Enable histograms for events/ftrace/timerlat and osnoise

Enable histograms for events/ftrace/timerlat and osnoise

[Daniel Bristot de Oliveira]

Hi Steven

I would like to collect histograms from timerlat and osnoise tracers event,
but I cannot collect them because they are not tracepoints but... tracer
events. So...

  # ls /sys/kernel/tracing/events/ftrace/timerlat/
  format	hist

no hist, filter, trigger files...

How hard would it be to enable histograms to the ftrace/osnoise and
ftrace/timerlat events?

This will allow me to reduce the overhead of rtla timerlat and rtla osnoise
tools...

Thanks in advance, Stephen!
-- Denny

Re: Enable histograms for events/ftrace/timerlat and osnoise

[Daniel Bristot de Oliveira]

On 3/24/23 17:13, Daniel Bristot de Oliveira wrote:
> Hi Steven
> 
> I would like to collect histograms from timerlat and osnoise tracers event,
> but I cannot collect them because they are not tracepoints but... tracer
> events. So...
> 
>   # ls /sys/kernel/tracing/events/ftrace/timerlat/
>   format	hist
> 
> no hist, filter, trigger files...

Ooop, the hist file exists, what is missing is the trigger file, which is
used to enable the histogram...

it seems that the "limitation" is around TRACE_EVENT_FL_IGNORE_ENABLE flag, and
the fact that ftrace events cannot be enabled/disabled... 

but wait, as the hist file exist, is there any other way to enable
the histogram? (or should we disable hist files for !TRACE_EVENT_FL_IGNORE_ENABLE)?

am I missing something?

> How hard would it be to enable histograms to the ftrace/osnoise and
> ftrace/timerlat events?
> 
> This will allow me to reduce the overhead of rtla timerlat and rtla osnoise
> tools...
> 
> Thanks in advance, Stephen!
> -- Denny

Re: Enable histograms for events/ftrace/timerlat and osnoise

[Steven Rostedt]

On Mon, 27 Mar 2023 14:37:11 +0200
Daniel Bristot de Oliveira <bristot@redhat.com> wrote:

> On 3/24/23 17:13, Daniel Bristot de Oliveira wrote:
> > Hi Steven
> > 
> > I would like to collect histograms from timerlat and osnoise tracers event,
> > but I cannot collect them because they are not tracepoints but... tracer
> > events. So...
> > 
> >   # ls /sys/kernel/tracing/events/ftrace/timerlat/
> >   format	hist
> > 
> > no hist, filter, trigger files...  
> 
> Ooop, the hist file exists, what is missing is the trigger file, which is
> used to enable the histogram...
> 
> it seems that the "limitation" is around TRACE_EVENT_FL_IGNORE_ENABLE flag, and
> the fact that ftrace events cannot be enabled/disabled... 
> 
> but wait, as the hist file exist, is there any other way to enable
> the histogram? (or should we disable hist files for !TRACE_EVENT_FL_IGNORE_ENABLE)?

No, I think we just missed not creating the "hist" file, which should not
actually exist.

> 
> am I missing something?
> 
> > How hard would it be to enable histograms to the ftrace/osnoise and
> > ftrace/timerlat events?

I'll try to get some time to look at this. No guarantees :-p

-- Steve

> > 
> > This will allow me to reduce the overhead of rtla timerlat and rtla osnoise
> > tools...
> > 
> > Thanks in advance, Stephen!
> > -- Denny