linux-kernel - Re: [PATCH] ftrace: introduce tracing_reset_online

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Date:	Fri, 19 Dec 2008 23:05:56 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Frédéric Weisbecker <fweisbec@...il.com>,
	Pekka J Enberg <penberg@...helsinki.fi>,
	linux-kernel@...r.kernel.org, Pekka Paalanen <pq@....fi>,
	Markus Metzger <markus.t.metzger@...il.com>
Subject: Re: [PATCH] ftrace: introduce tracing_reset_online_cpus() helper


* Steven Rostedt <rostedt@...dmis.org> wrote:

> 
> On Fri, 19 Dec 2008, Fr?d?ric Weisbecker wrote:
> > 
> > 
> > That's looks good.
> > By the past, I also suggested Steven to automatically reset the traces
> > buffer each time a tracer is started, that
> > would factor out the code a bit more. I don't think one tracer would
> > avoid to reset the buffer once it is started, and
> > I don't think it is needed to reset twice on tracer switching: on stop
> > of the old tracer and on start on the new. Only
> > on start should be enough.
> 
> I'm actually against the idea of reseting a trace everytime we enable it.
> That is:
> 
> echo 1 > /debug/tracing/tracing_enabled
> 
> This should not reset the tracer. I actually do tracing where I disable 
> and enable it around areas I am interested in. I want all tracing, not 
> just the last one.
> 
> Now we have recently added /debug/tracing/tracing_on which can quickly 
> stop tracing. I may be able to use that, and we can let the 
> tracing_enable" reset it too.
> 
> I'll have to take a look at my scripts to see if that would work.

yep, using /debug/tracing/tracing_enabled is the right model to quickly 
toggle tracing without losing the buffer.

The 'switch tracer' op is really a slow thing anyway. It can involve 
unpatching/repatching functions, etc. etc. - and while it works, it 
shouldnt really be encouraged as a dynamic tool coding pattern.

	Ingo
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