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