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

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Message-ID: <20100105060537.GA17310@elte.hu>
Date:	Tue, 5 Jan 2010 07:05:37 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Jiri Olsa <jolsa@...hat.com>, Jason Baron <jbaron@...hat.com>,
	rostedt@...dmis.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] dynamic debug - adding ring buffer storage support


* Frederic Weisbecker <fweisbec@...il.com> wrote:

> On Mon, Dec 28, 2009 at 10:24:02AM +0100, Ingo Molnar wrote:
> > > 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
> 
> 
> That said, I sometimes dream about one event per printk.

Yeah - but it's only really useful if we could properly encode/extract the 
record format as well.

The one person's printk would become another person's programmable tracepoint.

> Too bad that would bloat the memory.

Should be optional of course, and then developers/distros pick instrumentation 
landscape winners/losers. To most people memory overhead is not a big issue, 
if the result is sufficiently useful.

	Ingo
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