linux-kernel - Re: [PATCH] ftrace: add simple oneshot function tracer

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Message-ID: <20190617201627.647547c7@gandalf.local.home>
Date:   Mon, 17 Jun 2019 20:16:27 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Thomas Preisner <linux@...eisner.de>
Cc:     asdf@...f.de, Ingo Molnar <mingo@...hat.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ftrace: add simple oneshot function tracer

On Wed, 12 Jun 2019 23:29:35 +0200
Thomas Preisner <linux@...eisner.de> wrote:

Hi Thomas,

BTW, what email client do you use, because your replies seem to confuse
my email client (claws-mail) and it doesn't thread them at all.
Although they do look fine on mutt (when I view my LKML folder). Looks
like it doesn't create a "References:" header.

> On Tue, 11 Jun 2019 17:52:37 -0400
> Steven Rostedt <rostedt@...dmis.org> wrote:
> 
> > What do you mean? The function profile has its own file to enable it:
> > 
> >  echo 1 > /sys/kernel/tracing/function_profile_enabled
> >  
> >  And disable it:
> >  
> >   echo 0 > /sys/kernel/tracing/function_profile_enabled
> >   
> >   -- Steve  
> 
> Yes, I am aware of the function profiler providing a file operation for
> enabling and disabling itself. However, my oneshot profiler as of [PATCH
> v2] is a separate tracer/profiler without this file operation.
> 
> As this oneshot profiler is intended to be used for coverage/usage
> reports I want it to be able to record functions as soon as possible
> during bootup. Therefore, I just permanently activated the oneshot
> profiler since as of now there is no means to activate it or the
> function profiler via kernel commandline just like the normal tracers.
> 
> Still, if you want to I can add the file operation for
> enabling/disabling this new profiler together with a new kernel
> commandline argument for this profiler?
> 
> Or what would be your prefered way?
> 

Hmm, I guess I still need to think about exactly what this is for.
Perhaps we could add a "oneshot" option to the function tracer, and
when set it will only trace a function once? Is there a strong reason
to add a new event type "oneshot_entry"? It may be useful to record the
parent of the function that triggered the first instance as well.

I'm still trying to get a grip around exactly what use cases this would
be good for. Especially when adding new functionality like this.

-- Steve