From: "Fāng-ruì Sòng" <maskray@google.com>
To: Bill Wendling <morbo@google.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>,
Nathan Chancellor <natechancellor@gmail.com>,
Nick Desaulniers <ndesaulniers@google.com>,
Sami Tolvanen <samitolvanen@google.com>
Subject: Re: [PATCH] pgo: add clang's Profile Guided Optimization infrastructure
Date: Mon, 11 Jan 2021 16:44:34 -0800 [thread overview]
Message-ID: <CAFP8O3+knLmtt1M-wqdr0XGDFNbocz+1gmfTVUTtwDk5mS1FEA@mail.gmail.com> (raw)
In-Reply-To: <CAGG=3QVs8dUaqcnuHYiaqccMhp7OmkxewZ_PAhAr96todNJhfQ@mail.gmail.com>
On Mon, Jan 11, 2021 at 4:38 PM Bill Wendling <morbo@google.com> wrote:
>
> On Mon, Jan 11, 2021 at 12:31 PM Fangrui Song <maskray@google.com> wrote:
> > On 2021-01-11, Bill Wendling wrote:
> > >On Mon, Jan 11, 2021 at 12:12 PM Fangrui Song <maskray@google.com> wrote:
> > >>
> > >> On 2021-01-11, 'Bill Wendling' via Clang Built Linux 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 be used either by the compiler if LTO isn't enabled:
> > >> >
> > >> > ... -fprofile-use=vmlinux.profdata ...
> > >> >
> > >> >or by LLD if LTO is enabled:
> > >> >
> > >> > ... -lto-cs-profile-file=vmlinux.profdata ...
> > >>
> > >> This LLD option does not exist.
> > >> LLD does have some `--lto-*` options but the `-lto-*` form is not supported
> > >> (it clashes with -l) https://reviews.llvm.org/D79371
> > >>
> > >That's strange. I've been using that option for years now. :-) Is this
> > >a recent change?
> >
> > The more frequently used options (specifyed by the clang driver) are
> > -plugin-opt=... (options implemented by LLVMgold.so).
> > `-lto-*` is rare.
> >
> > >> (There is an earlier -fprofile-instr-generate which does
> > >> instrumentation in Clang, but the option does not have broad usage.
> > >> It is used more for code coverage, not for optimization.
> > >> Noticeably, it does not even implement the Kirchhoff's current law
> > >> optimization)
> > >>
> > >Right. I've been told outside of this email that -fprofile-generate is
> > >the prefered flag to use.
> > >
> > >> -fprofile-use= is used by both regular PGO and context-sensitive PGO (CSPGO).
> > >>
> > >> clang -flto=thin -fprofile-use= passes -plugin-opt=cs-profile-path= to the linker.
> > >> For regular PGO, this option is effectively a no-op (confirmed with CSPGO main developer).
> > >>
> > >> So I think the "or by LLD if LTO is enabled:" part should be removed.
> > >
> > >But what if you specify the linking step explicitly? Linux doesn't
> > >call "clang" when linking, but "ld.lld".
> >
> > Regular PGO+LTO does not need -plugin-opt=cs-profile-path=
> > CSPGO+LTO needs it.
> > Because -fprofile-use= may be used by both, Clang driver adds it.
> > CSPGO is relevant in this this patch, so the linker option does not need to be mentioned.
>
> I'm still a bit confused. Are you saying that when clang uses
> `-flto=thin -fprofile-use=foo` that the profile file "foo" is embedded
> into the bitcode file so that when the linker's run it'll be used?
>
> This is the workflow:
>
> clang ... -fprofile-use=vmlinux.profdata ... -c -o foo.o foo.c
> clang ... -fprofile-use=vmlinux.profdata ... -c -o bar.o bar.c
> ld.lld ... <output file> foo.o bar.o
>
> Are you saying that we don't need to have
> "-plugin-opt=cs-profile-path=vmlinux.profdata" on the "ld.lld ..."
