Re: [PATCH v7 1/7] Add AutoFDO support for Clang build

[Peter Jung <ptr1337@cachyos.org>]

You were right - reverting commit:
https://github.com/bminor/binutils-gdb/commit/b20ab53f81db7eefa0db00d14f06c04527ac324c 
from the 2.43 branch does fix the packaging.

I will forward this to an issue at their bugzilla.

On 05.11.24 15:33, Peter Jung wrote:
> Hi Rong,
> 
> Glad that you were able to reproduce the issue!
> Thanks for finding the root cause as well as the part of the code. This 
> really helps.
> 
> I was able to do a successful packaging with binutils 2.42.
> Lets forward this to the binutils tracker and hope this will be soon 
> solved. 🙂
> 
> I have tested this also on the latest commit 
> (e1e4078ac59740a79cd709d61872abe15aba0087) and the issue is also 
> reproducible there.
> 
> Thanks for your time! I dont see this as blocker. 🙂
> It gets time to get this series merged :P
> 
> Best regards,
> 
> Peter
> 
> 
> 
> On 05.11.24 08:25, Rong Xu wrote:
>> We debugged this issue and we found the failure seems to only happen
>> with strip (version 2.43) in binutil.
>>
>> For a profile-use compilation, either with -fprofile-use (PGO or
>> iFDO), or -fprofile-sample-use (AutoFDO),
>> an ELF section of .llvm.call-graph-profile is created for the object.
>> For some reasons (like to save space?),
>> the relocations in this section are of type "rel', rather the more
>> common "rela" type.
>>
>> In this case,
>> $ readelf -r kvm.ko |grep llvm.call-graph-profile
>> Relocation section '.rel.llvm.call-graph-profile' at offset 0xf62a00
>> contains 4 entries:
>>
>> strip (v2.43.0) has difficulty handling the relocations in
>> .rel.llvm.call-graph-profile -- it silently failed with --strip-debug.
>> But strip (v.2.42) has no issue with kvm.ko. The strip in llvm (i.e.
>> llvm-strip) also passes with kvm.ko
>>
>> I compared binutil/strip source code for version v2.43.0 and v2.42.
>> The different is around here:
>> In v2.42 of bfd/elfcode.h
>>     1618       if ((entsize == sizeof (Elf_External_Rela)
>>     1619            && ebd->elf_info_to_howto != NULL)
>>     1620           || ebd->elf_info_to_howto_rel == NULL)
>>     1621         res = ebd->elf_info_to_howto (abfd, relent, &rela);
>>     1622       else
>>     1623         res = ebd->elf_info_to_howto_rel (abfd, relent, &rela);
>>
>> In v2.43.0 of bfd/elfcode.h
>>     1618       if (entsize == sizeof (Elf_External_Rela)
>>     1619           && ebd->elf_info_to_howto != NULL)
>>     1620         res = ebd->elf_info_to_howto (abfd, relent, &rela);
>>     1621       else if (ebd->elf_info_to_howto_rel != NULL)
>>     1622         res = ebd->elf_info_to_howto_rel (abfd, relent, &rela);
>>
>> In the 2.43 strip, line 1618 is false and line 1621 is also false.
>> "res" is returned as false and the program exits with -1.
>>
>> While in 2.42, line 1620 is true and we get "res" from line 1621 and
>> program functions correctly.
>>
>> I'm not familiar with binutil code base and don't know the reason for
>> removing line 1620.
>> I can file a bug for binutil for people to further investigate this.
>>
>> It seems to me that this issue should not be a blocker for our patch.
>>
>> Regards,
>>
>> -Rong
>>
>>
>>
>>
>>
>> On Mon, Nov 4, 2024 at 12:24 PM Han Shen<shenhan@google.com> wrote:
>>> Hi Peter,
>>> Thanks for providing the detailed reproduce.
>>> Now I can see the error (after I synced to 6.12.0-rc6, I was using rc5).
>>> I'll look into that and report back.
>>>
>>>> I have tested your provided method, but the AutoFDO profile (lld does
>>> not get lto-sample-profile=$pathtoprofile passed)
>>>
>>> I see. You also turned on ThinLTO, which I didn't, so the profile was
>>> only used during compilation, not passed to lld.
>>>
>>> Thanks,
>>> Han
>>>
>>> On Mon, Nov 4, 2024 at 9:31 AM Peter Jung<ptr1337@cachyos.org> wrote:
>>>> Hi Han,
>>>>
>>>> I have tested your provided method, but the AutoFDO profile (lld does
>>>> not get lto-sample-profile=$pathtoprofile passed)  nor Clang as 
>>>> compiler
>>>> gets used.
>>>> Please replace following PKGBUILD and config from linux-mainline with
>>>> the provided one in the gist. The patch is also included there.
