arch/x86/purgatory/Makefile | 4 ++++ 1 file changed, 4 insertions(+)
With LTO enabled, ld.lld generates multiple .text sections for
purgatory.ro:
$ readelf -S purgatory.ro | grep " .text"
[ 1] .text PROGBITS 0000000000000000 00000040
[ 7] .text.purgatory PROGBITS 0000000000000000 000020e0
[ 9] .text.warn PROGBITS 0000000000000000 000021c0
[13] .text.sha256_upda PROGBITS 0000000000000000 000022f0
[15] .text.sha224_upda PROGBITS 0000000000000000 00002be0
[17] .text.sha256_fina PROGBITS 0000000000000000 00002bf0
[19] .text.sha224_fina PROGBITS 0000000000000000 00002cc0
This cause WARNING from kexec_purgatory_setup_sechdrs():
WARNING: CPU: 26 PID: 110894 at kernel/kexec_file.c:919
kexec_load_purgatory+0x37f/0x390
Fix this by disabling LTO for purgatory.
Fixes: 8652d44f466a ("kexec: support purgatories with .text.hot sections")
Cc: Ricardo Ribalda <ribalda@chromium.org>
Cc: Sami Tolvanen <samitolvanen@google.com>
Cc: kexec@lists.infradead.org
Cc: linux-kernel@vger.kernel.org
Cc: x86@kernel.org
Cc: llvm@lists.linux.dev
Signed-off-by: Song Liu <song@kernel.org>
---
AFAICT, x86 is the only arch that supports LTO and purgatory.
Changes in v2:
1. Use CC_FLAGS_LTO instead of hardcode -flto. (Nick Desaulniers)
---
arch/x86/purgatory/Makefile | 4 ++++
1 file changed, 4 insertions(+)
diff --git a/arch/x86/purgatory/Makefile b/arch/x86/purgatory/Makefile
index c2a29be35c01..08aa0f25f12a 100644
--- a/arch/x86/purgatory/Makefile
+++ b/arch/x86/purgatory/Makefile
@@ -19,6 +19,10 @@ CFLAGS_sha256.o := -D__DISABLE_EXPORTS -D__NO_FORTIFY
# optimization flags.
KBUILD_CFLAGS := $(filter-out -fprofile-sample-use=% -fprofile-use=%,$(KBUILD_CFLAGS))
+# When LTO is enabled, llvm emits many text sections, which is not supported
+# by kexec. Remove -flto=* flags.
+KBUILD_CFLAGS := $(filter-out $(CC_FLAGS_LTO),$(KBUILD_CFLAGS))
+
# When linking purgatory.ro with -r unresolved symbols are not checked,
# also link a purgatory.chk binary without -r to check for unresolved symbols.
PURGATORY_LDFLAGS := -e purgatory_start -z nodefaultlib
--
2.34.1Hi folks,
On Fri, Sep 8, 2023 at 4:13 PM Song Liu <song@kernel.org> wrote:
>
> With LTO enabled, ld.lld generates multiple .text sections for
> purgatory.ro:
>
> $ readelf -S purgatory.ro | grep " .text"
> [ 1] .text PROGBITS 0000000000000000 00000040
> [ 7] .text.purgatory PROGBITS 0000000000000000 000020e0
> [ 9] .text.warn PROGBITS 0000000000000000 000021c0
> [13] .text.sha256_upda PROGBITS 0000000000000000 000022f0
> [15] .text.sha224_upda PROGBITS 0000000000000000 00002be0
> [17] .text.sha256_fina PROGBITS 0000000000000000 00002bf0
> [19] .text.sha224_fina PROGBITS 0000000000000000 00002cc0
>
> This cause WARNING from kexec_purgatory_setup_sechdrs():
>
> WARNING: CPU: 26 PID: 110894 at kernel/kexec_file.c:919
> kexec_load_purgatory+0x37f/0x390
>
> Fix this by disabling LTO for purgatory.
