From: Nick Desaulniers <ndesaulniers@google.com>
To: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
Cc: Masahiro Yamada <masahiroy@kernel.org>,
Miguel Ojeda <ojeda@kernel.org>,
Linus Torvalds <torvalds@linux-foundation.org>,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
rust-for-linux@vger.kernel.org,
Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
Linux Doc Mailing List <linux-doc@vger.kernel.org>,
LKML <linux-kernel@vger.kernel.org>,
Alex Gaynor <alex.gaynor@gmail.com>,
Geoffrey Thomas <geofft@ldpreload.com>,
Finn Behrens <me@kloenk.de>,
Adam Bratschi-Kaye <ark.email@gmail.com>,
Wedson Almeida Filho <wedsonaf@google.com>,
Michael Ellerman <mpe@ellerman.id.au>
Subject: Re: [PATCH 04/13] Kbuild: Rust support
Date: Thu, 15 Apr 2021 11:03:22 -0700 [thread overview]
Message-ID: <CAKwvOdkyRkR0Jj5w5HWJ+o4YpOrLfTY1Vjho0bDn60fgRE-pkA@mail.gmail.com> (raw)
In-Reply-To: <CANiq72ksLeuL_uqoqbf3fhLP7M0j-7TdEvRDDmxThdmrEqD2Lw@mail.gmail.com>
On Wed, Apr 14, 2021 at 5:43 PM Miguel Ojeda
<miguel.ojeda.sandonis@gmail.com> wrote:
>
> On Thu, Apr 15, 2021 at 1:19 AM Nick Desaulniers
> <ndesaulniers@google.com> wrote:
> >
> > -Oz in clang typically generates larger kernel code than -Os; LLVM
> > seems to aggressively emit libcalls even when the setup for a call
> > would be larger than the inlined call itself. Is z smaller than s for
> > the existing rust examples?
>
> I will check if the `s`/`z` flags have the exact same semantics as
> they do in Clang, but as a quick test (quite late here, sorry!), yes,
> it seems `z` is smaller:
>
> text data bss dec hex filename
>
> 126568 8 104 126680 1eed8 drivers/android/rust_binder.o [s]
> 122923 8 104 123035 1e09b drivers/android/rust_binder.o [z]
>
> 212351 0 0 212351 33d7f rust/core.o [s]
> 207653 0 0 207653 32b25 rust/core.o [z]
cool, thanks for verifying. LGTM
> > This is a mess; who thought it would be a good idea to support
> > compiling the rust code at a different optimization level than the
> > rest of the C code in the kernel? Do we really need that flexibility
> > for Rust kernel code, or can we drop this feature?
>
> I did :P
>
> The idea is that, since it seemed to work out of the box when I tried,
> it could be nice to keep for debugging and for having another degree
> of freedom when testing the compiler/nightlies etc.
>
> Also, it is not intended for users, which is why I put it in the
> "hacking" menu -- users should still only modify the usual global
> option.
>
> However, it is indeed strange for the kernel and I don't mind dropping
> it if people want to see it out (one could still do it manually if
> needed...).
>
> (Related: from what I have been told, the kernel does not support
> lower levels in C just due to old problems with compilers; but those
> may be gone now).
IIRC the kernel (or at least x86_64 defconfig) cannot be built at -O0,
which is too bad if developers were myopically focused on build times.
It would have been nice to have something like
CONFIG_CC_OPTIMIZE_FOR_COMPILE_TIME to join
CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE and CONFIG_CC_OPTIMIZE_FOR_SIZE,
but maybe it's still possible to support one day. (¿Por qué no los
tres? Perhaps a false-trichotomy? Sorry, but those 3 are somewhat at
odds for compilation).
