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From:   Bill Wendling <morbo@google.com>
Date:   Wed, 13 Jan 2021 13:59:57 -0800
Message-ID: <CAGG=3QVK9KvxDi7b51Fyh=NLKVQka8ZxJeWzchaiDM=78_X3Tw@mail.gmail.com>
Subject: Re: [PATCH v4] pgo: add clang's Profile Guided Optimization infrastructure
To:     Nathan Chancellor <natechancellor@gmail.com>
Cc:     Jonathan Corbet <corbet@lwn.net>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Linux Doc Mailing List <linux-doc@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Sami Tolvanen <samitolvanen@google.com>
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On Wed, Jan 13, 2021 at 12:55 PM Nathan Chancellor
<natechancellor@gmail.com> wrote:
>
> Hi Bill,
>
> On Tue, Jan 12, 2021 at 10:19:58PM -0800, Bill Wendling wrote:
> > From: Sami Tolvanen <samitolvanen@google.com>
> >
> > Enable the use of clang's Profile-Guided Optimization[1]. To generate a
> > profile, the kernel is instrumented with PGO counters, a representative
> > workload is run, and the raw profile data is collected from
> > /sys/kernel/debug/pgo/profraw.
> >
> > The raw profile data must be processed by clang's "llvm-profdata" tool
> > before it can be used during recompilation:
> >
> >   $ cp /sys/kernel/debug/pgo/profraw vmlinux.profraw
> >   $ llvm-profdata merge --output=vmlinux.profdata vmlinux.profraw
> >
> > Multiple raw profiles may be merged during this step.
> >
> > The data can now be used by the compiler:
> >
> >   $ make LLVM=1 KCFLAGS=-fprofile-use=vmlinux.profdata ...
> >
> > This initial submission is restricted to x86, as that's the platform we
> > know works. This restriction can be lifted once other platforms have
> > been verified to work with PGO.
> >
> > Note that this method of profiling the kernel is clang-native, unlike
> > the clang support in kernel/gcov.
> >
> > [1] https://clang.llvm.org/docs/UsersManual.html#profile-guided-optimization
> >
> > Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
> > Co-developed-by: Bill Wendling <morbo@google.com>
> > Signed-off-by: Bill Wendling <morbo@google.com>
> > Change-Id: Ic78e69c682286d3a44c4549a0138578c98138b77
>
> Small nit: This should be removed.
>
Grrr....The git hook keeps adding it in there. :-(

> I applied this patch on top of v5.11-rc3, built it with LLVM 12
> (f1d5cbbdee5526bc86eac0a5652b115d9bc158e5 + D94470) with Microsoft's
> WSL 5.4 config [1] + CONFIG_PGO_CLANG=y, and ran it on WSL2.
>
> $ zgrep PGO /proc/config.gz
> # Profile Guided Optimization (PGO) (EXPERIMENTAL)
> CONFIG_ARCH_SUPPORTS_PGO_CLANG=y
> CONFIG_PGO_CLANG=y
> # end of Profile Guided Optimization (PGO) (EXPERIMENTAL)
>
> However, I see an issue with actually using the data:
>
> $ sudo -s
> # mount -t debugfs none /sys/kernel/debug
> # cp -a /sys/kernel/debug/pgo/profraw vmlinux.profraw
> # chown nathan:nathan vmlinux.profraw
> # exit
> $ tc-build/build/llvm/stage1/bin/llvm-profdata merge --output=vmlinux.profdata vmlinux.profraw
> warning: vmlinux.profraw: Invalid instrumentation profile data (bad magic)
> error: No profiles could be merged.
>
> Am I holding it wrong? :) Note, this is virtualized, I do not have any
> "real" x86 hardware that I can afford to test on right now.
>
> [1]: https://github.com/microsoft/WSL2-Linux-Kernel/raw/linux-msft-wsl-5.4.y/Microsoft/config-wsl
>
Could you send me the vmlinux.profraw file? (Don't CC this list.)

-bw