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

Steven Rostedt
2008-01-10T19:55:01Z

On Thu, 10 Jan 2008, Jan Kiszka wrote:

> 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.

Actually it's not ;-)

The original code had something like this:

#if CONFIG_MCOUNT
KBUILD_CFLAGS += ...
#else
#if CONFIG_FRAME_POINTER
KBUILD_CFLAGS += ...
#else
KBUILD_CFLAGS += ...
#endif
#endif

And Sam Ravnborg suggested to put that logic into the Kbuild system. For
which I did, but I put that comment there to just let others know that
MCOUNT expects the flags of FRAME_POINTER. But, I guess we can nuke that
comment anyway. It just leads to confusion.

>
> > +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.

Yeah, that's a relic from -rt. I never liked that, but I was just too lazy
to change it. I think I'll move the mcount_wrapper into entry_32.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.

Hmm, what about having frame pointers on? Isn't that a requirement?

>
> > + pushl %eax
> > + pushl %ecx
> > + pushl %edx
> > +
> > + call __mcount
>
> I think this indirection should be avoided, just like the 64-bit version
> and glibc do.

I thought about that too, but didn't have the time to look into the
calling convention for that. <does a quick look at glibc>

# objdump --start-address 0x`nm /lib/libc-2.7.so | sed -ne '/ mcount$/s/^\([0-9a-f]*\).*/\1/p'` -D /lib/libc-2.7.so |head -28 |tail -12
49201cd0 <_mcount>:
49201cd0: 50 push %eax
49201cd1: 51 push %ecx
49201cd2: 52 push %edx
49201cd3: 8b 54 24 0c mov 0xc(%esp),%edx
49201cd7: 8b 45 04 mov 0x4(%ebp),%eax
49201cda: e8 91 f4 ff ff call 49201170
<__mcount_internal>
49201cdf: 5a pop %edx
49201ce0: 59 pop %ecx
49201ce1: 58 pop %eax
49201ce2: c3 ret
49201ce3: 90 nop

Until I found out about the frame pointers, I'll leave in the ebp copy.

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

[...]

> > +/** __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.

I'll try that.

> > 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?

Will be used later on by the tracers. Actually, I wish this was in a more
generic kernel header, since I find myself typing
"__builtin_return_address" quite often.

>
> > +
> > +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.

Same checking for frame pointers too.

>
> > +
> > + 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?

Probably not. I'll see what happens without them.

>
> > + 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.

Yeah, it's probably a bit faster to do the mov instead. I'll add that.

>
> > +
> > + mov 0x0(%rbp),%rax
> > + mov 0x8(%rbp),%rdi
> > + mov 0x8(%rax),%rsi
>
> See [2] for saving one instruction here. :)

hehe, yeah, will do.

>
> > +
> > + 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.

Thanks,

-- Steve