Until then, I don't see why we need to permit developers to express
such flexibility for just the Rust code, or have it differ from the
intent of the C code. Does it make sense to set RUST_OPT_LEVEL_3 and
CC_OPTIMIZE_FOR_SIZE? I doubt it. That doesn't seem like a development
feature, but a mistake. YAGNI. Instead developers should clarify
what they care about in terms of high level intent; if someone wants
to micromanage optimization level flags in their forks they don't need
a Kconfig to do it (they're either going to hack KBUILD_CFLAGS,
CFLAGS_*.o, or KCFLAGS), and there's probably better mechanisms for
fine-tooth precision of optimizing actually hot code or not via PGO
and AutoFDO.
https://lore.kernel.org/lkml/20210407211704.367039-1-morbo@google.com/
--
Thanks,
~Nick Desaulniers
next prev parent reply other threads:[~2021-04-15 18:03 UTC|newest]
Thread overview: 199+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-14 18:45 [PATCH 00/13] [RFC] Rust support ojeda
2021-04-14 18:45 ` [PATCH 01/13] kallsyms: Support "big" kernel symbols (2-byte lengths) ojeda
2021-04-14 19:44 ` Matthew Wilcox
2021-04-14 19:59 ` Miguel Ojeda
2021-04-14 18:45 ` [PATCH 02/13] kallsyms: Increase maximum kernel symbol length to 512 ojeda
2021-04-14 23:48 ` Nick Desaulniers
2021-04-14 18:45 ` [PATCH 03/13] Makefile: Generate CLANG_FLAGS even in GCC builds ojeda
2021-04-14 18:59 ` Nathan Chancellor
2021-04-15 10:18 ` Miguel Ojeda
2021-04-14 23:46 ` Nick Desaulniers
2021-04-15 0:47 ` Miguel Ojeda
2021-04-14 18:45 ` [PATCH 04/13] Kbuild: Rust support ojeda
2021-04-14 23:19 ` Nick Desaulniers
2021-04-15 0:43 ` Miguel Ojeda
2021-04-15 18:03 ` Nick Desaulniers [this message]
2021-04-16 12:23 ` Miguel Ojeda
2021-04-17 19:35 ` Masahiro Yamada
2021-04-16 13:38 ` Peter Zijlstra
2021-04-16 17:05 ` Linus Torvalds
2021-04-16 17:47 ` Miguel Ojeda
2021-04-16 18:09 ` Al Viro
2021-04-16 18:57 ` Miguel Ojeda
2021-04-16 20:22 ` Willy Tarreau
2021-04-16 20:34 ` Connor Kuehl
2021-04-16 20:58 ` Willy Tarreau
2021-04-16 21:39 ` Miguel Ojeda
2021-04-16 22:04 ` Willy Tarreau
2021-04-16 22:45 ` Al Viro
2021-04-16 23:46 ` Miguel Ojeda
2021-04-17 4:24 ` Willy Tarreau
2021-04-17 15:38 ` Miguel Ojeda
2021-04-16 21:19 ` Miguel Ojeda
2021-04-16 17:34 ` Miguel Ojeda
2021-04-19 19:58 ` David Sterba
2021-04-19 20:17 ` Matthew Wilcox
2021-04-19 21:03 ` Miguel Ojeda
2021-04-19 20:54 ` Miguel Ojeda
2021-04-14 18:45 ` [PATCH 05/13] Rust: Compiler builtins crate ojeda
2021-04-14 19:19 ` Linus Torvalds
2021-04-14 19:34 ` Miguel Ojeda
2021-04-14 18:45 ` [PATCH 06/13] Rust: Module crate ojeda
2021-04-14 18:45 ` [PATCH 08/13] Rust: Export generated symbols ojeda
2021-04-14 18:46 ` [PATCH 09/13] Samples: Rust examples ojeda
2021-04-14 19:34 ` Linus Torvalds
2021-04-14 19:42 ` Miguel Ojeda
