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

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Message-ID: <53048B9D.2050606@linaro.org>
Date:	Wed, 19 Feb 2014 19:46:53 +0900
From:	AKASHI Takahiro <takahiro.akashi@...aro.org>
To:	Will Deacon <will.deacon@....com>
CC:	"rostedt@...dmis.org" <rostedt@...dmis.org>,
	"fweisbec@...il.com" <fweisbec@...il.com>,
	"mingo@...hat.com" <mingo@...hat.com>,
	Catalin Marinas <Catalin.Marinas@....com>,
	"gkulkarni@...iumnetworks.com" <gkulkarni@...iumnetworks.com>,
	"tim.bird@...ymobile.com" <tim.bird@...ymobile.com>,
	"arndb@...db.de" <arndb@...db.de>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"patches@...aro.org" <patches@...aro.org>
Subject: Re: [PATCH v3 1/6] arm64: Add ftrace support

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@...aro.org>
>> + *
>> + * 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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