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

[Russell King - ARM Linux <linux@arm.linux.org.uk>]

On Wed, Mar 25, 2009 at 11:45:05AM +0100, Uwe Kleine-König wrote:
> 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.

Correct.  But my point is that there's no way for ftrace_stub to ever call
mcount.  So the check there is pointless.

> > 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.

Ok - it would be nice if there was a comment to explain that.

Is someone going to fix the existing ftrace before trying to build stuff
on top of it?