linux-kernel - Re: Enable histograms for events/ftrace/timerlat and osnoise

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Message-ID: <9a1bb631-587c-9771-99b7-55623c67bb83@redhat.com>
Date:   Mon, 27 Mar 2023 14:37:11 +0200
From:   Daniel Bristot de Oliveira <bristot@...hat.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Masami Hiramatsu <mhiramat@...nel.org>,
        Tom Zanussi <zanussi@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: Enable histograms for events/ftrace/timerlat and osnoise

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