[PATCH v7 07/17] tracing: add dynamic function tracer support for MIPS

Wu Zhangjin
2009-11-09T15:32:36Z

With dynamic function tracer, by default, _mcount is defined as an
"empty" function, it just return without any more action, just like the
static function tracer does. When enabling it in user-space, it will
jump to a real tracing function(ftrace_caller), and do some real job for
us.

Differ from the static function tracer, dynamic function tracer provides
two functions ftrace_make_call()/ftrace_make_nop() to enable/disable the
tracing of some indicated kernel functions(set_ftrace_filter).

In the -v4 version, the implementation of this support is basically the same as
X86 version does: _mcount is implemented as an empty function and ftrace_caller
is implemented as a real tracing function respectively.

But in this version, to support module tracing with the help of
-mlong-calls. the stuff becomes a little more complex, let's have a look
at the new calling to _mcount with -mlong-calls(result of "objdump -hdr
vmlinux").

c: 3c030000 lui v1,0x0
c: R_MIPS_HI16 _mcount
c: R_MIPS_NONE *ABS*
c: R_MIPS_NONE *ABS*
10: 64630000 daddiu v1,v1,0
10: R_MIPS_LO16 _mcount
10: R_MIPS_NONE *ABS*
10: R_MIPS_NONE *ABS*
14: 03e0082d move at,ra
18: 0060f809 jalr v1

and the old version without -mlong-calls looks like this:

108: 03e0082d move at,ra
10c: 0c000000 jal 0 <fpcsr_pending>
10c: R_MIPS_26 _mcount
10c: R_MIPS_NONE *ABS*
10c: R_MIPS_NONE *ABS*
110: 00020021 nop

In the old version, there is only one "_mcount" string for every kernel
function, so, we just need to match this one in mcount_regex of
scripts/recordmcount.pl, but in the new version, we need to choose one
of the two to match. Herein, I choose the first one with "R_MIPS_HI16
_mcount".

and In the old verion, without module tracing support, we just need to replace
"jal _mcount" by "jal ftrace_caller" to do real tracing, and filter the tracing
of some kernel functions via replacing it by a nop instruction.

but as we have described before, the instruction "jal ftrace_caller" only left
32bit length for the address of ftrace_caller, it will fail when calling from
the module space. so, herein, we must replace something else.

the basic idea is loading the address of ftrace_caller to v1 via changing these
two instructions:

lui v1,0x0
addiu v1,v1,0

If we want to enable the tracing, we need to replace the above instructions to:

lui v1, HI_16BIT_ftrace_caller
addiu v1, v1, LOW_16BIT_ftrace_caller

If we want to stop the tracing of the indicated kernel functions, we
just need to replace the "jalr v1" to a nop instruction. but we need to
replace two instructions and encode the above two instructions
oursevles.

Is there a simpler solution, Yes! Here it is, in this version, we put _mcount
and ftrace_caller together, which means the address of _mcount and
ftrace_caller is the same:

_mcount:
ftrace_caller:
j ftrace_stub
nop

...(do real tracing here)...

ftrace_stub:
jr ra
move ra, at

By default, the kernel functions call _mcount, and then jump to ftrace_stub and
return. and when we want to do real tracing, we just need to remove that "j
ftrace_stub", and it will run through the two "nop" instructions and then do
the real tracing job.

what about filtering job? we just need to do this:

lui v1, hi_16bit_of_mcount <--> b 1f (0x10000004)
addiu v1, v1, low_16bit_of_mcount
move at, ra
jalr v1
nop
1f: (rec->ip + 12)

In linux-mips64, there will be some local symbols, whose name are
prefixed by $L, which need to be filtered. thanks goes to Steven for
writing the mips64-specific function_regex.

In a conclusion, with RISC, things becomes easier with such a "stupid"
trick, RISC is something like K.I.S.S, and also, there are lots of
"simple" tricks in the whole ftrace support, thanks goes to Steven and
the other folks for providing such a wonderful tracing framework!

Signed-off-by: Wu Zhangjin <wuzj@lemote.com>
---
arch/mips/Kconfig | 2 +
arch/mips/include/asm/ftrace.h | 9 ++++
arch/mips/kernel/Makefile | 3 +-
arch/mips/kernel/ftrace.c | 89 ++++++++++++++++++++++++++++++++++++++++
arch/mips/kernel/mcount.S | 29 +++++++++++++
scripts/recordmcount.pl | 41 ++++++++++++++++++
6 files changed, 172 insertions(+), 1 deletions(-)
create mode 100644 arch/mips/kernel/ftrace.c

