Re: trace-cmd: Using the python wrapping to read trace markers

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On Sun, Sep 11, 2022 at 11:03 PM Sharon Gabay <Sharon.Gabay@xxxxxxxxxxxx> wrote:
> Hi !

Hi Sharon,

> We are using this article to generate trace markers:
> I am responsible for reading the trace.dat binary into our tools and we are using the trace-cmd python wrapping.

The trace-cmd python wrappers are auto generated. The logic for that
may not be up to date, so I do not know what is the current state of
those wrappers.

> I'm trying to use the python wrapping in order to read trace markers, so far unsuccessfully (See below). I've noticed that the trace marker string is written directly into the event, without using pointers, indices etc, and that the size field in this case equals zero (in bold, below). So I'm guessing that the way to read it is a bit different (using the method for reading regular string field does not seem to work). Here's how the event description looks like inside trace.dat:
> name: print
> ID: 5
> format:
>         field:unsigned short common_type;       offset:0;       size:2; signed:0;
>         field:unsigned char common_flags;       offset:2;       size:1; signed:0;
>         field:unsigned char common_preempt_count;       offset:3;       size:1; signed:0;
>         field:int common_pid;   offset:4;       size:4; signed:1;
>         field:unsigned char common_migrate_disable;     offset:8;       size:1; signed:0;
>         field:unsigned char common_preempt_lazy_count;  offset:9;       size:1; signed:0;
>         field:unsigned long ip; offset:16;      size:8; signed:0;
>         field:char buf[];       offset:24;      size:0; signed:1;
> print fmt: "%ps: %s", (void *)REC->ip, REC->buf
> And here are some of the methods I've tried to access the "buf" field above:
> field_name = "buf"
> // method 1:
> f = tep_find_any_field(self._event, field_name)
> if f is None:
>     return None
> str = py_field_get_str(f, self._record) // (returns empty string)
> // method 2:
> f = tep_find_any_field(self._event, name)
> if f is None:
>     return None
> data = py_field_get_data(f, self._record) // (returns empty buffer)

This is the flow that should work in your case, but I do not know if
it can be implemented with the existing python wrappers :

 1. Get the ID of the trace marker event. Event IDs are assigned at
kernel boot time, so the ID can be different, depending on the ftace
     event = tep_find_event_by_name(tep, "ftrace", "print"), if you
are using the "trace_marker" file to inject marker events, or
     event = tep_find_event_by_name(tep, "ftrace", "raw_data"), if you
are using the "trace_marker_raw" file.
     marker_id = event->id
2. Iterate over the events from the file and look for events with id
== marker_id. For these events, you can the field with name "buf",
either as string or as binary buffer.

> Thanks in advance,

Thanks for using trace-cmd and helping to improve it!

If you are interested in writing Python programs for ftrace tracing, I
would encourage to try trace-cruncher:
Although it is still in its beta development stage, it is quite useful
for use cases such as yours. Take a look at examples/
file - it does exactly what you want to achieve. That example opens a
trace.dat file, iterates over all events from it looking for specific

> Sharon Gabay

Tzvetomir (Ceco) Stoyanov
VMware Open Source Technology Center

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