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

Nick Desaulniers
2021-06-21T20:44:12Z

On Mon, Jun 21, 2021 at 11:50 AM Bill Wendling <morbo@google.com> wrote:
>
> 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?

We need to be able to express via Kconfig "GCOV should not be enabled
for architectures that use noinstr when the toolchain does not support
__attribute__((no_profile_instrument_function))."

Where "architectures that use noinstr" are currently arm64, s390, and
x86. So I guess we could do:

+ depends on !ARM64 || !S390 || !X86 || CC_HAS_NO_PROFILE_FN_ATTR

(We could add a Kconfig for ARCH_WANTS_NO_INSTR, which might be more
informative than listed out architectures which might be non-obvious
to passers-by).

It would be most succinct to raise the requirements to: "GCOV should
not be enabled when the toolchain does not support
__attribute__((no_profile_instrument_function))." Then we could do:

+ depends on CC_HAS_NO_PROFILE_FN_ATTR

Assuming no one has the requirement to support GCOV on PPC with GCC <
7.1, for example.

>
> > 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

--
Thanks,
~Nick Desaulniers