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

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Date:	Fri, 11 Mar 2011 10:03:09 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Steven Rostedt <rostedt@...dmis.org>, linux-kernel@...r.kernel.org
Cc:	Ingo Molnar <mingo@...e.hu>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Yuanhan Liu <yuanhan.liu@...ux.intel.com>,
	chris <chris@...is-wilson.co.uk>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: Re: [PATCH][RFC] tracing: Enable tracepoints via module parameters

On Tue, 08 Mar 2011 17:18:04 -0500, Steven Rostedt <rostedt@...dmis.org> wrote:
> A few months ago it was suggested to have a way to enable tracepoints in
> a module when it is loaded. I tried various methods, but this one seems
> to be the least intrusive. In fact, it requires no modification to the
> module code.

This seems quite nice!

Importantly, do you like it better than the previous version?  I like it
because it doesn't touch my code, but that's not a fair test.

A few minor things:
1) Can we document this somewhere more persistent too? ftrace.txt?
2) Your documentation should probably just use "trace_foo_bar" and omit
   the "=1".  Because we can :)
3) Can we share more with param_set_bool?  Nice to have it in one place
   when someone decides the kernel really needs to accept "=true" or
   whatever.
4) =n should really unset the flag, so args fight correctly.

eg (wildly untested):

int ftrace_mod_param_set(const char *val, const struct kernel_param *kp)
{
	struct ftrace_event_call *call = kp->arg;
        struct kernel_param bool_kp;
        bool set;
        int err;

        /* We work exactly like param_set_bool. */
        bool_kp.arg = &set;
        bool_kp.flags = KPARAM_ISBOOL;
        err = param_set_bool(val, &bool_kp);
        if (err)
                return err;

       	/* Set flag to tell ftrace to enable this event on init */
        if (set)
                call->flags = TRACE_EVENT_FL_MOD_ENABLE;
        else
                call->flags &= ~TRACE_EVENT_FL_MOD_ENABLE;

	return 0;
}

Cheers,
Rusty.
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