Re: [PATCH v3 1/6] arm64: Add ftrace support

[AKASHI Takahiro]

On 02/18/2014 03:12 AM, Will Deacon wrote:
> On Fri, Feb 07, 2014 at 10:18:51AM +0000, AKASHI Takahiro wrote:
> > This patch implements arm64 specific part to support function tracers,
> > such as function (CONFIG_FUNCTION_TRACER), function_graph
> > (CONFIG_FUNCTION_GRAPH_TRACER) and function profiler
> > (CONFIG_FUNCTION_PROFILER).
> >
> > With 'function' tracer, all the functions in the kernel are traced with
> > timestamps in ${sysfs}/tracing/trace. If function_graph tracer is
> > specified, call graph is generated.
> >
> > The kernel must be compiled with -pg option so that _mcount() is inserted
> > at the beginning of functions. This function is called on every function's
> > entry as long as tracing is enabled.
> > In addition, function_graph tracer also needs to be able to probe function's
> > exit. ftrace_graph_caller() & return_to_handler do this by faking link
> > register's value to intercept function's return path.
> >
> > More details on architecture specific requirements are described in
> > Documentation/trace/ftrace-design.txt.
>
> [...]
>
> > diff --git a/arch/arm64/include/asm/ftrace.h b/arch/arm64/include/asm/ftrace.h
> > new file mode 100644
> > index 0000000..0d5dfdb
> > --- /dev/null
> > +++ b/arch/arm64/include/asm/ftrace.h
> > @@ -0,0 +1,23 @@
> > +/*
> > + * arch/arm64/include/asm/ftrace.h
> > + *
> > + * Copyright (C) 2013 Linaro Limited
> > + * Author: AKASHI Takahiro <takahiro.akashi@xxxxxxxxxx>
> > + *
> > + * This program is free software; you can redistribute it and/or modify
> > + * it under the terms of the GNU General Public License version 2 as
> > + * published by the Free Software Foundation.
> > + */
> > +#ifndef __ASM_FTRACE_H
> > +#define __ASM_FTRACE_H
> > +
> > +#ifdef CONFIG_FUNCTION_TRACER
> > +#define MCOUNT_ADDR            ((unsigned long)_mcount)
> > +#define MCOUNT_INSN_SIZE       4 /* sizeof mcount call */
>
> You can use AARCH64_INSN_SIZE here.

OK, but MCOUNT_INSN_SIZE can't be removed here because it is also used in
kernel/trace/ftrace.c. So I will redefine it as
#define MCOUNT_INSN_SIZE AARCH64_INSN_SIZE

In this case, I need to add __ASSEMBLY__ in asm/insn.h because
asm/ftrace.h (and so asm/insn.h) is included in entry-ftrace.S.

Is it OK?


> > +#include <linux/linkage.h>
> > +#include <asm/ftrace.h>
> > +
> > +/*
> > + * Gcc with -pg will put the following code in the beginning of each function:
> > + *      mov x0, x30
> > + *      bl _mcount
> > + *     [function's body ...]
> > + * "bl _mcount" may be replaced to "bl ftrace_caller" or NOP if dynamic
> > + * ftrace is enabled.
> > + *
> > + * Please note that x0 as an argument will not be used here because we can
> > + * get lr(x30) of insturmented function at any time by winding up call stack
>
> instrumented

I  will fix two occurences of "insturmented" :-)
Just a question: Is there any better word than "instrument"?

> > + * as long as the kernel is compiled without -fomit-frame-pointer.
> > + * (or CONFIG_FRAME_POINTER, this is forced on arm64)
> > + *
> > + * stack layout after mcount_enter:
> > + *
> > + *   0 ---------------------------------------- sp of _mcount()
> > + *      x29: fp of instrumented function       fp is not winded
> > + *     --------------------
> > + *      x30: lr of _mcount() (= instrumented function's pc)
> > + * +16 ---------------------------------------- sp of instrumented function
> > + *
> > + *     ....
> > + *
> > + * +xx ---------------------------------------- fp of instrumented function
> > + *      x29: fp of parent
> > + *     --------------------
> > + *      x30: lr of insturmented function (= parent's pc)
> > + *     --------------------
> > + *      xxx
> > + */
> > +
> > +       .macro mcount_enter
> > +       stp     x29, x30, [sp, #-48]!
> > +       .endm
>
> Can you elaborate in your comment about where this 48 comes from please? I
> can't join it up with your ascii art.

Right. It should be -16 as shown in the stack layout.
When I removed an operation of saving/restoring x0-x3 at v3 patch, this value
should also have been changed.

-Takahiro AKASHI

> Will
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