Re: [PATCH v2 net-next] net: introduce a new tracepoint for tcp_rcv_space_adjust

[Song Liu <songliubraving@fb.com>]

> 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? 

Thanks,
Song