diff --git a/arch/mips/Kconfig b/arch/mips/Kconfig
index a9bd992..147fbbc 100644
--- a/arch/mips/Kconfig
+++ b/arch/mips/Kconfig
@@ -6,6 +6,8 @@ config MIPS
select HAVE_ARCH_KGDB
select HAVE_FUNCTION_TRACER
select HAVE_FUNCTION_TRACE_MCOUNT_TEST
+ select HAVE_DYNAMIC_FTRACE
+ select HAVE_FTRACE_MCOUNT_RECORD
# Horrible source of confusion. Die, die, die ...
select EMBEDDED
select RTC_LIB if !LEMOTE_FULOONG2E
diff --git a/arch/mips/include/asm/ftrace.h b/arch/mips/include/asm/ftrace.h
index 5f8ebcf..7094a40 100644
--- a/arch/mips/include/asm/ftrace.h
+++ b/arch/mips/include/asm/ftrace.h
@@ -19,6 +19,15 @@
extern void _mcount(void);
#define mcount _mcount

+#ifdef CONFIG_DYNAMIC_FTRACE
+static inline unsigned long ftrace_call_adjust(unsigned long addr)
+{
+ return addr;
+}
+
+struct dyn_arch_ftrace {
+};
+#endif /* CONFIG_DYNAMIC_FTRACE */
#endif /* __ASSEMBLY__ */
#endif /* CONFIG_FUNCTION_TRACER */
#endif /* _ASM_MIPS_FTRACE_H */
diff --git a/arch/mips/kernel/Makefile b/arch/mips/kernel/Makefile
index 38e704a..3cda3ab 100644
--- a/arch/mips/kernel/Makefile
+++ b/arch/mips/kernel/Makefile
@@ -10,6 +10,7 @@ obj-y += cpu-probe.o branch.o entry.o genex.o irq.o process.o \

ifdef CONFIG_FUNCTION_TRACER
CFLAGS_REMOVE_trace_clock.o = -pg
+CFLAGS_REMOVE_ftrace.o = -pg
CFLAGS_REMOVE_early_printk.o = -pg
endif

@@ -30,7 +31,7 @@ obj-$(CONFIG_STACKTRACE) += stacktrace.o
obj-$(CONFIG_MODULES) += mips_ksyms.o module.o

obj-$(CONFIG_FTRACE) += trace_clock.o
-obj-$(CONFIG_FUNCTION_TRACER) += mcount.o
+obj-$(CONFIG_FUNCTION_TRACER) += mcount.o ftrace.o

obj-$(CONFIG_CPU_LOONGSON2) += r4k_fpu.o r4k_switch.o
obj-$(CONFIG_CPU_MIPS32) += r4k_fpu.o r4k_switch.o
diff --git a/arch/mips/kernel/ftrace.c b/arch/mips/kernel/ftrace.c
new file mode 100644
index 0000000..ac6647e
--- /dev/null
+++ b/arch/mips/kernel/ftrace.c
@@ -0,0 +1,89 @@
+/*
+ * Code for replacing ftrace calls with jumps.
+ *
+ * Copyright (C) 2007-2008 Steven Rostedt <srostedt@redhat.com>
+ * Copyright (C) 2009 DSLab, Lanzhou University, China
+ * Author: Wu Zhangjin <wuzj@lemote.com>
+ *
+ * Thanks goes to Steven Rostedt for writing the original x86 version.
+ */
+
+#include <linux/uaccess.h>
+#include <linux/init.h>
+#include <linux/ftrace.h>
+
+#include <asm/cacheflush.h>
+
+#ifdef CONFIG_DYNAMIC_FTRACE
+
+#define JAL 0x0c000000 /* jump & link: ip --> ra, jump to target */
+#define ADDR_MASK 0x03ffffff /* op_code|addr : 31...26|25 ....0 */
+#define jump_insn_encode(op_code, addr) \
+ ((unsigned int)((op_code) | (((addr) >> 2) & ADDR_MASK)))
+
+#ifdef CONFIG_CPU_LOONGSON2F
+static unsigned int ftrace_nop = 0x00020021;
+#else
+static unsigned int ftrace_nop = 0x00000000;
+#endif
+
+static int ftrace_modify_code(unsigned long ip, unsigned int new_code)
+{
+ *(unsigned int *)ip = new_code;
+
+ flush_icache_range(ip, ip + 8);
+
+ return 0;
+}
+
+static int lui_v1;
+
+int ftrace_make_nop(struct module *mod,
+ struct dyn_ftrace *rec, unsigned long addr)
+{
+ /* record it for ftrace_make_call */
+ if (lui_v1 == 0)
+ lui_v1 = *(unsigned int *)rec->ip;
+
+ /* lui v1, hi_16bit_of_mcount --> b 1f (0x10000004)
+ * addiu v1, v1, low_16bit_of_mcount
+ * move at, ra
+ * jalr v1
+ * nop
+ * 1f: (rec->ip + 12)
+ */
+ return ftrace_modify_code(rec->ip, 0x10000004);
+}
+
+static int modified; /* initialized as 0 by default */
+
+int ftrace_make_call(struct dyn_ftrace *rec, unsigned long addr)
+{
+ /* We just need to remove the "b ftrace_stub" at the fist time! */
+ if (modified == 0) {
+ modified = 1;
+ ftrace_modify_code(addr, ftrace_nop);
+ }
+ /* ftrace_make_nop have recorded lui_v1 for us */
+ return ftrace_modify_code(rec->ip, lui_v1);
+}
+
+#define FTRACE_CALL_IP ((unsigned long)(&ftrace_call))
+
+int ftrace_update_ftrace_func(ftrace_func_t func)
+{
+ unsigned int new;
+
+ new = jump_insn_encode(JAL, (unsigned long)func);
+
+ return ftrace_modify_code(FTRACE_CALL_IP, new);
+}
+
+int __init ftrace_dyn_arch_init(void *data)
+{
+ /* The return code is retured via data */
+ *(unsigned long *)data = 0;
+
+ return 0;
+}
+#endif /* CONFIG_DYNAMIC_FTRACE */
diff --git a/arch/mips/kernel/mcount.S b/arch/mips/kernel/mcount.S
index cbb45ed..ffc4259 100644
--- a/arch/mips/kernel/mcount.S
+++ b/arch/mips/kernel/mcount.S
@@ -58,6 +58,33 @@
move ra, AT
.endm