> line?
>
> -bw
The backend compile step -flto=thin -fprofile-use=foo has all the information.
-plugin-opt=cs-profile-path=vmlinux.profdata is not needed for regular PGO.
next prev parent reply other threads:[~2021-01-12 0:45 UTC|newest]
Thread overview: 116+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-01-11 8:18 [PATCH] pgo: add clang's Profile Guided Optimization infrastructure Bill Wendling
2021-01-11 8:39 ` Sedat Dilek
2021-01-11 8:42 ` Sedat Dilek
2021-01-11 9:17 ` Bill Wendling
2021-01-11 9:57 ` Sedat Dilek
2021-01-11 18:28 ` Nathan Chancellor
2021-01-11 20:12 ` Fangrui Song
2021-01-11 20:23 ` Bill Wendling
2021-01-11 20:31 ` Fangrui Song
2021-01-12 0:37 ` Bill Wendling
2021-01-12 0:44 ` Fāng-ruì Sòng [this message]
2021-01-11 21:04 ` Nathan Chancellor
2021-01-11 21:17 ` Nick Desaulniers
2021-01-11 21:32 ` Bill Wendling
2021-01-12 5:14 ` [PATCH v2] " Bill Wendling
2021-01-12 5:17 ` Sedat Dilek
2021-01-12 5:31 ` [PATCH v3] " Bill Wendling
[not found] ` <202101121755.pyYoRozB-lkp@intel.com>
2021-01-12 17:22 ` Nathan Chancellor
2021-01-13 6:19 ` [PATCH v4] " Bill Wendling
2021-01-13 20:55 ` Nathan Chancellor
2021-01-13 21:59 ` Bill Wendling
2021-01-14 4:07 ` Nick Desaulniers
2021-01-16 0:01 ` Nick Desaulniers
2021-01-16 0:13 ` Nick Desaulniers
2021-01-16 4:30 ` Sedat Dilek
2021-01-16 5:07 ` Sedat Dilek
2021-01-16 5:18 ` Sedat Dilek
2021-01-18 0:57 ` Sedat Dilek
2021-01-13 23:01 ` Nick Desaulniers
2021-01-16 9:43 ` [PATCH v5] " Bill Wendling
2021-01-16 17:38 ` Sedat Dilek
2021-01-16 18:36 ` Sedat Dilek
2021-01-16 20:23 ` Bill Wendling
2021-01-17 10:44 ` Sedat Dilek
2021-01-17 10:53 ` Sedat Dilek
2021-01-17 11:23 ` Sedat Dilek
2021-01-17 11:42 ` Sedat Dilek
2021-01-17 11:58 ` Sedat Dilek
[not found] ` <CA+icZUXmn15w=kSq2CZzQD5JggJw_9AEam=Sz13M0KpJ68MWZg@mail.gmail.com>
2021-01-17 17:42 ` Sedat Dilek
2021-01-17 20:34 ` Bill Wendling
[not found] ` <CA+icZUU1HihUFaEHzF69+01+Picg8aq6HAqHupxiRqyDGJ=Mpw@mail.gmail.com>
[not found] ` <CA+icZUUuzA5JEXyVzKbVX+T3xeOdRAU6-mntbo+VwwTxqmN7LA@mail.gmail.com>
2021-01-18 2:32 ` Bill Wendling
2021-01-18 12:39 ` Sedat Dilek
2021-01-18 17:26 ` Sedat Dilek
2021-01-18 21:56 ` Bill Wendling
2021-01-18 23:29 ` Sedat Dilek
2021-01-21 2:03 ` Sedat Dilek
2021-01-21 22:44 ` Sedat Dilek
2021-01-22 1:42 ` Nick Desaulniers
2021-01-22 1:49 ` Sedat Dilek
2021-01-22 1:52 ` Nick Desaulniers
