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[PATCH v8 16/16] tracing: make function graph tracer work with -mmcount-

To: rostedt@goodmis.org, Ralf Baechle <ralf@linux-mips.org>
Subject: [PATCH v8 16/16] tracing: make function graph tracer work with -mmcount-ra-address
From: Wu Zhangjin <wuzhangjin@gmail.com>
Date: Sat, 14 Nov 2009 14:33:31 +0800
Cc: Wu Zhangjin <wuzhangjin@gmail.com>, Frederic Weisbecker <fweisbec@gmail.com>, Ingo Molnar <mingo@elte.hu>, Nicholas Mc Guire <der.herr@hofr.at>, David Daney <ddaney@caviumnetworks.com>, Richard Sandiford <rdsandiford@googlemail.com>, Patrik Kluba <kpajko79@gmail.com>, Thomas Gleixner <tglx@linutronix.de>, Michal Simek <monstr@monstr.eu>, "Maciej W . Rozycki" <macro@linux-mips.org>, linux-mips@linux-mips.org, linux-kernel@vger.kernel.org, zhangfx@lemote.com, zhouqg@gmail.com
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Sender: linux-mips-bounce@linux-mips.org
From: Wu Zhangjin <wuzhangjin@gmail.com>

That thread "MIPS: Add option to pass return address location to
_mcount" from "David Daney <ddaney@caviumnetworks.com>" have added a new
option -mmcount-ra-address to gcc(4.5) for MIPS to transfer the location
of the return address to _mcount.

Benefit from this new feature, function graph tracer on MIPS will be
easier and safer to hijack the return address of the kernel function,
which will save some overhead and make the whole thing more reliable.

In this patch, at first, try to enable the option -mmcount-ra-address in
arch/mips/Makefile with cc-option, if gcc support it, it will be
enabled, otherwise, no side effect.

and then, we need to support this new option of gcc 4.5 and also support
the old gcc versions.

with _mcount in the old gcc versions, it's not easy to get the location
of return address(tracing: add function graph tracer support for MIPS),
   so, we do it in a C function: ftrace_get_parent_addr(ftrace.c), but
   with -mmcount-ra-address, only several instructions need to get what
   we want, so, I put into asm(mcount.S). and also, as the $12(t0) is
   used by -mmcount-ra-address for transferring the localtion of return
   address to _mcount, we need to save it into the stack and restore it
   when enabled dynamic function tracer, 'Cause we have called
   "ftrace_call" before "ftrace_graph_caller", which may destroy
   $12(t0).

(Thanks to David for providing that -mcount-ra-address and giving the
 idea of KBUILD_MCOUNT_RA_ADDRESS, both of them have made the whole
 thing more beautiful!)

Signed-off-by: Wu Zhangjin <wuzhangjin@gmail.com>
---
 arch/mips/Makefile        |    7 +++++++
 arch/mips/kernel/ftrace.c |   24 +++++++++++++++++-------
 arch/mips/kernel/mcount.S |   14 ++++++++++++++
 3 files changed, 38 insertions(+), 7 deletions(-)

diff --git a/arch/mips/Makefile b/arch/mips/Makefile
index 0a2073e..50039e8 100644
--- a/arch/mips/Makefile
+++ b/arch/mips/Makefile
@@ -51,6 +51,13 @@ endif
 ifndef CONFIG_FUNCTION_TRACER
 cflags-y := -ffunction-sections
 endif
+ifdef CONFIG_FUNCTION_GRAPH_TRACER
+  ifndef KBUILD_MCOUNT_RA_ADDRESS
+    ifeq ($(call cc-option-yn,-mmcount-ra-address), y)
+      cflags-y += -mmcount-ra-address -DKBUILD_MCOUNT_RA_ADDRESS
+    endif
+  endif
+endif
 cflags-y += $(call cc-option, -mno-check-zero-division)
 
 ifdef CONFIG_32BIT
diff --git a/arch/mips/kernel/ftrace.c b/arch/mips/kernel/ftrace.c
index e363fc6..68b0670 100644
--- a/arch/mips/kernel/ftrace.c
+++ b/arch/mips/kernel/ftrace.c
@@ -148,6 +148,7 @@ int ftrace_disable_ftrace_graph_caller(void)
 
