ss command uses hexadcimal output for socket cookie. Please use %llx for consistency.
>

OK