2021-04-14 19:49 ` Matthew Wilcox
2021-04-16 11:46 ` Andrej Shadura
2021-04-14 23:24 ` Nick Desaulniers
2021-04-15 7:10 ` Greg Kroah-Hartman
2021-04-15 7:39 ` Nick Desaulniers
2021-04-15 12:42 ` Miguel Ojeda
2021-04-16 13:07 ` Sven Van Asbroeck
2021-04-16 13:20 ` Greg Kroah-Hartman
2021-04-14 18:46 ` [PATCH 10/13] Documentation: Rust general information ojeda
2021-04-14 22:17 ` Nick Desaulniers
2021-04-14 23:34 ` Miguel Ojeda
2021-04-14 18:46 ` [PATCH 11/13] MAINTAINERS: Rust ojeda
2021-04-14 21:55 ` Nick Desaulniers
2021-04-14 22:02 ` Miguel Ojeda
2021-04-14 22:36 ` Nick Desaulniers
2021-04-14 18:46 ` [PATCH 12/13] Rust: add abstractions for Binder (WIP) ojeda
[not found] ` <20210414184604.23473-8-ojeda@kernel.org>
2021-04-14 19:31 ` [PATCH 07/13] Rust: Kernel crate Linus Torvalds
2021-04-14 19:50 ` Miguel Ojeda
2021-04-14 19:44 ` [PATCH 00/13] [RFC] Rust support Linus Torvalds
2021-04-14 20:20 ` Miguel Ojeda
2021-04-15 1:38 ` Kees Cook
2021-04-15 8:26 ` David Laight
2021-04-15 18:08 ` Kees Cook
2021-04-15 12:39 ` Miguel Ojeda
2021-04-14 20:09 ` Matthew Wilcox
2021-04-14 20:21 ` Linus Torvalds
2021-04-14 20:35 ` Josh Triplett
2021-04-14 22:08 ` David Laight
2021-04-14 20:29 ` Miguel Ojeda
2021-04-18 15:31 ` Wedson Almeida Filho
2021-04-15 0:22 ` Nick Desaulniers
2021-04-15 10:05 ` Miguel Ojeda
2021-04-15 18:58 ` Peter Zijlstra
2021-04-16 2:22 ` Wedson Almeida Filho
2021-04-16 4:25 ` Al Viro
2021-04-16 5:02 ` Wedson Almeida Filho
2021-04-16 5:39 ` Paul Zimmerman
2021-04-16 7:46 ` Peter Zijlstra
2021-04-16 7:09 ` Peter Zijlstra
2021-04-17 5:23 ` comex
2021-04-17 12:46 ` David Laight
2021-04-17 14:51 ` Paolo Bonzini
2021-04-19 7:32 ` Peter Zijlstra
2021-04-19 7:53 ` Paolo Bonzini
2021-04-19 8:26 ` Peter Zijlstra
2021-04-19 8:35 ` Peter Zijlstra
2021-04-19 9:02 ` Paolo Bonzini
2021-04-19 9:36 ` Peter Zijlstra
2021-04-19 9:40 ` Paolo Bonzini
2021-04-19 11:01 ` Will Deacon
2021-04-19 17:14 ` Linus Torvalds
2021-04-19 18:38 ` Paolo Bonzini
2021-04-19 18:50 ` Linus Torvalds
2021-04-22 10:03 ` Linus Walleij
2021-04-22 14:09 ` David Laight
2021-04-22 15:24 ` Wedson Almeida Filho
2021-04-26 0:18 ` Linus Walleij
2021-04-26 14:26 ` Miguel Ojeda
2021-04-26 14:40 ` Wedson Almeida Filho
2021-04-26 16:03 ` Miguel Ojeda
2021-04-27 10:54 ` Linus Walleij
2021-04-27 11:13 ` Robin Randhawa
2021-04-29 1:52 ` Wedson Almeida Filho
2021-04-26 18:01 ` Miguel Ojeda
2021-04-22 21:28 ` Miguel Ojeda
2021-04-26 0:31 ` Linus Walleij
2021-04-26 18:18 ` Miguel Ojeda
2021-04-27 11:13 ` Linus Walleij
2021-04-28 2:51 ` Kyle Strand
2021-04-28 3:10 ` Miguel Ojeda
2021-05-04 21:21 ` Linus Walleij
2021-05-04 23:30 ` Miguel Ojeda
2021-05-05 11:34 ` Linus Walleij
2021-05-05 14:17 ` Miguel Ojeda
2021-05-05 15:13 ` Enrico Weigelt, metux IT consult
