Re: [PATCH 3/6] Change the symbols order when --ffuntion-sections is enabled

[Rong Xu]

On Mon, Jul 29, 2024 at 2:34 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Sun, Jul 28, 2024 at 01:29:56PM -0700, Rong Xu wrote:
> > When the -ffunction-sections compiler option is enabled, each function
> > is placed in a separate section named .text.function_name rather than
> > putting all functions in a single .text section.
> >
> > However, using -function-sections can cause problems with the
> > linker script. The comments included in include/asm-generic/vmlinux.lds.h
> > note these issues.:
> >   “TEXT_MAIN here will match .text.fixup and .text.unlikely if dead
> >    code elimination is enabled, so these sections should be converted
> >    to use ".." first.”
> >
> > It is unclear whether there is a straightforward method for converting
> > a suffix to "..". This patch modifies the order of subsections within the
> > text output section when the -ffunction-sections flag is enabled.
> > Specifically, it repositions sections with certain fixed patterns (for
> > example .text.unlikely) before TEXT_MAIN, ensuring that they are grouped
> > and matched together.
> >
> > Note that the limitation arises because the linker script employs glob
> > patterns instead of regular expressions for string matching. While there
> > is a method to maintain the current order using complex patterns, this
> > significantly complicates the pattern and increases the likelihood of
> > errors.
> >
> > 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 | 19 ++++++++++++++++---
> >  1 file changed, 16 insertions(+), 3 deletions(-)
> >
> > diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> > index 5703526d6ebf..f3de66bda293 100644
> > --- a/include/asm-generic/vmlinux.lds.h
> > +++ b/include/asm-generic/vmlinux.lds.h
> > @@ -582,9 +582,21 @@
> >   * during second ld run in second ld pass when generating System.map
> >   *
> >   * TEXT_MAIN here will match .text.fixup and .text.unlikely if dead
> > - * code elimination is enabled, so these sections should be converted
> > - * to use ".." first.
> > + * code elimination or function-section is enabled. Match these symbols
> > + * first when in these builds.
> >   */
> > +#if defined(CONFIG_LD_DEAD_CODE_DATA_ELIMINATION) || defined(CONFIG_LTO_CLANG)
> > +#define TEXT_TEXT                                                    \
> > +             *(.text.asan.* .text.tsan.*)                            \
> > +             *(.text.unknown .text.unknown.*)                        \
> > +             *(.text.unlikely .text.unlikely.*)                      \
> > +             ALIGN_FUNCTION();                                       \
>
> Why leave the above text sections unaligned?

They are considered cold text. They are not aligned before the change.
But I have no objections to making it aligned.

(Sorry if you receive a duplicated message. I'm resending this in
plain text mode.)

>
> > +             *(.text.hot .text.hot.*)                                \
> > +             *(TEXT_MAIN .text.fixup)                                \
> > +             NOINSTR_TEXT                                            \
> > +             *(.ref.text)                                            \
> > +     MEM_KEEP(init.text*)
> > +#else
> >  #define TEXT_TEXT                                                    \
> >               ALIGN_FUNCTION();                                       \
> >               *(.text.hot .text.hot.*)                                \
> > @@ -594,7 +606,8 @@
> >               NOINSTR_TEXT                                            \
> >               *(.ref.text)                                            \
> >               *(.text.asan.* .text.tsan.*)                            \
> > -     MEM_KEEP(init.text*)                                            \
> > +     MEM_KEEP(init.text*)
> > +#endif
> >
> >
> >  /* sched.text is aling to function alignment to secure we have same
> > --
> > 2.46.0.rc1.232.g9752f9e123-goog
> >