linux-kernel - Re: [PATCH 3/4] tracing: Allow mmio tracer to display trace

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Message-ID: <20100515104602.59c1f0e1@daedalus.pq.iki.fi>
Date:	Sat, 15 May 2010 10:46:02 +0300
From:	Pekka Paalanen <pq@....fi>
To:	rostedt@...dmis.org
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Larry Finger <Larry.Finger@...inger.net>
Subject: Re: [PATCH 3/4] tracing: Allow mmio tracer to display
 trace_printk() and other events

Sorry, I'm not at my email every day. Real life...

On Thu, 13 May 2010 11:11:23 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:

> On Thu, 2010-05-13 at 15:29 +0300, Pekka Paalanen wrote:
> > On Thu, 13 May 2010 08:15:09 -0400
> 
> > > It only displays other events if the user enabled those
> > > events.
> > > 
> > > But that said, I don't want to break existing userspace
> > > tools. I can add a mmiotrace option "mmiotrace_all_events",
> > > if the user wants to see all events within the mmiotracer
> > > then they can just enable that option, otherwise, the
> > > mmiotracer will act like it currently does.
> > > 
> > > How does that sound?
> > 
> > That would be fine. Is it not redundant with what you said in
> > your first sentence?
> > 
> 
> Right now with this patch as is. When you enable the mmiotracer it
> clears the ring buffer. But if someone previously enabled an
> event (like sched_switch for example) then that event will appear
> in the output of the tracer.
> 
> The user will need to disable that event and restart the
> mmiotracer so the output will not break the userspace tools. Is
> this OK?

I think it is ok. No non-mmiotrace events are enabled automatically,
right? Except perhaps trace_printk()?

If a user enables other events while mmiotracing, I would
assume he knows what he is doing. End users doing dumps per
request never even know about other kinds of tracing than
mmiotrace.

> If not, then my suggestion is to have an mmiotracer option that
> keeps it from printing out any event except for the ones it knows
> about.
> 
> The reason I added this patch in the first place was because Larry
> Finger was using the mmiotrace with trace_printk() and the
> current code does not print out the trace_printk() when
> mmiotracer is active.

If this is *only* about trace_printk(), why not make a handler
for it to emit MARK lines? Actually, I somehow assumed that
would have been the case, but apparently the event type is
different. I do not recall these things too well anymore.


Thanks for keeping me in the loop.

-- 
Pekka Paalanen
http://www.iki.fi/pq/
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