linux-kernel - Re: [PATCH 1/2] tracing,mm - add kernel pagefault tracepoint for x86 & x86

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Date:	Wed, 10 Nov 2010 14:52:44 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Jiri Olsa <jolsa@...hat.com>
Cc:	Christoph Hellwig <hch@...radead.org>, rostedt@...dmis.org,
	andi@...stfloor.org, lwoodman@...hat.com,
	linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Frédéric Weisbecker <fweisbec@...il.com>
Subject: Re: [PATCH 1/2] tracing,mm - add kernel pagefault tracepoint for x86
 & x86_64


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

> On Wed, Nov 10, 2010 at 08:29:54AM -0500, Christoph Hellwig wrote:
> > On Wed, Nov 10, 2010 at 12:56:11PM +0100, Jiri Olsa wrote:
> > > +	TP_printk("task=%lx, address=%lx, regs=%lx",
> > > +		(unsigned long)__entry->task, (unsigned long)__entry->address,
> > > +			__entry->regs)
> > 
> > How exactly do you use the information in this trace point?  Especially
> > the undecoded pt_regs doesn't seem very useful to me at all.
> 
> agreed, the registers pointer are not very useful in the trace file output,
> and could be taken away.. just wanted to be complete I guess
> 
> but I believe they are useful when you register the mm_kernel_pagefault
> tracepoint and process the information by yourself

That would be expressed in a better and more generic fashion via adding 
PERF_SAMPLE_REGS to perf_event_sample_format, and add a ptregs dump in 
kernel/perf_event.c, perf_output_sample(). That way any tracepoint can request a 
(user-space)ptregs state snapshot, not just the pagefault ones.

Thanks,

	Ingo
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