Re: [PATCH 2/2] Kconfig: CC_HAS_NO_PROFILE_FN_ATTR, depend on for GCOV and PGO

Bill Wendling
2021-06-21T18:50:34Z

On Mon, Jun 21, 2021 at 11:22 AM Nick Desaulniers
<ndesaulniers@google.com> wrote:
>
> On Fri, Jun 18, 2021 at 11:23 PM Marco Elver <elver@google.com> wrote:
> >
> > On Sat, 19 Jun 2021 at 01:30, Nick Desaulniers <ndesaulniers@google.com> wrote:
> > >
> > > We don't want compiler instrumentation to touch noinstr functions, which
> > > are annotated with the no_profile function attribute. Add a Kconfig test
> > > for this and make PGO and GCOV depend on it.
> > >
> > > Cc: Masahiro Yamada <masahiroy@kernel.org>
> > > Cc: Peter Oberparleiter <oberpar@linux.ibm.com>
> > > Link: https://lore.kernel.org/lkml/YMTn9yjuemKFLbws@hirez.programming.kicks-ass.net/
> > > Link: https://lore.kernel.org/lkml/YMcssV%2Fn5IBGv4f0@hirez.programming.kicks-ass.net/
> > > Suggested-by: Peter Zijlstra <peterz@infradead.org>
> > > Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
> > > ---
> > > init/Kconfig | 3 +++
> > > kernel/gcov/Kconfig | 1 +
> > > kernel/pgo/Kconfig | 3 ++-
> > > 3 files changed, 6 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/init/Kconfig b/init/Kconfig
> > > index 1ea12c64e4c9..540f862b40c6 100644
> > > --- a/init/Kconfig
> > > +++ b/init/Kconfig
> > > @@ -83,6 +83,9 @@ config TOOLS_SUPPORT_RELR
> > > config CC_HAS_ASM_INLINE
> > > def_bool $(success,echo 'void foo(void) { asm inline (""); }' | $(CC) -x c - -c -o /dev/null)
> > >
> > > +config CC_HAS_NO_PROFILE_FN_ATTR
> > > + def_bool $(success,echo '__attribute__((no_profile)) int x();' | $(CC) -x c - -c -o /dev/null -Werror)
> > > +
> > > config CONSTRUCTORS
> > > bool
> > >
> > > diff --git a/kernel/gcov/Kconfig b/kernel/gcov/Kconfig
> > > index 58f87a3092f3..19facd4289cd 100644
> > > --- a/kernel/gcov/Kconfig
> > > +++ b/kernel/gcov/Kconfig
> > > @@ -5,6 +5,7 @@ config GCOV_KERNEL
> > > bool "Enable gcov-based kernel profiling"
> > > depends on DEBUG_FS
> > > depends on !CC_IS_CLANG || CLANG_VERSION >= 110000
> > > + depends on !X86 || (X86 && CC_HAS_NO_PROFILE_FN_ATTR)
> >
> > [+Cc Mark]
> >
> > arm64 is also starting to rely on noinstr working properly.
>
> Sure,
> Will, Catalin, other arm64 folks:
> Any thoughts on requiring GCC 7.1+/Clang 13.0+ for GCOV support? That
> way we can better guarantee that GCOV (and eventually, PGO) don't
> touch noinstr functions?
>
> If that's ok, I'll add modify the above like:
>
> + depends on !ARM64 || (ARM64 && CC_HAS_NO_PROFILE_FN_ATTR)
>
Wouldn't "!ARM64 || CC_HAS_NO_PROFILE_FN_ATTR" be more succinct?

> to the above hunk in v2. Oh, looks like arch/s390 also uses noinstr.
> Same question applies then:
>
> + depends on !S390 || (S390 && CC_HAS_NO_PROFILE_FN_ATTR)
>
> Or, we could just do
>
> + depends on CC_HAS_NO_PROFILE_FN_ATTR
>
> Though that will penalize architectures not using noinstr, that still
> would like to use GCOV with versions of GCC older than 7.1. Perhaps
> there are no such such users, or they should consider upgrading their
> tools to we can stick with the simpler Kconfig? Thoughts?
>
> >
> > This should probably be a 'select ARCH_HAS_GCOV_PROFILE_ALL if
> > CC_HAS_NO_PROFILE_FN_ATTR' in the relevant arch/../Kconfig.
> >
> > Alternatively, using:
> > https://lkml.kernel.org/r/YMcssV/n5IBGv4f0@hirez.programming.kicks-ass.net
> >
> > But I'd probably not overcomplicate things at this point and just use
> > ARCH_HAS_GCOV_PROFILE_ALL, because GCOV seems to be a) rarely used,
> > and b) if someone decides to selectively instrument stuff like entry
> > code, we can just say it's user error.
> >
> >
> > > select CONSTRUCTORS
> > > default n
> > > help
> > > diff --git a/kernel/pgo/Kconfig b/kernel/pgo/Kconfig
> > > index d2053df1111c..26f75ac4c6c1 100644
> > > --- a/kernel/pgo/Kconfig
> > > +++ b/kernel/pgo/Kconfig
> > > @@ -8,7 +8,8 @@ config PGO_CLANG
> > > bool "Enable clang's PGO-based kernel profiling"
> > > depends on DEBUG_FS
> > > depends on ARCH_SUPPORTS_PGO_CLANG
> > > - depends on CC_IS_CLANG && CLANG_VERSION >= 120000
> > > + depends on CC_IS_CLANG
> > > + depends on CC_HAS_NO_PROFILE_FN_ATTR
> > > help
> > > This option enables clang's PGO (Profile Guided Optimization) based
> > > code profiling to better optimize the kernel.
> > > --
> > > 2.32.0.288.g62a8d224e6-goog
> > >
>
>
>
> --
> Thanks,
> ~Nick Desaulniers