Re: [PATCH 0/5] ftrace: to kill a daemon

[Steven Rostedt]

On Sat, 9 Aug 2008, Andi Kleen wrote:

> > I wish we had a true 5 byte nop. The alternative is a jmp 0, which is 
> > measurable.  This is replacing mcount from a kernel compile with the -pg 
> 
> Scary usage.  How much is the difference? 
> 
> BTW a good way to get most of that back would be to not use frame 
> pointers if you have them enabled ;-) Unfortunately recently
> the "select FRAME_POINTER"s keep creeping all over so far too many kernels
> suffer from this slow mode now :-/

Funny, CONFIG_FTRACE happens to select that. Now the question is, would 
mcount work without it?

i386:
ENTRY(mcount)
        pushl %eax
        pushl %ecx
        pushl %edx
        movl 0xc(%esp), %eax
        movl 0x4(%ebp), %edx  <-- we record the parent ip here
        subl $MCOUNT_INSN_SIZE, %eax

        call *ftrace_trace_function

        popl %edx
        popl %ecx
        popl %eax
	ret


x86_64:
ENTRY(mcount)
        subq $0x38, %rsp
        movq %rax, (%rsp)
        movq %rcx, 8(%rsp)
        movq %rdx, 16(%rsp)
        movq %rsi, 24(%rsp)
        movq %rdi, 32(%rsp)
        movq %r8, 40(%rsp)
        movq %r9, 48(%rsp)

        movq 0x38(%rsp), %rdi
        movq 8(%rbp), %rsi <-- we record the parent pointer here
        subq $MCOUNT_INSN_SIZE, %rdi

        call   *ftrace_trace_function

        movq 48(%rsp), %r9
        movq 40(%rsp), %r8
        movq 32(%rsp), %rdi
        movq 24(%rsp), %rsi
        movq 16(%rsp), %rdx
        movq 8(%rsp), %rcx
        movq (%rsp), %rax
        addq $0x38, %rsp
	retq

-- Steve

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