linux-kernel - Re: [v2 PATCH -tip 3/6] net: sctp: Add SCTP ACK tracking trace event

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Date:   Mon, 18 Dec 2017 12:05:16 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Ian McDonald <ian.mcdonald@...di.co.nz>,
        Vlad Yasevich <vyasevich@...il.com>,
        Stephen Hemminger <stephen@...workplumber.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        "H . Peter Anvin" <hpa@...or.com>,
        Gerrit Renker <gerrit@....abdn.ac.uk>,
        "David S . Miller" <davem@...emloft.net>,
        Neil Horman <nhorman@...driver.com>, dccp@...r.kernel.org,
        netdev@...r.kernel.org, linux-sctp@...r.kernel.org,
        Stephen Rothwell <sfr@...b.auug.org.au>
Subject: Re: [v2 PATCH -tip 3/6] net: sctp: Add SCTP ACK tracking trace
 event

On Mon, 18 Dec 2017 17:12:15 +0900
Masami Hiramatsu <mhiramat@...nel.org> wrote:

> Add SCTP ACK tracking trace event to trace the changes of SCTP
> association state in response to incoming packets.
> It is used for debugging SCTP congestion control algorithms,
> and will replace sctp_probe module.
> 
> Note that this event a bit tricky. Since this consists of 2
> events (sctp_probe and sctp_probe_path) so you have to enable
> both events as below.
> 
>   # cd /sys/kernel/debug/tracing
>   # echo 1 > events/sctp/sctp_probe/enable
>   # echo 1 > events/sctp/sctp_probe_path/enable
> 
> Or, you can enable all the events under sctp.
> 
>   # echo 1 > events/sctp/enable
> 
> Since sctp_probe_path event is always invoked from sctp_probe
> event, you can not see any output if you only enable
> sctp_probe_path.

I have to ask, why did you do it this way?


> +#include <trace/define_trace.h>
> diff --git a/net/sctp/sm_statefuns.c b/net/sctp/sm_statefuns.c
> index 8f8ccded13e4..c5f92b2cc5c3 100644
> --- a/net/sctp/sm_statefuns.c
> +++ b/net/sctp/sm_statefuns.c
> @@ -59,6 +59,9 @@
>  #include <net/sctp/sm.h>
>  #include <net/sctp/structs.h>
>  
> +#define CREATE_TRACE_POINTS
> +#include <trace/events/sctp.h>
> +
>  static struct sctp_packet *sctp_abort_pkt_new(
>  					struct net *net,
>  					const struct sctp_endpoint *ep,
> @@ -3219,6 +3222,8 @@ enum sctp_disposition sctp_sf_eat_sack_6_2(struct net *net,
>  	struct sctp_sackhdr *sackh;
>  	__u32 ctsn;
>  
> +	trace_sctp_probe(ep, asoc, chunk);

What about doing this right after this probe:

	if (trace_sctp_probe_path_enabled()) {
		struct sctp_transport *sp;

		list_for_each_entry(sp, &asoc->peer.transpor_addr_list,
				    transports) {
			trace_sctp_probe_path(sp, asoc);
		}
	}

The "trace_sctp_probe_path_enabled()" is a static branch, which means
it's a nop just like a tracepoint is, and will not add any overhead if
the trace_sctp_probe_path is not enabled.

-- Steve

> +
>  	if (!sctp_vtag_verify(chunk, asoc))
>  		return sctp_sf_pdiscard(net, ep, asoc, type, arg, commands);
>