2021-01-20 1:02 ` Nick Desaulniers
2021-01-21 0:51 ` Nick Desaulniers
2021-01-21 8:24 ` Bill Wendling
2021-01-21 8:24 ` [PATCH v6] " Bill Wendling
2021-01-21 10:34 ` Sedat Dilek
2021-01-22 1:44 ` Nick Desaulniers
2021-01-22 1:51 ` Sedat Dilek
2021-01-22 0:14 ` Sedat Dilek
2021-01-22 0:58 ` Sedat Dilek
2021-01-22 1:29 ` Nick Desaulniers
2021-01-22 10:11 ` [PATCH v7] " Bill Wendling
2021-01-22 11:31 ` Sedat Dilek
2021-01-22 18:41 ` Nick Desaulniers
[not found] ` <CA+icZUU=XfwqMcXYonQKcD4QgqTBW-mA+d_84b7cU2R3HYPOSQ@mail.gmail.com>
2021-01-28 21:12 ` Nick Desaulniers
2021-01-28 21:19 ` Sedat Dilek
2021-01-28 21:24 ` Nick Desaulniers
2021-01-28 21:39 ` Sedat Dilek
2021-01-29 7:43 ` Sedat Dilek
2021-01-29 21:48 ` Nick Desaulniers
2021-02-10 23:25 ` Bill Wendling
2021-02-22 21:52 ` Bill Wendling
2021-02-26 22:20 ` [PATCH v8] " Bill Wendling
2021-02-26 22:55 ` Bill Wendling
2021-02-28 18:52 ` Fangrui Song
2021-02-28 21:50 ` Fangrui Song
2021-01-12 17:37 ` [PATCH v2] " Nick Desaulniers
2021-01-12 17:45 ` Fāng-ruì Sòng
2021-01-21 2:21 ` [PATCH] " Sedat Dilek
2021-01-22 1:34 ` Nick Desaulniers
2021-01-22 1:43 ` Sedat Dilek
2021-04-07 21:17 ` [PATCH v9] " Bill Wendling
2021-04-07 21:22 ` Kees Cook
2021-04-07 21:44 ` Fāng-ruì Sòng
2021-04-07 21:47 ` Nathan Chancellor
2021-04-07 21:58 ` Bill Wendling
2021-05-19 21:37 ` Kees Cook
2021-05-22 23:51 ` Bill Wendling
2021-05-31 21:12 ` Nathan Chancellor
2021-06-01 17:31 ` Nick Desaulniers
2021-06-12 16:59 ` Peter Zijlstra
2021-06-12 17:25 ` Bill Wendling
2021-06-12 18:15 ` Peter Zijlstra
2021-06-12 19:10 ` Bill Wendling
2021-06-12 19:28 ` Bill Wendling
2021-06-12 20:25 ` Peter Zijlstra
2021-06-12 20:56 ` Bill Wendling
2021-06-12 22:47 ` Bill Wendling
2021-06-13 18:07 ` Bill Wendling
2021-06-14 9:43 ` Peter Zijlstra
2021-06-14 10:18 ` Peter Zijlstra
2021-06-14 7:51 ` Peter Zijlstra
2021-06-14 9:01 ` Peter Zijlstra
2021-06-14 9:39 ` Bill Wendling
2021-06-14 10:44 ` Peter Zijlstra
2021-06-14 11:41 ` Bill Wendling
2021-06-14 11:43 ` Bill Wendling
2021-06-14 14:16 ` Marco Elver
2021-06-14 15:26 ` Kees Cook
2021-06-14 15:35 ` Peter Zijlstra
2021-06-14 16:22 ` Kees Cook
2021-06-14 18:07 ` Nick Desaulniers
2021-06-14 20:49 ` Nick Desaulniers
2021-06-14 15:46 ` Peter Zijlstra
2021-06-14 16:03 ` Nick Desaulniers
2021-06-12 20:20 ` Fangrui Song
2021-06-12 20:31 ` Peter Zijlstra
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