Re: [RFC PATCH 01/22 -v2] Add basic support for gcc profiler instrumentation

Jan Kiszka
2008-01-11T00:07:28Z

Steven Rostedt wrote:
> Index: linux-compile-i386.git/Makefile
> ===================================================================
> --- linux-compile-i386.git.orig/Makefile 2008-01-09 14:09:36.000000000 -0500
> +++ linux-compile-i386.git/Makefile 2008-01-09 14:10:07.000000000 -0500
> @@ -509,6 +509,10 @@ endif
>
> include $(srctree)/arch/$(SRCARCH)/Makefile
>
> +# MCOUNT expects frame pointer

This comment looks stray.

> +ifdef CONFIG_MCOUNT
> +KBUILD_CFLAGS += -pg
> +endif
> ifdef CONFIG_FRAME_POINTER
> KBUILD_CFLAGS += -fno-omit-frame-pointer -fno-optimize-sibling-calls
> else
> Index: linux-compile-i386.git/arch/x86/Kconfig
> ===================================================================
> --- linux-compile-i386.git.orig/arch/x86/Kconfig 2008-01-09 14:09:36.000000000 -0500
> +++ linux-compile-i386.git/arch/x86/Kconfig 2008-01-09 14:10:07.000000000 -0500
> @@ -28,6 +28,10 @@ config GENERIC_CMOS_UPDATE
> bool
> default y
>
> +config ARCH_HAS_MCOUNT
> + bool
> + default y
> +
> config CLOCKSOURCE_WATCHDOG
> bool
> default y
> Index: linux-compile-i386.git/arch/x86/kernel/Makefile_32
> ===================================================================
> --- linux-compile-i386.git.orig/arch/x86/kernel/Makefile_32 2008-01-09 14:09:36.000000000 -0500
> +++ linux-compile-i386.git/arch/x86/kernel/Makefile_32 2008-01-09 14:10:07.000000000 -0500
> @@ -23,6 +23,7 @@ obj-$(CONFIG_APM) += apm_32.o
> obj-$(CONFIG_X86_SMP) += smp_32.o smpboot_32.o tsc_sync.o
> obj-$(CONFIG_SMP) += smpcommon_32.o
> obj-$(CONFIG_X86_TRAMPOLINE) += trampoline_32.o
> +obj-$(CONFIG_MCOUNT) += mcount-wrapper.o

So far the code organization is different for 32 and 64 bit. I would
suggest to either

o move both trampolines into entry_*.S or
o put them in something like mcount-wrapper_32/64.S.

> obj-$(CONFIG_X86_MPPARSE) += mpparse_32.o
> obj-$(CONFIG_X86_LOCAL_APIC) += apic_32.o nmi_32.o
> obj-$(CONFIG_X86_IO_APIC) += io_apic_32.o
> Index: linux-compile-i386.git/arch/x86/kernel/mcount-wrapper.S
> ===================================================================
> --- /dev/null 1970-01-01 00:00:00.000000000 +0000
> +++ linux-compile-i386.git/arch/x86/kernel/mcount-wrapper.S 2008-01-09 14:10:07.000000000 -0500
> @@ -0,0 +1,25 @@
> +/*
> + * linux/arch/x86/mcount-wrapper.S
> + *
> + * Copyright (C) 2004 Ingo Molnar
> + */
> +
> +.globl mcount
> +mcount:
> + cmpl $0, mcount_enabled
> + jz out
> +
> + push %ebp
> + mov %esp, %ebp

What is the benefit of having a call frame in this trampoline? We used
to carry this in the i386 mcount tracer for Adeos/I-pipe too (it was
derived from the -rt code), but I just successfully tested a removal
patch. Also glibc [1] doesn't include it.

> + pushl %eax
> + pushl %ecx
> + pushl %edx
> +
> + call __mcount

I think this indirection should be avoided, just like the 64-bit version
and glibc do.

> +
> + popl %edx
> + popl %ecx
> + popl %eax
> + popl %ebp
> +out:
> + ret

...

> Index: linux-compile-i386.git/lib/tracing/mcount.c
> ===================================================================
> --- /dev/null 1970-01-01 00:00:00.000000000 +0000
> +++ linux-compile-i386.git/lib/tracing/mcount.c 2008-01-09 14:10:07.000000000 -0500
> @@ -0,0 +1,77 @@
> +/*
> + * Infrastructure for profiling code inserted by 'gcc -pg'.
> + *
> + * Copyright (C) 2007 Arnaldo Carvalho de Melo <acme@redhat.com>
> + *
> + * Converted to be more generic:
> + * Copyright (C) 2007-2008 Steven Rostedt <srostedt@redhat.com>
> + *
> + * From code in the latency_tracer, that is:
> + *
> + * Copyright (C) 2004-2006 Ingo Molnar
> + * Copyright (C) 2004 William Lee Irwin III
> + */
> +
> +#include <linux/module.h>
> +#include <linux/mcount.h>
> +
> +/*
> + * Since we have nothing protecting between the test of
> + * mcount_trace_function and the call to it, we can't
> + * set it to NULL without risking a race that will have
> + * the kernel call the NULL pointer. Instead, we just
> + * set the function pointer to a dummy function.
> + */
> +notrace void dummy_mcount_tracer(unsigned long ip,
> + unsigned long parent_ip)
> +{
> + /* do nothing */
> +}
> +
> +mcount_func_t mcount_trace_function __read_mostly = dummy_mcount_tracer;
> +int mcount_enabled __read_mostly;
> +
> +/** __mcount - hook for profiling
> + *
> + * This routine is called from the arch specific mcount routine, that in turn is
> + * called from code inserted by gcc -pg.
> + */
> +notrace void __mcount(void)
> +{
> + mcount_trace_function(CALLER_ADDR1, CALLER_ADDR2);
> +}

mcount_trace_function should always be called from the assembly
trampoline, IMO.

