Message-ID: <CAK7LNARpXOm1R_BVsH-fSC4ZzQqstHj0amzX8fu6=USwTD91Tw@mail.gmail.com>
Date: Thu, 7 Nov 2024 23:57:20 +0900
From: Masahiro Yamada <masahiroy@...nel.org>
To: Rong Xu <xur@...gle.com>
Cc: Alice Ryhl <aliceryhl@...gle.com>, Andrew Morton <akpm@...ux-foundation.org>, 
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	llvm@...ts.linux.dev
Subject: Re: [PATCH v7 0/7] Add AutoFDO and Propeller support for Clang build

On Thu, Nov 7, 2024 at 4:00 AM Rong Xu <xur@...gle.com> wrote:
>
> On Wed, Nov 6, 2024 at 8:09 AM Masahiro Yamada <masahiroy@...nel.org> wrote:
> >
> > On Sun, Nov 3, 2024 at 2:51 AM Rong Xu <xur@...gle.com> wrote:
> > >
> > > Hi,
> > >
> > > This patch series is to integrate AutoFDO and Propeller support into
> > > the Linux kernel. AutoFDO is a profile-guided optimization technique
> > > that leverages hardware sampling to enhance binary performance.
> > > Unlike Instrumentation-based FDO (iFDO), AutoFDO offers a user-friendly
> > > and straightforward application process. While iFDO generally yields
> > > superior profile quality and performance, our findings reveal that
> > > AutoFDO achieves remarkable effectiveness, bringing performance close
> > > to iFDO for benchmark applications.
> > >
> > > Propeller is a profile-guided, post-link optimizer that improves
> > > the performance of large-scale applications compiled with LLVM. It
> > > operates by relinking the binary based on an additional round of runtime
> > > profiles, enabling precise optimizations that are not possible at
> > > compile time.  Similar to AutoFDO, Propeller too utilizes hardware
> > > sampling to collect profiles and apply post-link optimizations to improve
> > > the benchmark’s performance over and above AutoFDO.
> > >
> > > Our empirical data demonstrates significant performance improvements
> > > with AutoFDO and Propeller, up to 10% on microbenchmarks and up to 5%
> > > on large warehouse-scale benchmarks. This makes a strong case for their
> > > inclusion as supported features in the upstream kernel.
> > >
> > > Background
> > >
> > > A significant fraction of fleet processing cycles (excluding idle time)
> > > from data center workloads are attributable to the kernel. Ware-house
> > > scale workloads maximize performance by optimizing the production kernel
> > > using iFDO (a.k.a instrumented PGO, Profile Guided Optimization).
> > >
> > > iFDO can significantly enhance application performance but its use
> > > within the kernel has raised concerns. AutoFDO is a variant of FDO that
> > > uses the hardware’s Performance Monitoring Unit (PMU) to collect
> > > profiling data. While AutoFDO typically yields smaller performance
> > > gains than iFDO, it presents unique benefits for optimizing kernels.
> > >
> > > AutoFDO eliminates the need for instrumented kernels, allowing a single
> > > optimized kernel to serve both execution and profile collection. It also
> > > minimizes slowdown during profile collection, potentially yielding
> > > higher-fidelity profiling, especially for time-sensitive code, compared
> > > to iFDO. Additionally, AutoFDO profiles can be obtained from production
> > > environments via the hardware’s PMU whereas iFDO profiles require
> > > carefully curated load tests that are representative of real-world
> > > traffic.
> > >
> > > AutoFDO facilitates profile collection across diverse targets.
> > > Preliminary studies indicate significant variation in kernel hot spots
> > > within Google’s infrastructure, suggesting potential performance gains
> > > through target-specific kernel customization.
> > >
> > > Furthermore, other advanced compiler optimization techniques, including
> > > ThinLTO and Propeller can be stacked on top of AutoFDO, similar to iFDO.
> > > ThinLTO achieves better runtime performance through whole-program
> > > analysis and cross module optimizations. The main difference between
> > > traditional LTO and ThinLTO is that the latter is scalable in time and
> > > memory.
> > >
> > > This patch series adds AutoFDO and Propeller support to the kernel. The
> > > actual solution comes in six parts:
> > >
> > > [P 1] Add the build support for using AutoFDO in Clang
> > >
> > >       Add the basic support for AutoFDO build and provide the
> > >       instructions for using AutoFDO.
> > >
> > > [P 2] Fix objtool for bogus warnings when -ffunction-sections is enabled
> > >
> > > [P 3] Adjust symbol ordering in text output sections
> > >
> > > [P 4] Add markers for text_unlikely and text_hot sections
> > >
> > > [P 5] Enable –ffunction-sections for the AutoFDO build
> > >
> > > [P 6] Enable Machine Function Split (MFS) optimization for AutoFDO
> > >
> > > [P 7] Add Propeller configuration to the kernel build
> > >
> > > Patch 1 provides basic AutoFDO build support. Patches 2 to 6 further
> > > enhance the performance of AutoFDO builds and are functionally dependent
> > > on Patch 1. Patch 7 enables support for Propeller and is dependent on
> > > patch 2 to patch 4.
> > >
> > > Caveats
> > >
> > > AutoFDO is compatible with both GCC and Clang, but the patches in this
> > > series are exclusively applicable to LLVM 17 or newer for AutoFDO and
> > > LLVM 19 or newer for Propeller. For profile conversion, two different
> > > tools could be used, llvm_profgen or create_llvm_prof. llvm_profgen
> > > needs to be the LLVM 19 or newer, or just the LLVM trunk. Alternatively,
> > > create_llvm_prof v0.30.1 or newer can be used instead of llvm-profgen.
> > >
> > > Additionally, the build is only supported on x86 platforms equipped
> > > with PMU capabilities, such as LBR on Intel machines. More
> > > specifically:
> > >  * Intel platforms: works on every platform that supports LBR;
> > >    we have tested on Skylake.
> > >  * AMD platforms: tested on AMD Zen3 with the BRS feature. The kernel
> > >    needs to be configured with “CONFIG_PERF_EVENTS_AMD_BRS=y", To
> > >    check, use
> > >    $ cat /proc/cpuinfo | grep “ brs”
> > >    For the AMD Zen4, AMD LBRV2 is supported, but we suspect a bug with
> > >    AMD LBRv2 implementation in Genoa which blocks the usage.
> > >
> > > For ARM, we plan to send patches for SPE-based Propeller when
> > > AutoFDO for Arm is ready.
> > >
> > > Experiments and Results
> > >
> > > Experiments were conducted to compare the performance of AutoFDO-optimized
> > > kernel images (version 6.9.x) against default builds.. The evaluation
> > > encompassed both open source microbenchmarks and real-world production
> > > services from Google and Meta. The selected microbenchmarks included Neper,
> > > a network subsystem benchmark, and UnixBench which is a comprehensive suite
> > > for assessing various kernel operations.
> > >
> > > For Neper, AutoFDO optimization resulted in a 6.1% increase in throughput
> > > and a 10.6% reduction in latency. UnixBench saw a 2.2% improvement in its
> > > index score under low system load and a 2.6% improvement under high system
> > > load.
> > >
> > > For further details on the improvements observed in Google and Meta's
> > > production services, please refer to the LLVM discourse post:
> > > https://discourse.llvm.org/t/optimizing-the-linux-kernel-with-autofdo-including-thinlto-and-propeller/79108
> > >
> > > Thanks,
> > >
> > > Rong Xu and Han Shen
> >
> >
> > I applied this series to linux-kbuild.
> >
>
> Thanks for taking the patch!
>
> > As I mentioned before, I do not like #ifdef because
> > it hides (not fixes) issues only for default cases.
>
> We followed the suggestion and removed most of the #if (or #ifdef) in
> the linker script.
> I just checked: there are two #ifdef remaining:
> (1) in the propeller patch for .llvm_bb_addr_map
> (2) in linker script patch for arch/sparc/kernel/vmlinux.lds.S.
>
> I think it's likely safe to remove the checks for head_64.o in
> non-SPARC64 builds and .llvm_bb_addr_map symbols in non-propeller builds.
>
> SPARC64 builds should always produce head_64.o, and non-SPARC64
> builds shouldn't.
>
> Propeller builds always generate .llvm_bb_addr_map symbols, and the
> linker will omit the section if it's empty in non-propeller builds.
>
> Keeping the checks is harmless and might slightly reduce linker
> workload for matching.
> But If you'd prefer to remove them, I'm happy to provide a patch.


