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

Russell King - ARM Linux
2009-03-25T11:22:21Z

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?