Re: how to register an event handler for the instance

[Tzvetomir Stoyanov <tz.stoyanov@gmail.com>]

On Wed, May 11, 2022 at 9:33 PM christopher lee
<christopher.lee.eu@gmail.com> wrote:
>
> On Sat, 2022-05-07 at 08:02 +0300, Tzvetomir Stoyanov wrote:
> > On Fri, May 6, 2022 at 7:18 PM christopher lee
> > <christopher.lee.eu@gmail.com> wrote:
> > >
> > > Hi all,
> > >
> > > I use libtraceevent in my trace tool to parse the event log. Now I
> > > allocated an instance, but no idea how to use
> > > tep_register_event_handler() to register a handler for the
> > > instance. Is
> > > it possilbe to register handler for the event in the instance?
> > >
> >
> > Hi Chrit,
> > All trace events are the same across all trace instances, thus the
> > handler registered with tep_register_event_handler() handles events
> > from all instances with a given id. That's why there are no
> > "instance"
> > oriented APIs in that library. I would suggest looking at the tracefs
> > library, which is instance aware. It can be used to allocate trace
> > instances and read events from a given instance.
> >
> > https://trace-cmd.org/Documentation/libtracefs/
> >
>
>
> Hi Tzvetomir Stoyanov,
>
> Thanks,  instance event handlers work now, but I'm confused with these
> codes, do I have to read trace_pipe_raw? I found that if I didn't read
> this file, the handler would not be able to get trace information.
> Becuase if I read this file, and also call tep_print_event() will
> consume lots of CPU resoruce. how can I parse the trace event info
> without reading this file?

Hi Christ,
All recorded events are in trace_pipe_raw file, so yes - if you want
to parse the trace event info, the first step is to read the events.
It consumes lots of CPU resources because there are a lot of events to
read and parse. I would suggest using the tracefs_iterate_raw_events()
API to read the events, as it is optimized to read the trace_pipe_raw
file in an efficient way, or at least to look at its implementation if
you do not want to use the tracefs library.

static int read_callback(struct tep_event *event, struct tep_record
*record, int cpu, void *context)
{
     ...
     tep_print_event();
     ...
}

tracefs_iterate_raw_events(tep, instance, NULL, 0, read_callback, ...);

>
>
>         for (i = 0; ; i++) {
>                 char *raw_buf;
>                 char *cpu;
>
>                 ret = asprintf(&cpu, "%s/cpu%d", per_cpu, i);
>                 if (ret < 0)
>                         pdie("Could not allocate memory for cpu buffer
> %d name", i);
>
>                 ret = stat(cpu, &st);
>                 if (ret < 0 || !S_ISDIR(st.st_mode)) {
>                         free(cpu);
>                         goto start;
>                 }
>
>                 ret = asprintf(&raw_buf, "%s/trace_pipe_raw", cpu);
>                 if (ret < 0)
>                         pdie("Could not allocate memory for cpu %d raw
> buffer name", i);
>
>                 read_raw_buffer(i, raw_buf);
>                 free(raw_buf);
>                 free(cpu);
>         }
>
> Best regards,
> Christ
>
> >



-- 
Tzvetomir (Ceco) Stoyanov
VMware Open Source Technology Center