>
> Fixes: 8652d44f466a ("kexec: support purgatories with .text.hot sections")
> Cc: Ricardo Ribalda <ribalda@chromium.org>
> Cc: Sami Tolvanen <samitolvanen@google.com>
> Cc: kexec@lists.infradead.org
> Cc: linux-kernel@vger.kernel.org
> Cc: x86@kernel.org
> Cc: llvm@lists.linux.dev
> Signed-off-by: Song Liu <song@kernel.org>
What would be the next step for this work? We hope to back port it
to our production kernel soon.
Thanks,
Song
>
> ---
> AFAICT, x86 is the only arch that supports LTO and purgatory.
>
> Changes in v2:
> 1. Use CC_FLAGS_LTO instead of hardcode -flto. (Nick Desaulniers)
> ---
> arch/x86/purgatory/Makefile | 4 ++++
> 1 file changed, 4 insertions(+)
>
> diff --git a/arch/x86/purgatory/Makefile b/arch/x86/purgatory/Makefile
> index c2a29be35c01..08aa0f25f12a 100644
> --- a/arch/x86/purgatory/Makefile
> +++ b/arch/x86/purgatory/Makefile
> @@ -19,6 +19,10 @@ CFLAGS_sha256.o := -D__DISABLE_EXPORTS -D__NO_FORTIFY
> # optimization flags.
> KBUILD_CFLAGS := $(filter-out -fprofile-sample-use=% -fprofile-use=%,$(KBUILD_CFLAGS))
>
> +# When LTO is enabled, llvm emits many text sections, which is not supported
> +# by kexec. Remove -flto=* flags.
> +KBUILD_CFLAGS := $(filter-out $(CC_FLAGS_LTO),$(KBUILD_CFLAGS))
> +
> # When linking purgatory.ro with -r unresolved symbols are not checked,
> # also link a purgatory.chk binary without -r to check for unresolved symbols.
> PURGATORY_LDFLAGS := -e purgatory_start -z nodefaultlib
> --
> 2.34.1
>
On Thu, Sep 14, 2023 at 9:31 AM Song Liu <song@kernel.org> wrote: > > What would be the next step for this work? We hope to back port it > to our production kernel soon. Please send a v3 with the fixes tag updated. I wouldn't mind if you added a comment to the commit message to the effect of: We could also add the use of an explicit linker script to rejoin .text.* sections back into .text. Simply disable the production of more .text.* sections for now; -flto* implies -ffunction-sections. -- Thanks, ~Nick Desaulniers
On Thu, Sep 14, 2023 at 9:34 AM Nick Desaulniers <ndesaulniers@google.com> wrote: > > On Thu, Sep 14, 2023 at 9:31 AM Song Liu <song@kernel.org> wrote: > > > > What would be the next step for this work? We hope to back port it > > to our production kernel soon. > > Please send a v3 with the fixes tag updated. I wouldn't mind if you > added a comment to the commit message to the effect of: > > We could also add the use of an explicit linker script to rejoin > .text.* sections back into .text. Simply disable the production of > more .text.* sections for now; -flto* implies -ffunction-sections. Got it. I just sent v3 with these changes. Thanks, Song
On Fri, Sep 8, 2023 at 4:13 PM Song Liu <song@kernel.org> wrote:
>
> With LTO enabled, ld.lld generates multiple .text sections for
> purgatory.ro:
>
> $ readelf -S purgatory.ro | grep " .text"
> [ 1] .text PROGBITS 0000000000000000 00000040
> [ 7] .text.purgatory PROGBITS 0000000000000000 000020e0
> [ 9] .text.warn PROGBITS 0000000000000000 000021c0
> [13] .text.sha256_upda PROGBITS 0000000000000000 000022f0
> [15] .text.sha224_upda PROGBITS 0000000000000000 00002be0
> [17] .text.sha256_fina PROGBITS 0000000000000000 00002bf0
> [19] .text.sha224_fina PROGBITS 0000000000000000 00002cc0
>
> This cause WARNING from kexec_purgatory_setup_sechdrs():
>
> WARNING: CPU: 26 PID: 110894 at kernel/kexec_file.c:919
> kexec_load_purgatory+0x37f/0x390
>
> Fix this by disabling LTO for purgatory.
Thanks for the v2!
>
> Fixes: 8652d44f466a ("kexec: support purgatories with .text.hot sections")
Dunno that this fixes tag is precise. I think perhaps
Fixes: b33fff07e3e3 ("x86, build: allow LTO to be selected")
would be more precise.
