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Re: [PATCH] arm: ftrace: Adds support for CONFIG_DYNAMIC_FTRACE_WITH_REGS

Started byPetr Mladek <pmladek@suse.com>
First post2017-01-10 17:00 +0100
Last post2017-01-13 10:10 +0100
Articles 4 — 3 participants

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  Re: [PATCH] arm: ftrace: Adds support for  CONFIG_DYNAMIC_FTRACE_WITH_REGS Petr Mladek <pmladek@suse.com> - 2017-01-10 17:00 +0100
    Re: [PATCH] arm: ftrace: Adds support for  CONFIG_DYNAMIC_FTRACE_WITH_REGS Abel Vesa <abelvesa@gmail.com> - 2017-01-12 01:30 +0100
      Re: [PATCH] arm: ftrace: Adds support for CONFIG_DYNAMIC_FTRACE_WITH_REGS Jean-Jacques Hiblot <jjhiblot@traphandler.com> - 2017-01-12 22:50 +0100
        Re: [PATCH] arm: ftrace: Adds support for CONFIG_DYNAMIC_FTRACE_WITH_REGS Jean-Jacques Hiblot <jjhiblot@traphandler.com> - 2017-01-13 10:10 +0100

#1555640 — Re: [PATCH] arm: ftrace: Adds support for CONFIG_DYNAMIC_FTRACE_WITH_REGS

FromPetr Mladek <pmladek@suse.com>
Date2017-01-10 17:00 +0100
SubjectRe: [PATCH] arm: ftrace: Adds support for CONFIG_DYNAMIC_FTRACE_WITH_REGS
Message-ID<sY70b-3cI-19@gated-at.bofh.it>
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

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.

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

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#1557028

FromAbel Vesa <abelvesa@gmail.com>
Date2017-01-12 01:30 +0100
Message-ID<sYBrd-5sK-25@gated-at.bofh.it>
In reply to#1555640
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.

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

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#1557837 — Re: [PATCH] arm: ftrace: Adds support for CONFIG_DYNAMIC_FTRACE_WITH_REGS

FromJean-Jacques Hiblot <jjhiblot@traphandler.com>
Date2017-01-12 22:50 +0100
SubjectRe: [PATCH] arm: ftrace: Adds support for CONFIG_DYNAMIC_FTRACE_WITH_REGS
Message-ID<sYVpT-DA-11@gated-at.bofh.it>
In reply to#1557028
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

        @ 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

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#1558141 — Re: [PATCH] arm: ftrace: Adds support for CONFIG_DYNAMIC_FTRACE_WITH_REGS

FromJean-Jacques Hiblot <jjhiblot@traphandler.com>
Date2017-01-13 10:10 +0100
SubjectRe: [PATCH] arm: ftrace: Adds support for CONFIG_DYNAMIC_FTRACE_WITH_REGS
Message-ID<sZ61Y-7rG-31@gated-at.bofh.it>
In reply to#1557837
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

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