linux-kernel - Re: [PATCH] Add function graph tracer support for ARM

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Date:	Fri, 24 Apr 2009 08:44:00 +0200
From:	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>
To:	Tim Bird <tim.bird@...sony.com>
Cc:	linux kernel <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...e.hu>,
	linux-arm-kernel <linux-arm-kernel@...ts.arm.linux.org.uk>,
	Russell King <rmk@....linux.org.uk>,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [PATCH] Add function graph tracer support for ARM

Hello,

On Thu, Apr 23, 2009 at 02:49:56PM -0700, Tim Bird wrote:
> >> @@ -23,6 +23,7 @@ obj-$(CONFIG_ISA_DMA)		+= dma-isa.o
> >>  obj-$(CONFIG_PCI)		+= bios32.o isa.o
> >>  obj-$(CONFIG_SMP)		+= smp.o
> >>  obj-$(CONFIG_DYNAMIC_FTRACE)	+= ftrace.o
> >> +obj-$(CONFIG_FUNCTION_GRAPH_TRACER)	+= ftrace.o
> > I there a reason that CONFIG_DYNAMIC_FTRACE and
> > CONFIG_FUNCTION_GRAPH_TRACER share the same source file?
> ftrace.c has stuff from both, so you want it "turned on"
> if one or the other (or both) are configed on.
I got this ...
> 
> I'll look at whether it makes sense to split these two into
> separate files.  This issue may be resolved by the code
> movement that Frederic Weisbecker mentioned in his e-mail.
... and this was my intention.
 
> >> --- a/arch/arm/kernel/entry-common.S
> >> +++ b/arch/arm/kernel/entry-common.S
> >> @@ -135,8 +135,16 @@ ENTRY(mcount)
> >>  	adr r0, ftrace_stub
> >>  	cmp r0, r2
> >>  	bne trace
> >> +
> >> +#ifdef CONFIG_FUNCTION_GRAPH_TRACER
> >> +	ldr r1, =ftrace_graph_return
> >> +	ldr r2, [r1]
> >> +	cmp r0, r2		@ if *ftrace_graph_return != ftrace_stub
> >> +	bne ftrace_graph_caller
> >> +#endif /* CONFIG_FUNCTION_GRAPH_TRACER */
> >> +
> >>  	ldr lr, [fp, #-4]			@ restore lr
> >> -	ldmia sp!, {r0-r3, pc}
> >> +	ldmia sp!, {r0-r3, pc}			@ return doing nothing
> > If ftrace_trace_function != ftrace_stub then ftrace_graph_caller isn't
> > called.  Is this correct?
> My comment is possibly misleading here.  The sense of the
> '!=' includes the 'n' portion of the 'bne' on the following line.
> So the logic is:
> if *ftrace_graph_return != ftrace stub, branch to ftrace_graph_caller
> or in other words
> if *ftrace_graph_return == ftrace_stub, return doing nothing
> 
> Maybe I should just remove the comment and let the code
> speak for itself?  (or maybe move the comment down one line?)
Yes, the comments are as good as the comments for ftrace_trace_function.
What I meant is that with your code using both ftrace_trace_function and
ftrace_graph_caller doesn't work, because if ftrace_trace_function !=
ftrace_stub then the check for ftrace_trace_function is simply skipped.

And I just noticed something else:  There is a 2nd implementation of
mcount in arch/arm/kernel/entry-common.S for CONFIG_DYNAMIC_FTRACE
(which is currently not selectable for ARM).  Maybe add a note to this
mcount that it needs fixing for graph tracing when it is revived?
 
Best regards
Uwe 

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