>>>>
>>>> https://gist.github.com/ptr1337/c92728bb273f7dbc2817db75eedec9ed
>>>>
>>>> The main change I am doing here, is passing following to the build 
>>>> array
>>>> and replacing "make all":
>>>>
>>>> make LLVM=1 LLVM_IAS=1 CLANG_AUTOFDO_PROFILE=${srcdir}/perf.afdo all
>>>>
>>>> When compiling the kernel with makepkg, this results at the 
>>>> packaging to
>>>> following issue and can be reliable reproduced.
>>>>
>>>> Regards,
>>>>
>>>> Peter
>>>>
>>>>
>>>> On 04.11.24 05:50, Han Shen wrote:
>>>>> Hi Peter, thanks for reporting the issue. I am trying to reproduce it
>>>>> in the up-to-date archlinux environment. Below is what I have:
>>>>>     0. pacman -Syu
>>>>>     1. cloned archlinux build files from
>>>>> https://aur.archlinux.org/linux-mainline.git the newest mainline
>>>>> version is 6.12rc5-1.
>>>>>     2. changed the PKGBUILD file to include the patches series
>>>>>     3. changed the "config" to turn on clang autofdo
>>>>>     4. collected afdo profiles
>>>>>     5. MAKEFLAGS="-j48 V=1 LLVM=1 CLANG_AUTOFDO_PROFILE=$(pwd)/ 
>>>>> perf.afdo" \
>>>>>           makepkg -s --skipinteg --skippgp
>>>>>     6. install and reboot
>>>>> The above steps succeeded.
>>>>> You mentioned the error happens at "module_install", can you instruct
>>>>> me how to execute the "module_install" step?
>>>>>
>>>>> Thanks,
>>>>> Han
>>>>>
>>>>> On Sat, Nov 2, 2024 at 12:53 PM Peter Jung<ptr1337@cachyos.org> wrote:
>>>>>>
>>>>>> On 02.11.24 20:46, Peter Jung wrote:
>>>>>>> On 02.11.24 18:51, Rong Xu wrote:
>>>>>>>> Add the build support for using Clang's AutoFDO. Building the 
>>>>>>>> kernel
>>>>>>>> with AutoFDO does not reduce the optimization level from the
>>>>>>>> compiler. AutoFDO uses hardware sampling to gather information 
>>>>>>>> about
>>>>>>>> the frequency of execution of different code paths within a binary.
>>>>>>>> This information is then used to guide the compiler's optimization
>>>>>>>> decisions, resulting in a more efficient binary. Experiments
>>>>>>>> showed that the kernel can improve up to 10% in latency.
>>>>>>>>
>>>>>>>> The support requires a Clang compiler after LLVM 17. This 
>>>>>>>> submission
>>>>>>>> is limited to x86 platforms that support PMU features like LBR on
>>>>>>>> Intel machines and AMD Zen3 BRS. Support for SPE on ARM 1,
>>>>>>>>     and BRBE on ARM 1 is part of planned future work.
>>>>>>>>
>>>>>>>> Here is an example workflow for AutoFDO kernel:
>>>>>>>>
>>>>>>>> 1) Build the kernel on the host machine with LLVM enabled, for 
>>>>>>>> example,
>>>>>>>>           $ make menuconfig LLVM=1
>>>>>>>>        Turn on AutoFDO build config:
>>>>>>>>          CONFIG_AUTOFDO_CLANG=y
>>>>>>>>        With a configuration that has LLVM enabled, use the 
>>>>>>>> following
>>>>>>>>        command:
>>>>>>>>           scripts/config -e AUTOFDO_CLANG
>>>>>>>>        After getting the config, build with
>>>>>>>>          $ make LLVM=1
>>>>>>>>
>>>>>>>> 2) Install the kernel on the test machine.
>>>>>>>>
>>>>>>>> 3) Run the load tests. The '-c' option in perf specifies the sample
>>>>>>>>       event period. We suggest     using a suitable prime number,
>>>>>>>>       like 500009, for this purpose.
>>>>>>>>       For Intel platforms:
>>>>>>>>          $ perf record -e BR_INST_RETIRED.NEAR_TAKEN:k -a -N -b -c
>>>>>>>> <count> \
>>>>>>>>            -o <perf_file> -- <loadtest>
>>>>>>>>       For AMD platforms:
>>>>>>>>          The supported system are: Zen3 with BRS, or Zen4 with 
>>>>>>>> amd_lbr_v2
>>>>>>>>         For Zen3:
>>>>>>>>          $ cat proc/cpuinfo | grep " brs"
>>>>>>>>          For Zen4:
>>>>>>>>          $ cat proc/cpuinfo | grep amd_lbr_v2
>>>>>>>>          $ perf record --pfm-events 
>>>>>>>> RETIRED_TAKEN_BRANCH_INSTRUCTIONS:k
>>>>>>>> -a \
>>>>>>>>            -N -b -c <count> -o <perf_file> -- <loadtest>
>>>>>>>>
>>>>>>>> 4) (Optional) Download the raw perf file to the host machine.