> +EXPORT_SYMBOL_GPL(mcount);
> +/*
> + * The above EXPORT_SYMBOL is for the gcc call of mcount and not the
> + * function __mcount that it is underneath. I put the export there
> + * to fool checkpatch.pl. It wants that export to be with the
> + * function, but that function happens to be in assembly.
> + */
> +
> +/**
> + * register_mcount_function - register a function for profiling
> + * @func - the function for profiling.
> + *
> + * Register a function to be called by all functions in the
> + * kernel.
> + *
> + * Note: @func and all the functions it calls must be labeled
> + * with "notrace", otherwise it will go into a
> + * recursive loop.
> + */
> +int register_mcount_function(mcount_func_t func)
> +{
> + mcount_trace_function = func;
> + return 0;
> +}
> +
> +/**
> + * clear_mcount_function - reset the mcount function
> + *
> + * This NULLs the mcount function and in essence stops
> + * tracing. There may be lag
> + */
> +void clear_mcount_function(void)
> +{
> + mcount_trace_function = dummy_mcount_tracer;
> +}
> Index: linux-compile-i386.git/include/linux/mcount.h
> ===================================================================
> --- /dev/null 1970-01-01 00:00:00.000000000 +0000
> +++ linux-compile-i386.git/include/linux/mcount.h 2008-01-09 15:17:20.000000000 -0500
> @@ -0,0 +1,21 @@
> +#ifndef _LINUX_MCOUNT_H
> +#define _LINUX_MCOUNT_H
> +
> +#ifdef CONFIG_MCOUNT
> +extern int mcount_enabled;
> +
> +#include <linux/linkage.h>
> +
> +#define CALLER_ADDR0 ((unsigned long)__builtin_return_address(0))
> +#define CALLER_ADDR1 ((unsigned long)__builtin_return_address(1))
> +#define CALLER_ADDR2 ((unsigned long)__builtin_return_address(2))

Still used when __mcount would be gone?

> +
> +typedef void (*mcount_func_t)(unsigned long ip, unsigned long parent_ip);
> +
> +extern void mcount(void);
> +
> +int register_mcount_function(mcount_func_t func);
> +void clear_mcount_function(void);
> +
> +#endif /* CONFIG_MCOUNT */
> +#endif /* _LINUX_MCOUNT_H */
> Index: linux-compile-i386.git/arch/x86/kernel/entry_64.S
> ===================================================================
> --- linux-compile-i386.git.orig/arch/x86/kernel/entry_64.S 2008-01-09 14:09:36.000000000 -0500
> +++ linux-compile-i386.git/arch/x86/kernel/entry_64.S 2008-01-09 14:10:07.000000000 -0500
> @@ -53,6 +53,46 @@
>
> .code64
>
> +#ifdef CONFIG_MCOUNT
> +
> +ENTRY(mcount)
> + cmpl $0, mcount_enabled
> + jz out
> +
> + push %rbp
> + mov %rsp,%rbp

Same as for x86_32.

> +
> + push %r11
> + push %r10

glibc [2] doesn't save those two, and we were also happy without them so
far. Or are there nasty corner-cases in the kernel?

> + push %r9
> + push %r8
> + push %rdi
> + push %rsi
> + push %rdx
> + push %rcx
> + push %rax

SAVE_ARGS/RESTORE_ARGS and glibc use explicit rsp manipulation + movq
instead of push/pop. I wonder if there is a small advantage, but I'm not
that deep into this arch.

> +
> + mov 0x0(%rbp),%rax
> + mov 0x8(%rbp),%rdi
> + mov 0x8(%rax),%rsi

See [2] for saving one instruction here. :)

> +
> + call *mcount_trace_function
> +
> + pop %rax
> + pop %rcx
> + pop %rdx
> + pop %rsi
> + pop %rdi
> + pop %r8
> + pop %r9
> + pop %r10
> + pop %r11
> +
> + pop %rbp
> +out:
> + ret
> +#endif
> +
> #ifndef CONFIG_PREEMPT
> #define retint_kernel retint_restore_args
> #endif

This generic approach is very appreciated here as well. It would take
away the burden of maintaining the arch-dependent stubs within I-pipe.

What we could contribute later on is a blackfin trampoline, there is
just still a bug in their toolchain which breaks mcount for modules. But
I could check with the bfin guys again about the progress and underline
the importance of this long-pending issue.

Jan

[1]http://sources.redhat.com/cgi-bin/cvsweb.cgi/libc/sysdeps/i386/i386-mcount.S?rev=1.6&content-type=text/x-cvsweb-markup&cvsroot=glibc
[2]http://sources.redhat.com/cgi-bin/cvsweb.cgi/libc/sysdeps/x86_64/_mcount.S?rev=1.5&content-type=text/x-cvsweb-markup&cvsroot=glibc