Ping. at 1:15 PM, Nadav Amit <namit@xxxxxxxxxx> 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@xxxxxxxxxxxxx> > Cc: Sam Ravnborg <sam@xxxxxxxxxxxx> > Cc: Alok Kataria <akataria@xxxxxxxxxx> > Cc: Christopher Li <sparse@xxxxxxxxxxx> > Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> > Cc: "H. Peter Anvin" <hpa@xxxxxxxxx> > Cc: Ingo Molnar <mingo@xxxxxxxxxx> > Cc: Jan Beulich <JBeulich@xxxxxxxx> > Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx> > Cc: Juergen Gross <jgross@xxxxxxxx> > Cc: Kate Stewart <kstewart@xxxxxxxxxxxxxxxxxxx> > Cc: Kees Cook <keescook@xxxxxxxxxxxx> > Cc: linux-sparse@xxxxxxxxxxxxxxx > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> > Cc: Philippe Ombredanne <pombredanne@xxxxxxxx> > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> > Cc: virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx > Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> > Cc: x86@xxxxxxxxxx > Cc: Chris Zankel <chris@xxxxxxxxxx> > Cc: Max Filippov <jcmvbkbc@xxxxxxxxx> > Cc: linux-xtensa@xxxxxxxxxxxxxxxx > > 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
