Re: [PATCH] tracing: Add a debug_trace_printk() function

[Steven Rostedt]

On Wed, 14 Jun 2023 09:57:33 +0000
David Laight <David.Laight@xxxxxxxxxx> wrote:

> From: Steven Rostedt
> > Sent: 13 June 2023 00:34
> > 
> > From: "Steven Rostedt (Google)" <rostedt@xxxxxxxxxxx>
> > 
> > While doing some tracing and kernel debugging, I found that some of my
> > trace_printk()s were being lost in the noise of the other code that was
> > being traced. Having a way to write trace_printk() not in the top level
> > trace buffer would have been useful.
> > 
> > There was also a time I needed to debug ftrace itself, where
> > trace_printk() did not hit the paths that were being debugged. But because
> > the trace that was being debugged, was going into the top level ring
> > buffer, it was causing issues for seeing what is to be traced.
> > 
> > To solve both of the above, add a debug_trace_printk() that can be used
> > just like trace_printk() except that it goes into a "debug" instance
> > buffer instead. This can be used at boot up as well.  
> ...
> > +#ifdef CONFIG_FTRACE_DEBUG_PRINT
> > +	debug_trace = trace_array_get_by_name("debug");
> > +	if (WARN_ON(!debug_trace))
> > +		return;
> > +	trace_array_init_printk(debug_trace);
> > +#endif  
> 
> I was wondering if that could be done whenever the "debug"
> trace_array is created?
> (perhaps only if trace_prink() has been used?)
> Since (AFAICT) it could be created at any time??
> 
> So you wouldn't really need an extra kernel knob?
> (Except to get the boot time trace diverted.)
> The trace could go to the global buffer if the debug one
> isn't created.

I'd rather not touch trace_printk(), that would just confuse people more.

Anyway, I'm not going to apply this. I have other ideas on how to
accomplish this. But for now, I wanted it in my patchwork to remind me to
do those other methods. In the mean time, I can just apply this patch
when I need to.

> 
> OTOH I'm missing what trace_array_init_prink() does?
> It seems to just call alloc_percpu_trace_buffer() with
> no arguments.
> 
> It looks like alloc_percpu_trace_buffer() is called if there
> are any trace_printk() formats in the main kernel.
> Hopefully they aren't just in modules??

No, they are allocated if a module uses them too. Try it out. Load a module
with trace_printk() and you'll see that banner print out.

-- Steve