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SubjectRe: [PATCH v7 00/10] x86: macrofying inline asm for better compilation
Date
Ping.

at 1:15 PM, Nadav Amit <namit@vmware.com> wrote:

> This patch-set deals with an interesting yet stupid problem: kernel code
> that does not get inlined despite its simplicity. There are several
> causes for this behavior: "cold" attribute on __init, different function
> optimization levels; conditional constant computations based on
> __builtin_constant_p(); and finally large inline assembly blocks.
>
> This patch-set deals with the inline assembly problem. I separated these
> patches from the others (that were sent in the RFC) for easier
> inclusion. I also separated the removal of unnecessary new-lines which
> would be sent separately.
>
> The problem with inline assembly is that inline assembly is often used
> by the kernel for things that are other than code - for example,
> assembly directives and data. GCC however is oblivious to the content of
> the blocks and assumes their cost in space and time is proportional to
> the number of the perceived assembly "instruction", according to the
> number of newlines and semicolons. Alternatives, paravirt and other
> mechanisms are affected, causing code not to be inlined, and degrading
> compilation quality in general.
>
> The solution that this patch-set carries for this problem is to create
> an assembly macro, and then call it from the inline assembly block. As
> a result, the compiler sees a single "instruction" and assigns the more
> appropriate cost to the code.
>
> To avoid uglification of the code, as many noted, the macros are first
> precompiled into an assembly file, which is later assembled together
> with the C files. This also enables to avoid duplicate implementation
> that was set before for the asm and C code. This can be seen in the
> exception table changes.
>
> Overall this patch-set slightly increases the kernel size (my build was
> done using my Ubuntu 18.04 config + localyesconfig for the record):
>
> text data bss dec hex filename
> 18140829 10224724 2957312 31322865 1ddf2f1 ./vmlinux before
> 18163608 10227348 2957312 31348268 1de562c ./vmlinux after (+0.1%)
>
> The number of static functions in the image is reduced by 379, but
> actually inlining is even better, which does not always shows in these
> numbers: a function may be inlined causing the calling function not to
> be inlined.
>
> I ran some limited number of benchmarks, and in general the performance
> impact is not very notable. You can still see >10 cycles shaved off some
> syscalls that manipulate page-tables (e.g., mprotect()), in which
> paravirt caused many functions not to be inlined. In addition this
> patch-set can prevent issues such as [1], and improves code readability
> and maintainability.
>
> [1] https://patchwork.kernel.org/patch/10450037/
>
> v6->v7: * Fix context switch tracking (Ingo)
> * Fix xtensa build error (Ingo)
> * Rebase on 4.18-rc8
>
> v5->v6: * Removing more code from jump-labels (PeterZ)
> * Fix build issue on i386 (0-day, PeterZ)
>
> v4->v5: * Makefile fixes (Masahiro, Sam)
>
> v3->v4: * Changed naming of macros in 2 patches (PeterZ)
> * Minor cleanup of the paravirt patch
>
> v2->v3: * Several build issues resolved (0-day)
> * Wrong comments fix (Josh)
> * Change asm vs C order in refcount (Kees)
>
> v1->v2: * Compiling the macros into a separate .s file, improving
> readability (Linus)
> * Improving assembly formatting, applying most of the comments
> according to my judgment (Jan)
> * Adding exception-table, cpufeature and jump-labels
> * Removing new-line cleanup; to be submitted separately
>
> Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
> Cc: Sam Ravnborg <sam@ravnborg.org>
> Cc: Alok Kataria <akataria@vmware.com>
> Cc: Christopher Li <sparse@chrisli.org>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: "H. Peter Anvin" <hpa@zytor.com>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Jan Beulich <JBeulich@suse.com>
> Cc: Josh Poimboeuf <jpoimboe@redhat.com>
> Cc: Juergen Gross <jgross@suse.com>
> Cc: Kate Stewart <kstewart@linuxfoundation.org>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: linux-sparse@vger.kernel.org
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Philippe Ombredanne <pombredanne@nexb.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: virtualization@lists.linux-foundation.org
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: x86@kernel.org
> Cc: Chris Zankel <chris@zankel.net>
> Cc: Max Filippov <jcmvbkbc@gmail.com>
> Cc: linux-xtensa@linux-xtensa.org
>
> Nadav Amit (10):
> xtensa: defining LINKER_SCRIPT for the linker script
> Makefile: Prepare for using macros for inline asm
> x86: objtool: use asm macro for better compiler decisions
> x86: refcount: prevent gcc distortions
> x86: alternatives: macrofy locks for better inlining
> x86: bug: prevent gcc distortions
> x86: prevent inline distortion by paravirt ops
> x86: extable: use macros instead of inline assembly
> x86: cpufeature: use macros instead of inline assembly
> x86: jump-labels: use macros instead of inline assembly
>
> Makefile | 9 ++-
> arch/x86/Makefile | 11 ++-
> arch/x86/entry/calling.h | 2 +-
> arch/x86/include/asm/alternative-asm.h | 20 ++++--
> arch/x86/include/asm/alternative.h | 11 +--
> arch/x86/include/asm/asm.h | 61 +++++++---------
> arch/x86/include/asm/bug.h | 98 +++++++++++++++-----------
> arch/x86/include/asm/cpufeature.h | 82 ++++++++++++---------
> arch/x86/include/asm/jump_label.h | 77 ++++++++------------
> arch/x86/include/asm/paravirt_types.h | 56 +++++++--------
> arch/x86/include/asm/refcount.h | 74 +++++++++++--------
> arch/x86/kernel/macros.S | 16 +++++
> arch/xtensa/kernel/Makefile | 4 +-
> include/asm-generic/bug.h | 8 +--
> include/linux/compiler.h | 56 +++++++++++----
> scripts/Kbuild.include | 4 +-
> scripts/mod/Makefile | 2 +
> 17 files changed, 333 insertions(+), 258 deletions(-)
> create mode 100644 arch/x86/kernel/macros.S
>
> --
> 2.17.1


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