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Date:   Thu, 26 Dec 2019 17:53:02 -0300
From:   Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
To:     Kevin Kou <qdkevin.kou@...il.com>
Cc:     linux-sctp@...r.kernel.org, netdev@...r.kernel.org,
        nhorman@...driver.com, davem@...emloft.net
Subject: Re: [PATCH net-next] sctp: move trace_sctp_probe_path into
 sctp_outq_sack

On Thu, Dec 26, 2019 at 12:29:17PM +0000, Kevin Kou wrote:
> The original patch bringed in the "SCTP ACK tracking trace event"
> feature was committed at Dec.20, 2017, it replaced jprobe usage
> with trace events, and bringed in two trace events, one is
> TRACE_EVENT(sctp_probe), another one is TRACE_EVENT(sctp_probe_path).
> The original patch intended to trigger the trace_sctp_probe_path in
> TRACE_EVENT(sctp_probe) as below code,
> 
> +TRACE_EVENT(sctp_probe,
> +
> +	TP_PROTO(const struct sctp_endpoint *ep,
> +		 const struct sctp_association *asoc,
> +		 struct sctp_chunk *chunk),
> +
> +	TP_ARGS(ep, asoc, chunk),
> +
> +	TP_STRUCT__entry(
> +		__field(__u64, asoc)
> +		__field(__u32, mark)
> +		__field(__u16, bind_port)
> +		__field(__u16, peer_port)
> +		__field(__u32, pathmtu)
> +		__field(__u32, rwnd)
> +		__field(__u16, unack_data)
> +	),
> +
> +	TP_fast_assign(
> +		struct sk_buff *skb = chunk->skb;
> +
> +		__entry->asoc = (unsigned long)asoc;
> +		__entry->mark = skb->mark;
> +		__entry->bind_port = ep->base.bind_addr.port;
> +		__entry->peer_port = asoc->peer.port;
> +		__entry->pathmtu = asoc->pathmtu;
> +		__entry->rwnd = asoc->peer.rwnd;
> +		__entry->unack_data = asoc->unack_data;
> +
> +		if (trace_sctp_probe_path_enabled()) {
> +			struct sctp_transport *sp;
> +
> +			list_for_each_entry(sp, &asoc->peer.transport_addr_list,
> +					    transports) {
> +				trace_sctp_probe_path(sp, asoc);
> +			}
> +		}
> +	),
> 
> But I found it did not work when I did testing, and trace_sctp_probe_path
> had no output, I finally found that there is trace buffer lock
> operation(trace_event_buffer_reserve) in include/trace/trace_events.h:
> 
> static notrace void							\
> trace_event_raw_event_##call(void *__data, proto)			\
> {									\
> 	struct trace_event_file *trace_file = __data;			\
> 	struct trace_event_data_offsets_##call __maybe_unused __data_offsets;\
> 	struct trace_event_buffer fbuffer;				\
> 	struct trace_event_raw_##call *entry;				\
> 	int __data_size;						\
> 									\
> 	if (trace_trigger_soft_disabled(trace_file))			\
> 		return;							\
> 									\
> 	__data_size = trace_event_get_offsets_##call(&__data_offsets, args); \
> 									\
> 	entry = trace_event_buffer_reserve(&fbuffer, trace_file,	\
> 				 sizeof(*entry) + __data_size);		\
> 									\
> 	if (!entry)							\
> 		return;							\
> 									\
> 	tstruct								\
> 									\
> 	{ assign; }							\
> 									\
> 	trace_event_buffer_commit(&fbuffer);				\
> }
> 
> The reason caused no output of trace_sctp_probe_path is that
> trace_sctp_probe_path written in TP_fast_assign part of
> TRACE_EVENT(sctp_probe), and it will be placed( { assign; } ) after the
> trace_event_buffer_reserve() when compiler expands Macro,
> 
>         entry = trace_event_buffer_reserve(&fbuffer, trace_file,        \
>                                  sizeof(*entry) + __data_size);         \
>                                                                         \
>         if (!entry)                                                     \
>                 return;                                                 \
>                                                                         \
>         tstruct                                                         \
>                                                                         \
>         { assign; }                                                     \
> 
> so trace_sctp_probe_path finally can not acquire trace_event_buffer
> and return no output, that is to say the nest of tracepoint entry function
> is not allowed. The function call flow is:
> 
> trace_sctp_probe()
> -> trace_event_raw_event_sctp_probe()
>  -> lock buffer
>  -> trace_sctp_probe_path()
>    -> trace_event_raw_event_sctp_probe_path()  --nested
>    -> buffer has been locked and return no output.
> 
> This patch is to remove trace_sctp_probe_path from the TP_fast_assign
> part of TRACE_EVENT(sctp_probe) to avoid the nest of entry function,
> and trigger sctp_probe_path_trace in sctp_outq_sack.
> 
> After this patch, you can enable both events individually,
>   # 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
> 
> Signed-off-by: Kevin Kou <qdkevin.kou@...il.com>

Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@...il.com>

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