>>>>>>>>
>>>>>>>> 5) To generate an AutoFDO profile, two offline tools are available:
>>>>>>>>       create_llvm_prof and llvm_profgen. The create_llvm_prof 
>>>>>>>> tool is part
>>>>>>>>       of the AutoFDO project and can be found on GitHub
>>>>>>>>       (https://github.com/google/autofdo), version v0.30.1 or 
>>>>>>>> later. The
>>>>>>>>       llvm_profgen tool is included in the LLVM compiler itself. 
>>>>>>>> It's
>>>>>>>>       important to note that the version of llvm_profgen doesn't 
>>>>>>>> need to
>>>>>>>>       match the version of Clang. It needs to be the LLVM 19 
>>>>>>>> release or
>>>>>>>>       later, or from the LLVM trunk.
>>>>>>>>          $ llvm-profgen --kernel --binary=<vmlinux> --
>>>>>>>> perfdata=<perf_file> \
>>>>>>>>            -o <profile_file>
>>>>>>>>       or
>>>>>>>>          $ create_llvm_prof --binary=<vmlinux> -- 
>>>>>>>> profile=<perf_file> \
>>>>>>>>            --format=extbinary --out=<profile_file>
>>>>>>>>
>>>>>>>>       Note that multiple AutoFDO profile files can be merged 
>>>>>>>> into one via:
>>>>>>>>          $ llvm-profdata merge -o <profile_file>  <profile_1> ...
>>>>>>>> <profile_n>
>>>>>>>>
>>>>>>>> 6) Rebuild the kernel using the AutoFDO profile file with the 
>>>>>>>> same config
>>>>>>>>       as step 1, (Note CONFIG_AUTOFDO_CLANG needs to be enabled):
>>>>>>>>          $ make LLVM=1 CLANG_AUTOFDO_PROFILE=<profile_file>
>>>>>>>>
>>>>>>>> Co-developed-by: Han Shen<shenhan@google.com>
>>>>>>>> Signed-off-by: Han Shen<shenhan@google.com>
>>>>>>>> Signed-off-by: Rong Xu<xur@google.com>
>>>>>>>> Suggested-by: Sriraman Tallam<tmsriram@google.com>
>>>>>>>> Suggested-by: Krzysztof Pszeniczny<kpszeniczny@google.com>
>>>>>>>> Suggested-by: Nick Desaulniers<ndesaulniers@google.com>
>>>>>>>> Suggested-by: Stephane Eranian<eranian@google.com>
>>>>>>>> Tested-by: Yonghong Song<yonghong.song@linux.dev>
>>>>>>>> Tested-by: Yabin Cui<yabinc@google.com>
>>>>>>>> Tested-by: Nathan Chancellor<nathan@kernel.org>
>>>>>>>> Reviewed-by: Kees Cook<kees@kernel.org>
>>>>>>> Tested-by: Peter Jung<ptr1337@cachyos.org>
>>>>>>>
>>>>>> The compilations and testing with the "make pacman-pkg" function from
>>>>>> the kernel worked fine.
>>>>>>
>>>>>> One problem I do face:
>>>>>> When I apply a AutoFDO profile together with the PKGBUILD [1] from
>>>>>> archlinux im running into issues at "module_install" at the 
>>>>>> packaging.
>>>>>>
>>>>>> See following log:
>>>>>> ```
>>>>>> make[2]: *** [scripts/Makefile.modinst:125:
>>>>>> /tmp/makepkg/linux-cachyos-rc-autofdo/pkg/linux-cachyos-rc- 
>>>>>> autofdo/usr/lib/modules/6.12.0-rc5-5-cachyos-rc-autofdo/kernel/ 
>>>>>> arch/x86/kvm/kvm.ko]
>>>>>> Error 1
>>>>>> make[2]: *** Deleting file
>>>>>> '/tmp/makepkg/linux-cachyos-rc-autofdo/pkg/linux-cachyos-rc- 
>>>>>> autofdo/usr/lib/modules/6.12.0-rc5-5-cachyos-rc-autofdo/kernel/ 
>>>>>> arch/x86/kvm/kvm.ko'
>>>>>>      INSTALL
>>>>>> /tmp/makepkg/linux-cachyos-rc-autofdo/pkg/linux-cachyos-rc- 
>>>>>> autofdo/usr/lib/modules/6.12.0-rc5-5-cachyos-rc-autofdo/kernel/ 
>>>>>> crypto/cryptd.ko
>>>>>> make[2]: *** Waiting for unfinished jobs....
>>>>>> ```
>>>>>>
>>>>>>
>>>>>> This can be fixed with removed "INSTALL_MOD_STRIP=1" to the passed
>>>>>> parameters of module_install.
>>>>>>
>>>>>> This explicitly only happens, if a profile is passed - otherwise the
>>>>>> packaging works without problems.
>>>>>>
>>>>>> Regards,
>>>>>>
>>>>>> Peter Jung
>>>>>>
>