2021-05-06 12:47 ` Linus Walleij
2021-05-07 18:23 ` Miguel Ojeda
2021-04-16 4:27 ` Boqun Feng
2021-04-16 6:04 ` Nick Desaulniers
2021-04-16 18:47 ` Paul E. McKenney
2021-04-19 20:35 ` Nick Desaulniers
2021-04-19 21:37 ` Paul E. McKenney
2021-04-19 22:03 ` Miguel Ojeda
2021-04-16 20:48 ` Josh Triplett
2021-04-16 8:16 ` Michal Kubecek
2021-04-16 9:29 ` Willy Tarreau
2021-04-16 11:24 ` Peter Zijlstra
2021-04-16 13:07 ` Wedson Almeida Filho
2021-04-16 14:19 ` Peter Zijlstra
2021-04-16 15:04 ` Miguel Ojeda
2021-04-16 15:43 ` Peter Zijlstra
2021-04-16 16:21 ` Miguel Ojeda
2021-04-16 15:33 ` Wedson Almeida Filho
2021-04-16 16:14 ` Willy Tarreau
2021-04-16 17:10 ` Miguel Ojeda
2021-04-16 17:18 ` Peter Zijlstra
2021-04-16 18:08 ` Matthew Wilcox
2021-04-17 11:17 ` Peter Zijlstra
2021-04-17 11:46 ` Willy Tarreau
2021-04-17 14:24 ` Peter Zijlstra
2021-04-17 14:36 ` Willy Tarreau
2021-04-17 13:46 ` David Laight
2021-04-16 17:37 ` Willy Tarreau
2021-04-16 17:46 ` Connor Kuehl
2021-04-20 0:24 ` Nick Desaulniers
2021-04-20 3:47 ` Willy Tarreau
2021-04-20 5:56 ` Greg Kroah-Hartman
2021-04-20 6:16 ` Willy Tarreau
2021-04-29 15:38 ` peter enderborg
2021-04-17 13:53 ` Wedson Almeida Filho
2021-04-17 14:21 ` Willy Tarreau
2021-04-17 15:23 ` Miguel Ojeda
2021-04-18 15:51 ` Wedson Almeida Filho
2021-04-17 12:41 ` David Laight
2021-04-17 13:01 ` Wedson Almeida Filho
2021-04-16 15:03 ` Matthew Wilcox
2021-04-17 13:29 ` Wedson Almeida Filho
2021-04-16 15:58 ` Theodore Ts'o
2021-04-16 16:21 ` Wedson Almeida Filho
2021-04-17 15:11 ` Paolo Bonzini
2021-04-16 14:21 ` Miguel Ojeda
2021-04-17 20:42 ` Richard Weinberger
2021-04-28 18:34 ` Mariusz Ceier
2021-04-28 20:25 ` Nick Desaulniers
2021-04-28 21:21 ` David Laight
2021-04-29 11:14 ` Kajetan Puchalski
2021-04-29 11:25 ` Kajetan Puchalski
2021-04-29 14:06 ` Mariusz Ceier
2021-04-29 14:13 ` Sven Van Asbroeck
2021-04-29 14:26 ` Willy Tarreau
2021-04-29 15:06 ` Al Viro
2021-04-29 16:09 ` Mariusz Ceier
2021-04-30 6:39 ` Thomas Schoebel-Theuer
2021-04-30 8:30 ` David Laight
2021-05-05 13:58 ` Enrico Weigelt, metux IT consult
2021-05-05 14:41 ` Miguel Ojeda
2022-06-20 15:11 ` Olliver Schinagl
2022-06-27 17:44 ` Miguel Ojeda
2022-07-18 6:56 ` Olliver Schinagl
2022-07-20 19:23 ` Miguel Ojeda
2022-07-20 20:21 ` Nicolas Pitre
2022-07-27 7:47 ` Olliver Schinagl
2022-07-27 13:32 ` Nicolas Pitre
2022-07-27 8:05 ` Olliver Schinagl
2022-07-28 10:21 ` Gary Guo
2022-07-28 12:09 ` Greg Kroah-Hartman
2022-07-28 12:28 ` Gary Guo
2022-07-28 20:45 ` Olliver Schinagl
2022-07-29 8:04 ` Greg Kroah-Hartman
2022-07-28 20:43 ` Olliver Schinagl
2022-10-15 14:16 ` Olliver Schinagl
2022-10-16 1:44 ` Bagas Sanjaya
2022-10-16 1:50 ` Bagas Sanjaya
2022-10-16 13:23 ` Josh Triplett
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