+#ifdef CONFIG_DYNAMIC_FTRACE
+
+NESTED(ftrace_caller, PT_SIZE, ra)
+ .globl _mcount
+_mcount:
+ b ftrace_stub
+ nop
+ lw t0, function_trace_stop
+ bnez t0, ftrace_stub
+ nop
+
+ MCOUNT_SAVE_REGS
+
+ move a0, ra /* arg1: next ip, selfaddr */
+ .globl ftrace_call
+ftrace_call:
+ nop /* a placeholder for the call to a real tracing function */
+ move a1, AT /* arg2: the caller's next ip, parent */
+
+ MCOUNT_RESTORE_REGS
+ .globl ftrace_stub
+ftrace_stub:
+ RETURN_BACK
+ END(ftrace_caller)
+
+#else /* ! CONFIG_DYNAMIC_FTRACE */
+
NESTED(_mcount, PT_SIZE, ra)
lw t0, function_trace_stop
bnez t0, ftrace_stub
@@ -82,5 +109,7 @@ ftrace_stub:
RETURN_BACK
END(_mcount)

+#endif /* ! CONFIG_DYNAMIC_FTRACE */
+
.set at
.set reorder
diff --git a/scripts/recordmcount.pl b/scripts/recordmcount.pl
index 62a0001..11ac245 100755
--- a/scripts/recordmcount.pl
+++ b/scripts/recordmcount.pl
@@ -287,6 +287,47 @@ if ($arch eq "x86_64") {
$ld .= " -m elf64_sparc";
$cc .= " -m64";
$objcopy .= " -O elf64-sparc";
+
+} elsif ($arch eq "mips") {
+ # To enable module support, we need to enable the -mlong-calls option
+ # of gcc for MIPS, after using this option, we can not get the real
+ # offset of the calling to _mcount, but the offset of the lui
+ # instruction or the addiu one. herein, we record the address of the
+ # first one, and then we can replace this instruction by a branch
+ # instruction to jump over the profiling function to filter the
+ # indicated functions, or swith back to the lui instruction to trace
+ # them, which means dynamic tracing.
+ #
+ # c: 3c030000 lui v1,0x0
+ # c: R_MIPS_HI16 _mcount
+ # c: R_MIPS_NONE *ABS*
+ # c: R_MIPS_NONE *ABS*
+ # 10: 64630000 daddiu v1,v1,0
+ # 10: R_MIPS_LO16 _mcount
+ # 10: R_MIPS_NONE *ABS*
+ # 10: R_MIPS_NONE *ABS*
+ # 14: 03e0082d move at,ra
+ # 18: 0060f809 jalr v1
+ $mcount_regex = "^\\s*([0-9a-fA-F]+): R_MIPS_HI16\\s+_mcount\$";
+ $objdump .= " -Melf-trad".$endian."mips ";
+
+ if ($endian eq "big") {
+ $endian = " -EB ";
+ $ld .= " -melf".$bits."btsmip";
+ } else {
+ $endian = " -EL ";
+ $ld .= " -melf".$bits."ltsmip";
+ }
+
+ $cc .= " -mno-abicalls -fno-pic -mabi=" . $bits . $endian;
+ $ld .= $endian;
+
+ if ($bits == 64) {
+ $function_regex =
+ "^([0-9a-fA-F]+)\\s+<(.|[^\$]L.*?|\$[^L].*?|[^\$][^L].*?)>:";
+ $type = ".dword";
+ }
+
} else {
die "Arch $arch is not supported with CONFIG_FTRACE_MCOUNT_RECORD";
}
--
1.6.2.1