Message-ID: <20241020033116.GA3653827@thelio-3990X>
Date: Sat, 19 Oct 2024 20:31:16 -0700
From: Nathan Chancellor <nathan@...nel.org>
To: Rong Xu <xur@...gle.com>, Andrew Morton <akpm@...ux-foundation.org>,
	Masahiro Yamada <masahiroy@...nel.org>
Cc: Alice Ryhl <aliceryhl@...gle.com>, 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>, Han Shen <shenhan@...gle.com>,
	Heiko Carstens <hca@...ux.ibm.com>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
	Jann Horn <jannh@...gle.com>, Jonathan Corbet <corbet@....net>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	Juergen Gross <jgross@...e.com>,
	Justin Stitt <justinstitt@...gle.com>, Kees Cook <kees@...nel.org>,
	"Mike Rapoport (IBM)" <rppt@...nel.org>,
	Nick Desaulniers <ndesaulniers@...gle.com>,
	Nicolas Schier <nicolas@...sle.eu>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Sami Tolvanen <samitolvanen@...gle.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Wei Yang <richard.weiyang@...il.com>, workflows@...r.kernel.org,
	Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
	Maksim Panchenko <max4bolt@...il.com>, x86@...nel.org,
	linux-arch@...r.kernel.org, linux-doc@...r.kernel.org,
	linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org,
	llvm@...ts.linux.dev
Subject: Re: [PATCH v4 0/6] Add AutoFDO and Propeller support for Clang build

Hi Masahiro and Andrew,

Top posting only for visibility. Would it make more sense to have this
land via the Kbuild tree or -mm? The core of the series really touches
Kbuild and I think the x86 stuff can just land with Acks, unless the
-tip folks feel differently. I would like Rong to have a relatively
clear path forward to mainline once the requisite review and testing has
accomplished, which requires a shepherd :)

Cheers,
Nathan

On Mon, Oct 14, 2024 at 02:33:34PM -0700, Rong Xu 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] Change the subsection ordering when -ffunction-sections is enabled
> 
> [P 4] Enable –ffunction-sections for the AutoFDO build
> 
> [P 5] Enable Machine Function Split (MFS) optimization for AutoFDO
> 
> [P 6] Add Propeller configuration to the kernel build
> 
> Patch 1 provides basic AutoFDO build support. Patches 2 to 5 further
> enhance the performance of AutoFDO builds and are functionally dependent
> on Patch 1. Patch 6 enables support for Propeller and is dependent on
> patch 2 and patch 3.
> 
> 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.
> 
> 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
...
> Rong Xu (6):
>   Add AutoFDO support for Clang build
>   objtool: Fix unreachable instruction warnings for weak funcitons
>   Change the symbols order when --ffuntion-sections is enabled
>   AutoFDO: Enable -ffunction-sections for the AutoFDO build
>   AutoFDO: Enable machine function split optimization for AutoFDO
>   Add Propeller configuration for kernel build.
> 
>  Documentation/dev-tools/autofdo.rst   | 165 ++++++++++++++++++++++++++
>  Documentation/dev-tools/index.rst     |   2 +
>  Documentation/dev-tools/propeller.rst | 161 +++++++++++++++++++++++++
>  MAINTAINERS                           |  14 +++
>  Makefile                              |   2 +
>  arch/Kconfig                          |  42 +++++++
>  arch/x86/Kconfig                      |   2 +
>  arch/x86/kernel/vmlinux.lds.S         |   4 +
>  include/asm-generic/vmlinux.lds.h     |  54 +++++++--
>  scripts/Makefile.autofdo              |  25 ++++
>  scripts/Makefile.lib                  |  20 ++++
>  scripts/Makefile.propeller            |  28 +++++
>  tools/objtool/check.c                 |   2 +
>  tools/objtool/elf.c                   |  15 ++-
>  14 files changed, 524 insertions(+), 12 deletions(-)
>  create mode 100644 Documentation/dev-tools/autofdo.rst
>  create mode 100644 Documentation/dev-tools/propeller.rst
>  create mode 100644 scripts/Makefile.autofdo
>  create mode 100644 scripts/Makefile.propeller
> 
> 
> base-commit: eb952c47d154ba2aac794b99c66c3c45eb4cc4ec
> -- 
> 2.47.0.rc1.288.g06298d1525-goog
> 

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