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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