Re: Anyone working on ftrace function graph support on ARM?

["Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>]

Hi Russell,

On Wed, Mar 25, 2009 at 09:57:51AM +0000, Russell King - ARM Linux wrote:
> On Wed, Mar 25, 2009 at 09:54:18AM +0100, Ingo Molnar wrote:
> > Unwinding is not realistic or desired for the function tracer - it 
> > runs in every kernel function so performance is paramount.
> 
> Which would also include the unwinder itself as well.
> 
> > So, if i understood you correctly, an OABI_COMPAT and FRAME_POINTERS 
> > dependency has to be added to the ARM HAVE_FUNCTION_GRAPH_TRACER 
> > Kconfig rule.
> 
> If we have frame pointers enabled with EABI, then it looks like it will
> work as well.  So the dependency should be on FRAME_POINTERS for _every_
> feature using the mcount code.
> 
> Hmm, and it looks like the ftrace code is rather crap:
> 
> ENTRY(mcount)
>         stmdb sp!, {r0-r3, lr}
>         ldr r0, =ftrace_trace_function
>         ldr r2, [r0]
>         adr r0, ftrace_stub
>         cmp r0, r2
>         bne trace
>         ldr lr, [fp, #-4]                       @ restore lr
>         ldmia sp!, {r0-r3, pc}
> 
> trace:
>         ldr r1, [fp, #-4]                       @ lr of instrumented routine
>         mov r0, lr
>         sub r0, r0, #MCOUNT_INSN_SIZE
>         mov lr, pc
>         mov pc, r2
>  XXX calling a C function results in r0-r3,ip,lr being clobbered XXX
> 
>         mov lr, r1                              @ restore lr
>  XXX not necessarily, r1 might be some other random value
> 
>         ldmia sp!, {r0-r3, pc}
> 
> In fact, to me the above code looks totally crap, because it's checking
> whether the caller is 'ftrace_stub'.  It can never be 'ftrace_stub'
> because that is an assembly function:
> 
>         .globl ftrace_stub
> ftrace_stub:
>         mov pc, lr
> 
> and therefore gcc has no hand in adding a mcount call to it.
Hhhm.  Isn't the equivalent C-Code ~ as follows:

	if (ftrace_trace_function != ftrace_stub)
		trace(some, args);
	return;
?  ftrace_trace_function is initialised to ftrace_stub at compile time
and is changed when a tracerfunction is registered.

> Moreover, the _dynamic_ ftrace code does this:
> 
> ENTRY(mcount)
>         stmdb sp!, {r0-r3, lr}
>         mov r0, lr
>         sub r0, r0, #MCOUNT_INSN_SIZE
> 
>         .globl mcount_call
> mcount_call:
>         bl ftrace_stub
>         ldr lr, [fp, #-4]                       @ restore lr
>         ldmia sp!, {r0-r3, pc}
> 
> ENTRY(ftrace_caller)
>         stmdb sp!, {r0-r3, lr}
>         ldr r1, [fp, #-4]
>         mov r0, lr
>         sub r0, r0, #MCOUNT_INSN_SIZE
> 
>         .globl ftrace_call
> ftrace_call:
>         bl ftrace_stub
>         ldr lr, [fp, #-4]                       @ restore lr
>         ldmia sp!, {r0-r3, pc}
> 
> In other words, it pushes some words onto the stack, sets r0, calls
> an assembly function which does nothing but just returns, reloads lr,
> restores the stack and returns.  This ftrace implementation looks like
> an exercise in slowing down execution to me with no added value.
The idea is that the instruction at address mcount_call (and
ftrace_call IIRC) is rewritten at run time.
Still the code is not active currently (because CONFIG_DYNAMIC_FTRACE
isn't selectable for ARM) and needs some care anyhow on reactivation
because the way how dynamic ftrace works changed somehow.  Didn't look
at it up to now though.

Best regards
Uwe

-- 
Pengutronix e.K.                              | Uwe Kleine-König            |
Industrial Linux Solutions                    | http://www.pengutronix.de/  |