linux-kernel - Re: [RFC PATCH] dynamic debug - adding ring buffer storage support

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Date:	Mon, 28 Dec 2009 10:24:02 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Jiri Olsa <jolsa@...hat.com>
Cc:	Jason Baron <jbaron@...hat.com>, rostedt@...dmis.org,
	fweisbec@...il.com, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] dynamic debug - adding ring buffer storage support


* Jiri Olsa <jolsa@...hat.com> wrote:

> On Tue, Dec 22, 2009 at 10:39:56AM -0500, Jason Baron wrote:
> > On Tue, Dec 22, 2009 at 12:32:06PM +0100, Jiri Olsa wrote:
> > > Hi,
> > > 
> > > as I use dynamic debug sometimes, I thought it could be useful having
> > > the possibility to store the output somewhere else than dmesg.
> > > 
> > > The attached patch implements support for storing dynamic debug
> > > messages to the ring buffer.
> > > 
> > > The dynamic debug allows simple addition of new flags,
> > > so I added 'r' flag for ring buffer storage.
> > > 
> > > I used the ring buffer implementation from trace framework.
> > > 
> > > hopefuly this could be any use for others as well...
> > > plz let me know what you think,
> > > 
> > > jirka
> > > 
> > 
> > hi,
> > 
> > I like the idea of using the ring buffer for output. However, I'm not
> > sure why we need a separate one for this. I like adding the 'r' for ring
> > buffer storage, by why not just make this path call 'trace_printk()', and
> > store the string in the existing kernel tracing ring buffer? That way it
> > can interleave with other trace data as well.
> 
> that way you need to enable tracing as well... but thats ok I guess :)
> 
> I was investigating trace events for this, but did not find a way
> to put variable length argument inside... and I overlooked the
> trace_printk, I'll look on it and see how it fits, thanks
> 
> also having separate ring buffer makes the 'trace'/'trace_pipe' code
> really simple (suprissingly) compared to ftrace, and I thought
> on this place it could last for some time.. ;)

I think what we want is a unified channel of events, of which printk (and 
dynamic-printk) is one form. I.e. we should add printk events and 
dynamic-printk events as well, which would show up in /debug/tracing/events/ 
in a standard ftrace event form and would be accessible to tooling that way.

For printk a single event would be enough i suspect (we dont want a separate 
event for every printk), and for dynamic-printk we want to map the existing 
dyn-printk topologies into /debug/tracing/events, to preserve the distinctions 
and controls available there.

This way in the long run we'd have one unified facility.

	Ingo
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