linux-kernel - Re: [PATCH][RFC] tracing: Enable tracepoints via module parameters

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Date:   Tue, 20 Apr 2021 10:55:11 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        fweisbec <fweisbec@...il.com>, Jessica Yu <jeyu@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...e.hu>, chris@...is-wilson.co.uk,
        yuanhan liu <yuanhan.liu@...ux.intel.com>,
        "Grumbach, Emmanuel" <emmanuel.grumbach@...el.com>
Subject: Re: [PATCH][RFC] tracing: Enable tracepoints via module parameters

On Tue, 20 Apr 2021 09:29:27 -0400 (EDT)
Mathieu Desnoyers <mathieu.desnoyers@...icios.com> wrote:

> ----- On Apr 20, 2021, at 8:55 AM, rostedt rostedt@...dmis.org wrote:
> [...]
> > 
> > Would adding automatic module parameters be an issue? That is, you can add
> > in the insmod command line a parameter that will enable tracepoints. We
> > could have a way to even see them from the modinfo. I think I had that
> > working once, and it wasn't really that hard to do.  
> 
> There is one thing we should consider here in terms of namespacing: those module
> command line parameters should be specific to each tracer (e.g. ftrace, perf, ebpf).
> 
> LTTng for instance already tackles early module load tracing in a different
> way: users can enable instrumentation of yet-to-be loaded kernel modules. So
> it would not make sense in that scheme to have module load parameters.
> 
> It's a different trade-off in terms of error reporting though: for instance,
> LTTng won't report an error if a user does a typo when entering an event name.
> 
> So I think those command line parameters should be tracer-specific, do you agree ?


No, I do not agree. I would like to make it consistent with the kernel
command line. As you can put in: "trace_event=sched_switch" and the
sched_switch trace point will be enable (for the tracefs directory) on boot
up. The same should be for modules as well.

It shouldn't affect LTTng, as you already have a way to enable them as they
get loaded.

-- Steve