Hi Yafang,
> After commit 3087c61ed2c4 ("tools/testing/selftests/bpf: replace open-coded 16 with TASK_COMM_LEN"),
> the content of the format file under
> /sys/kernel/debug/tracing/events/task/task_newtask was changed from
> field:char comm[16]; offset:12; size:16; signed:0;
> to
> field:char comm[TASK_COMM_LEN]; offset:12; size:16; signed:0;
>
> John reported that this change breaks older versions of perfetto.
> Then Mathieu pointed out that this behavioral change was caused by the
> use of __stringify(_len), which happens to work on macros, but not on enum
> labels. And he also gave the suggestion on how to fix it:
> :One possible solution to make this more robust would be to extend
> :struct trace_event_fields with one more field that indicates the length
> :of an array as an actual integer, without storing it in its stringified
> :form in the type, and do the formatting in f_show where it belongs.
>
> The result as follows after this change,
> $ cat /sys/kernel/debug/tracing/events/task/task_newtask/format
> field:char comm[16]; offset:12; size:16; signed:0;
>
> Fixes: 3087c61ed2c4 ("tools/testing/selftests/bpf: replace open-coded 16 with TASK_COMM_LEN")
> Reported-by: John Stultz <jstultz@xxxxxxxxxx>
> Debugged-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> Suggested-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> Signed-off-by: Yafang Shao <laoar.shao@xxxxxxxxx>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> Cc: Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx>
> Cc: Kajetan Puchalski <kajetan.puchalski@xxxxxxx>
> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Cc: John Stultz <jstultz@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx # v5.17+
>From my testing this works the same with older Perfetto as the previous
diff you sent in the older thread, ie this type of errors:
Name Value Type
mismatched_sched_switch_tids 2439 error (analysis)
systrace_parse_failure 3853 error (analysis)
Meaning that applying this patch on top of the old one ends up with
different results than just reverting the old one so not a 100% fix
but as I said before, still an improvement.
Thanks,
Kajetan