 #endif                         /* !CONFIG_DYNAMIC_FTRACE */
 
+#ifndef KBUILD_MCOUNT_RA_ADDRESS
 #define S_RA_SP        (0xafbf << 16)  /* s{d,w} ra, offset(sp) */
 #define S_R_SP (0xafb0 << 16)  /* s{d,w} R, offset(sp) */
 #define OFFSET_MASK    0xffff  /* stack offset range: 0 ~ PT_SIZE */
@@ -201,6 +202,8 @@ unsigned long ftrace_get_parent_addr(unsigned long 
self_addr,
        return 0;
 }
 
+#endif
+
 /*
  * Hook the return address and push it in the stack of return addrs
  * in current thread info.
@@ -218,19 +221,26 @@ void prepare_ftrace_return(unsigned long *parent, 
unsigned long self_addr,
                return;
 
        /* "parent" is the stack address saved the return address of the caller
-        * of _mcount, for a leaf function not save the return address in the
-        * stack address, so, we "emulate" one in _mcount's stack space, and
-        * hijack it directly, but for a non-leaf function, it will save the
-        * return address to the its stack space, so, we can not hijack the
-        * "parent" directly, but need to find the real stack address,
+        * of _mcount.
+        *
+        * if the gcc < 4.5, a leaf function does not save the return address
+        * in the stack address, so, we "emulate" one in _mcount's stack space,
+        * and hijack it directly, but for a non-leaf function, it save the
+        * return address to the its own stack space, we can not hijack it
+        * directly, but need to find the real stack address,
         * ftrace_get_parent_addr() does it!
+        *
+        * if gcc>= 4.5, with the new -mmcount-ra-address option, for a
+        * non-leaf function, the location of the return address will be saved
+        * to $12 for us, and for a leaf function, only put a zero into $12. we
+        * do it in ftrace_graph_caller of mcount.S.
         */
 
        /* old = *parent; */
        safe_load_stack(old, parent, faulted);
        if (unlikely(faulted))
                goto out;
-
+#ifndef KBUILD_MCOUNT_RA_ADDRESS
        parent = (unsigned long *)ftrace_get_parent_addr(self_addr, old,
                                                         (unsigned long)parent,
                                                         fp);
@@ -238,7 +248,7 @@ void prepare_ftrace_return(unsigned long *parent, unsigned 
long self_addr,
         * ra, stop function graph tracer and return */
        if (parent == 0)
                goto out;
-
+#endif
        /* *parent = return_hooker; */
        safe_store_stack(return_hooker, parent, faulted);
        if (unlikely(faulted))
diff --git a/arch/mips/kernel/mcount.S b/arch/mips/kernel/mcount.S
index 522e91c..0a9cfdb 100644
--- a/arch/mips/kernel/mcount.S
+++ b/arch/mips/kernel/mcount.S
@@ -70,6 +70,9 @@ _mcount:
         nop
 
        MCOUNT_SAVE_REGS
+#ifdef KBUILD_MCOUNT_RA_ADDRESS
+       PTR_S   t0, PT_R12(sp)  /* t0 saved the location of the return 
address(at) by -mmcount-ra-address */
+#endif
 
        move    a0, ra          /* arg1: next ip, selfaddr */
        .globl ftrace_call
@@ -133,11 +136,22 @@ ftrace_stub:
 NESTED(ftrace_graph_caller, PT_SIZE, ra)
 #ifdef CONFIG_DYNAMIC_FTRACE
        PTR_L   a1, PT_R31(sp)  /* load the original ra from the stack */
+#ifdef KBUILD_MCOUNT_RA_ADDRESS
+       PTR_L   t0, PT_R12(sp)  /* load the original t0 from the stack */
+#endif
 #else
        MCOUNT_SAVE_REGS
        move    a1, ra          /* arg2: next ip, selfaddr */
 #endif
+
+#ifdef KBUILD_MCOUNT_RA_ADDRESS
+       bnez    t0, 1f          /* non-leaf func: t0 saved the location of the 
return address */
+        nop
+       PTR_LA  t0, PT_R1(sp)   /* leaf func: get the location of at(old ra) 
from our own stack */
+1:     move    a0, t0          /* arg1: the location of the return address */
+#else
        PTR_LA  a0, PT_R1(sp)   /* arg1: &AT -> a0 */
+#endif
        jal     prepare_ftrace_return
 #ifdef CONFIG_FRAME_POINTER
         move   a2, fp          /* arg3: frame pointer */
-- 
1.6.2.1


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