On Tue, Oct 22, 2024 at 8:28 AM Rong Xu <xur@xxxxxxxxxx> wrote: > > On Sun, Oct 20, 2024 at 8:18 PM Masahiro Yamada <masahiroy@xxxxxxxxxx> wrote: > > > > On Tue, Oct 15, 2024 at 6:33 AM Rong Xu <xur@xxxxxxxxxx> wrote: > > > > > > Enable the machine function split optimization for AutoFDO in Clang. > > > > > > Machine function split (MFS) is a pass in the Clang compiler that > > > splits a function into hot and cold parts. The linker groups all > > > cold blocks across functions together. This decreases hot code > > > fragmentation and improves iCache and iTLB utilization. > > > > > > MFS requires a profile so this is enabled only for the AutoFDO builds. > > > > > > Co-developed-by: Han Shen <shenhan@xxxxxxxxxx> > > > Signed-off-by: Han Shen <shenhan@xxxxxxxxxx> > > > Signed-off-by: Rong Xu <xur@xxxxxxxxxx> > > > Suggested-by: Sriraman Tallam <tmsriram@xxxxxxxxxx> > > > Suggested-by: Krzysztof Pszeniczny <kpszeniczny@xxxxxxxxxx> > > > --- > > > include/asm-generic/vmlinux.lds.h | 6 ++++++ > > > scripts/Makefile.autofdo | 2 ++ > > > 2 files changed, 8 insertions(+) > > > > > > diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h > > > index ace617d1af9b..20e46c0917db 100644 > > > --- a/include/asm-generic/vmlinux.lds.h > > > +++ b/include/asm-generic/vmlinux.lds.h > > > @@ -565,9 +565,14 @@ defined(CONFIG_AUTOFDO_CLANG) > > > __unlikely_text_start = .; \ > > > *(.text.unlikely .text.unlikely.*) \ > > > __unlikely_text_end = .; > > > +#define TEXT_SPLIT \ > > > + __split_text_start = .; \ > > > + *(.text.split .text.split.[0-9a-zA-Z_]*) \ > > > + __split_text_end = .; > > > #else > > > #define TEXT_HOT *(.text.hot .text.hot.*) > > > #define TEXT_UNLIKELY *(.text.unlikely .text.unlikely.*) > > > +#define TEXT_SPLIT > > > #endif > > > > > > Why conditional? > > The condition is to ensure that we don't change the default kernel > build by any means. > The new code will introduce a few new symbols. Same. Adding two __split_text_start and __split_text_end markers do not affect anything. It just increases the kallsyms table slightly. You can do it unconditionally. > > > > > > > Where are __unlikely_text_start and __unlikely_text_end used? > > These new symbols are currently unreferenced within the kernel source tree. > However, they provide a valuable means of identifying hot and cold > sections of text, > and how large they are. I think they are useful information. Should be explained in the commit description. -- Best Regards Masahiro Yamada
