linux-kernel - Re: tracing and mmiotrace

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Date:	Sat, 22 Mar 2008 16:16:48 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Pekka Paalanen <pq@....fi>
cc:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org
Subject: Re: tracing and mmiotrace


Hi Pekka,

I'm on vacation at the moment, but will start up helping Ingo with ftrace
on Monday.

On Sat, 22 Mar 2008, Pekka Paalanen wrote:

> >
> > that's weird - _entry_ is NULL?
>
> Yes. See the patch, and my kernel log at
> http://jumi.lut.fi/~paalanen/scratch/tracing-dmesg1.txt
>
> First you see my check in __trace_special() trigger, and then NULL
> pointer dereference BUG at ffffffff80259332 which translates to
> kernel/trace/trace.c:639 in tracing_generic_entry_update(), line:
> 	entry->preempt_count	= pc & 0xff;
> I don't understand why it tries to dereference NULL even when I
> check for it.

Interesting. I'll have to look into that more on Monday.

>
> ---
> Oh, and during bootup there is an error about -fstack-protector-all
> test failed.
>
> > > btw. currently 'cat /debug/tracing/trace' gives the
> > > print_func_help_header() and quits, it never stays waiting for more
> > > data.
> >
> > yeah - try "cat /debug/tracing/trace_pipe" for the waiting variant -
> > does that work better?
>
> Not really. IIRC it said "resource temporarily unavailable" until
> mmiotrace actually tried to produce data. No header there. So I
> cannot make "cat" ready to read and after that start tracing.

I'll have to clean up the trace_pipe code to be a bit more robust.

Thanks for doing this. I'll be getting more involved starting Monday.

-- Steve

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