Message-ID: <449fddd2-342f-48cc-9a11-8a34814f1284@cachyos.org>
Date: Tue, 5 Nov 2024 15:33:39 +0100
From: Peter Jung <ptr1337@...hyos.org>
To: Rong Xu <xur@...gle.com>, Han Shen <shenhan@...gle.com>
Cc: Alice Ryhl <aliceryhl@...gle.com>,
 Andrew Morton <akpm@...ux-foundation.org>, Arnd Bergmann <arnd@...db.de>,
 Bill Wendling <morbo@...gle.com>, Borislav Petkov <bp@...en8.de>,
 Breno Leitao <leitao@...ian.org>, Brian Gerst <brgerst@...il.com>,
 Dave Hansen <dave.hansen@...ux.intel.com>, David Li <davidxl@...gle.com>,
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 Jonathan Corbet <corbet@....net>, Josh Poimboeuf <jpoimboe@...nel.org>,
 Juergen Gross <jgross@...e.com>, Justin Stitt <justinstitt@...gle.com>,
 Kees Cook <kees@...nel.org>, Masahiro Yamada <masahiroy@...nel.org>,
 "Mike Rapoport (IBM)" <rppt@...nel.org>,
 Nathan Chancellor <nathan@...nel.org>,
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 <tglx@...utronix.de>, Wei Yang <richard.weiyang@...il.com>,
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 <davem@...emloft.net>, Andreas Larsson <andreas@...sler.com>,
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 Sriraman Tallam <tmsriram@...gle.com>, Stephane Eranian
 <eranian@...gle.com>, x86@...nel.org, linux-arch@...r.kernel.org,
 sparclinux@...r.kernel.org, linux-doc@...r.kernel.org,
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 llvm@...ts.linux.dev
Subject: Re: [PATCH v7 1/7] Add AutoFDO support for Clang build

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@...gle.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=$pathtoprofile passed)
>>
>> I see. You also turned on ThinLTO, which I didn't, so the profile was
>> only used during compilation, not passed to lld.
>>
>> Thanks,
>> Han
>>
>> On Mon, Nov 4, 2024 at 9:31 AM Peter Jung<ptr1337@...hyos.org> wrote:
>>> Hi Han,
>>>
>>> I have tested your provided method, but the AutoFDO profile (lld does
>>> not get lto-sample-profile=$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@...hyos.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@...gle.com>
>>>>>>> Signed-off-by: Han Shen<shenhan@...gle.com>
>>>>>>> Signed-off-by: Rong Xu<xur@...gle.com>
>>>>>>> Suggested-by: Sriraman Tallam<tmsriram@...gle.com>
>>>>>>> Suggested-by: Krzysztof Pszeniczny<kpszeniczny@...gle.com>
>>>>>>> Suggested-by: Nick Desaulniers<ndesaulniers@...gle.com>
>>>>>>> Suggested-by: Stephane Eranian<eranian@...gle.com>
>>>>>>> Tested-by: Yonghong Song<yonghong.song@...ux.dev>
>>>>>>> Tested-by: Yabin Cui<yabinc@...gle.com>
>>>>>>> Tested-by: Nathan Chancellor<nathan@...nel.org>
>>>>>>> Reviewed-by: Kees Cook<kees@...nel.org>
>>>>>> Tested-by: Peter Jung<ptr1337@...hyos.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
>>>>>

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