Date: Thu, 15 Nov 2007 20:00:22 -0500 (EST)
From: Steven Rostedt <rostedt@goodmis.org>
To: Tim Bird <tim.bird@am.sony.com>
cc: john cooper <john.cooper@third-harmonic.com>,
       Thomas Gleixner <tglx@linutronix.de>,
       linux kernel <linux-kernel@vger.kernel.org>,
       Ingo Molnar <mingo@redhat.com>
Subject: Re: MIPS RT debug support
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 <473BADD4.5030400@third-harmonic.com> <473CD6E1.1060607@am.sony.com>
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On Thu, 15 Nov 2007, Tim Bird wrote:

> john cooper wrote:
> > The more daunting problem stems from limitations in the MIPS
> > ABI which makes the latency trace support problematic.
> > Rather than rehash the issue:
> >
> >     http://lists.linuxcoding.com/kernel/2005-q4/msg10163.html
> >
> > Until we have a usable instrumentation solution in place,
> > characterization, debug, and support of PREEMPT_RT for MIPS
> > is going to be a challenge.
>
> Agreed.  I have been using KFT (Kernel Function Trace)
> on MIPS, and it has decent support for function traceback
> reporting, but it's not currently integrated with latency-trace
> at all.   We should discuss if this could possibly be
> used to debug RT-preempt.  It is much heavier weight than
> the mcount stuff, but uses similar (but not identical)
> gcc profiling instrumentation.  I'm not sure if the
> two can be turned on together, or how hard it would
> be to move latency-trace onto -finstrument_functions.

I'm not familiar with the KFT but I'm sure it would be easy to port
latency_trace to it. Really, all the mcount does is make a wrapper to pass
to the trace calls.

Here's the code for mcount in arch/x86/kernel/entry_64.S:

ENTRY(mcount)
	cmpl $0, mcount_enabled
	jz out

	push %rbp
	mov %rsp,%rbp

	push %r11
	push %r10
	push %r9
	push %r8
	push %rdi
	push %rsi
	push %rdx
	push %rcx
	push %rax

	mov 0x0(%rbp),%rax
	mov 0x8(%rbp),%rdi
	mov 0x8(%rax),%rsi

	call   __trace

	pop %rax
	pop %rcx
	pop %rdx
	pop %rsi
	pop %rdi
	pop %r8
	pop %r9
	pop %r10
	pop %r11

	pop %rbp
out:
	ret


Which simply passes to __trace the rip that jumped here, and (if possible)
the rip of that caller. The parent rip is not necessary.

 >
> But it's probably worth researching a little.  We'll
> need something to give insight into the problem paths.

If the KFT could do the above, it should be trivial to adapt.

Hmm, if someone is willing to send me a free mips box, I may do it myself
;-)

-- Steve

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