I am talking about the #ifdef in include/asm-generic/vmlinux.lds.h


Yeah, it is me who (reluctantly) accepted cb87481ee89d.

Now, the #ifdef has become a little more complicated.
The default case is safe, but there are hidden issues.

Some issues are easy to fix, so I sent some patches.
https://lore.kernel.org/linux-kbuild/20241106161445.189399-1-masahiroy@kernel.org/T/#t
https://lore.kernel.org/linux-kbuild/20241106161445.189399-1-masahiroy@kernel.org/T/#m4e4fa70386696e903b68d3fe1d7277e9a63fbefe
https://lore.kernel.org/linux-kbuild/20241107111519.GA15424@willie-the-truck/T/#mccf6d49ddd11c90dcc583d7a68934bb3311da880

For example, see e41f501d3912.

When CONFIG_LD_DEAD_CODE_DATA_ELIMINATION=y or
CONFIG_LTO_CLANG=y or CONFIG_AUTOFDO_CLANG=y or
CONFIG_PROPELLER_CLANG=y, the .text.startup sections
will go to TEXT_MAIN instead of INIT_TEXT.
This is not a fatal issue, but we cannot reuse memory for .text.startup
sections.

Removing the #ifdef (i.e. reverting cb87481ee89d) is more difficult
because we need to take a closer look at potential impacts for all
architectures.

I understood you did not want to take a risk to break random architectures,
so I decided to postpone the #ifdef issue and accept your patch set.

-- 
Best Regards
Masahiro Yamada

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