| Message ID | CACh+v5M-BDru3OyhrROBqPKJuSN=yoxVstN-fqZGh8SU1EHQiw@mail.gmail.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
2017-01-12 15:30 GMT+01:00 Jean-Jacques Hiblot <jjhiblot@traphandler.com>: > 2017-01-12 1:19 GMT+01:00 Abel Vesa <abelvesa@gmail.com>: >> On Tue, Jan 10, 2017 at 04:51:12PM +0100, Petr Mladek wrote: >>> On Thu 2016-12-08 22:46:55, Abel Vesa wrote: >>> > On Thu, Dec 08, 2016 at 09:46:35PM +0000, Abel Vesa wrote: >>> > > From: Jean-Jacques Hiblot <jjhiblot@traphandler.com> >>> > > >>> > > From: Jean-Jacques Hiblot <jjhiblot@traphandler.com> >>> > >>> > >From statement twice in the commit message. Will resend. >>> > > >>> > > The DYNAMIC_FTRACE_WITH_REGS configuration makes it possible for a ftrace >>> > > operation to specify if registers need to saved/restored by the ftrace handler. >>> > > This is needed by kgraft and possibly other ftrace-based tools, and the ARM >>> > > architecture is currently lacking this feature. It would also be the first step >>> > > to support the "Kprobes-on-ftrace" optimization on ARM. >>> > > >>> > > This patch introduces a new ftrace handler that stores the registers on the >>> > > stack before calling the next stage. The registers are restored from the stack >>> > > before going back to the instrumented function. >>> > > >>> > > A side-effect of this patch is to activate the support for ftrace_modify_call() >>> > > as it defines ARCH_SUPPORTS_FTRACE_OPS for the ARM architecture >>> > > >>> > > Signed-off-by: Jean-Jacques Hiblot <jjhiblot@traphandler.com> >>> > > Signed-off-by: Abel Vesa <abelvesa@linux.com> >>> > > --- >>> > > arch/arm/Kconfig | 2 ++ >>> > > arch/arm/include/asm/ftrace.h | 4 +++ >>> > > arch/arm/kernel/entry-ftrace.S | 78 ++++++++++++++++++++++++++++++++++++++++++ >>> > > arch/arm/kernel/ftrace.c | 33 ++++++++++++++++++ >>> > > 4 files changed, 117 insertions(+) >>> > > >>> > > diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig >>> > > index b5d529f..87f1a9f 100644 >>> > > --- a/arch/arm/Kconfig >>> > > +++ b/arch/arm/Kconfig >>> > > @@ -50,6 +50,7 @@ config ARM >>> > > select HAVE_DMA_API_DEBUG >>> > > select HAVE_DMA_CONTIGUOUS if MMU >>> > > select HAVE_DYNAMIC_FTRACE if (!XIP_KERNEL) && !CPU_ENDIAN_BE32 && MMU >>> > > + select HAVE_DYNAMIC_FTRACE_WITH_REGS if HAVE_DYNAMIC_FTRACE >>> > > select HAVE_EFFICIENT_UNALIGNED_ACCESS if (CPU_V6 || CPU_V6K || CPU_V7) && MMU >>> > > select HAVE_EXIT_THREAD >>> > > select HAVE_FTRACE_MCOUNT_RECORD if (!XIP_KERNEL) >>> > > @@ -90,6 +91,7 @@ config ARM >>> > > select PERF_USE_VMALLOC >>> > > select RTC_LIB >>> > > select SYS_SUPPORTS_APM_EMULATION >>> > > + select FRAME_POINTER if DYNAMIC_FTRACE_WITH_REGS && FUNCTION_GRAPH_TRACER >> Hi Petr, >>> >>> FRAME_POINTER is not for free. It takes space on the stack. Also there >>> is a performance penalty. Do we really need to depend on it? If so, >>> it might be worth a note in the commit message. >> > > FRAME_POINTER is not needed. the dependency is wrong and should be removed. > The code must be modified to not use fp register: > > --- a/arch/arm/kernel/entry-ftrace.S > +++ b/arch/arm/kernel/entry-ftrace.S > @@ -130,7 +130,8 @@ ftrace_graph_regs_call: > #ifdef CONFIG_FUNCTION_GRAPH_TRACER > .macro __ftrace_graph_regs_caller > > - sub r0, fp, #4 @ lr of instrumented > routine (parent) > + add r0, sp, #64 @ r0 is now a pointer to lr of > + @ instrumented routine I made some tests after sending this email. And it turns out that it doesn't work if we change "sub r0, fp, #4" to "add r0, sp, #64 " here. So it looks like there is a dependency on FRAME_POINTER after all. Note that the same is true for __ftrace_graph_caller. I don't know if the 'graph' feature of ftrace requires intrinsically FRAME_POINTER but it looks like it currently does on ARM (with or without register saving) I'll try to spend some time on the subject next week. > > @ called from __ftrace_regs_caller > ldr r1, [sp, #56] @ instrumented routine (func) > @@ -139,8 +140,9 @@ ftrace_graph_regs_call: > mov r2, fp @ frame pointer > bl prepare_ftrace_return > > - ldr lr, [fp, #-4] @ restore lr from the stack > - ldmia sp, {r0-r11, ip, sp} @ restore r0 through sp > + ldr lr, [sp, #64] @ get the previous LR value from stack > + ldmia sp, {r0-r11, ip, sp} @ pop the saved registers INCLUDING > + @ the stack pointer > ret ip > .endm > #endif > > > Jean-Jacques > > >> I was trying to create my own patch when I found this work done by >> Jean-Jacques, so I haven't looked specifically for the FRAME_POINTER >> part. I looked now at it and you seem to be right, FRAME_POINTER is >> not needed. >> >> I will get rid of the FRAME_POINTER part, change the authorship and >> send it again in the following days. >>> >>> I made only a quick look at the patch. It looks reasonable. But I do >>> not have enough knowledge about the arm architecture, assembly, and >>> ftrace-specifics. Also I cannot test it easily. So issues might >>> be hidden to my eyes. >>> >>> Best Regards, >>> Petr >> Thanks, >> Abel
Hi Jean-Jacques,
Here is the implementation I've made for ftrace_graph_regs_caller:
.macro __ftrace_graph_regs_caller
sub r0, fp, #4 @ lr of
instrumented routine (parent)
@ called from __ftrace_regs_caller
ldr r1, [sp, #56] @ instrumented routine (func)
mcount_adjust_addr r1, r1
sub r2, r0, #4 @ frame pointer
bl prepare_ftrace_return
ldr lr, [sp, #64] @ restore lr from the stack
ldmia sp, {r0-r11, ip, sp} @ restore r0 through sp
ret ip
.endm
AFAIK, you can still use fp (see the implementation without regs) since
it is an alternative name for r11. The FRAME_POINTER config options does
something else entirely and has nothing to do with what we need here.
I tested it but unfortunately, my current setup is with qemu and I don't have
a real hardware here close to test it properly. Could you please tell me if this
works on your setup?
Also the way I tested it was to comment out the code that uses the default
ftrace_graph_caller in ftrace_modify_graph_caller and enforced the usage of
the one with regs.
I will create a proper patch later today (I need to cleanup some stuff
first) and
send it.
Thanks,
Abel
--- a/arch/arm/kernel/entry-ftrace.S +++ b/arch/arm/kernel/entry-ftrace.S @@ -130,7 +130,8 @@ ftrace_graph_regs_call: #ifdef CONFIG_FUNCTION_GRAPH_TRACER .macro __ftrace_graph_regs_caller - sub r0, fp, #4 @ lr of instrumented routine (parent) + add r0, sp, #64 @ r0 is now a pointer to lr of + @ instrumented routine @ called from __ftrace_regs_caller