> Cc: Ricardo Ribalda <ribalda@chromium.org>
> Cc: Sami Tolvanen <samitolvanen@google.com>
> Cc: kexec@lists.infradead.org
> Cc: linux-kernel@vger.kernel.org
> Cc: x86@kernel.org
> Cc: llvm@lists.linux.dev
> Signed-off-by: Song Liu <song@kernel.org>
>
> ---
> AFAICT, x86 is the only arch that supports LTO and purgatory.
>
> Changes in v2:
> 1. Use CC_FLAGS_LTO instead of hardcode -flto. (Nick Desaulniers)
> ---
> arch/x86/purgatory/Makefile | 4 ++++
> 1 file changed, 4 insertions(+)
>
> diff --git a/arch/x86/purgatory/Makefile b/arch/x86/purgatory/Makefile
> index c2a29be35c01..08aa0f25f12a 100644
> --- a/arch/x86/purgatory/Makefile
> +++ b/arch/x86/purgatory/Makefile
> @@ -19,6 +19,10 @@ CFLAGS_sha256.o := -D__DISABLE_EXPORTS -D__NO_FORTIFY
> # optimization flags.
> KBUILD_CFLAGS := $(filter-out -fprofile-sample-use=% -fprofile-use=%,$(KBUILD_CFLAGS))
>
> +# When LTO is enabled, llvm emits many text sections, which is not supported
> +# by kexec. Remove -flto=* flags.
-flto* in LLVM implies -ffunction-sections, which creates a .text.<fn
name> section per function definition to facilitate -Wl,--gc-sections.
Overall the question here is "do we really need to optimize kexec?"
If the answer is yes, then this patch AND 8652d44f466a are both
pessimizing kexec (though having it work at all is strictly better
than not at all). The best fix IMO would be to provide a linker
script for this purgatory image that rejoins the text sections back
into one .text. For example:
commit eff8728fe698 ("vmlinux.lds.h: Add PGO and AutoFDO input sections")
I assume people do care about the time to kexec, hence the raison
d'etre for projects like kpatch.
I'm fine to sign off on this approach if we don't really care, or want
to care more later, but we can do better here.
> +KBUILD_CFLAGS := $(filter-out $(CC_FLAGS_LTO),$(KBUILD_CFLAGS))
> +
> # When linking purgatory.ro with -r unresolved symbols are not checked,
> # also link a purgatory.chk binary without -r to check for unresolved symbols.
> PURGATORY_LDFLAGS := -e purgatory_start -z nodefaultlib
> --
> 2.34.1
>
--
Thanks,
~Nick Desaulniers
On Mon, Sep 11, 2023 at 9:00 AM Nick Desaulniers
<ndesaulniers@google.com> wrote:
>
> On Fri, Sep 8, 2023 at 4:13 PM Song Liu <song@kernel.org> wrote:
> >
> > With LTO enabled, ld.lld generates multiple .text sections for
> > purgatory.ro:
> >
> > $ readelf -S purgatory.ro | grep " .text"
> > [ 1] .text PROGBITS 0000000000000000 00000040
> > [ 7] .text.purgatory PROGBITS 0000000000000000 000020e0
> > [ 9] .text.warn PROGBITS 0000000000000000 000021c0
> > [13] .text.sha256_upda PROGBITS 0000000000000000 000022f0
> > [15] .text.sha224_upda PROGBITS 0000000000000000 00002be0
> > [17] .text.sha256_fina PROGBITS 0000000000000000 00002bf0
> > [19] .text.sha224_fina PROGBITS 0000000000000000 00002cc0
> >
> > This cause WARNING from kexec_purgatory_setup_sechdrs():
> >
> > WARNING: CPU: 26 PID: 110894 at kernel/kexec_file.c:919
> > kexec_load_purgatory+0x37f/0x390
> >
> > Fix this by disabling LTO for purgatory.
>
> Thanks for the v2!
>
> >
> > Fixes: 8652d44f466a ("kexec: support purgatories with .text.hot sections")
>
> Dunno that this fixes tag is precise. I think perhaps
>
> Fixes: b33fff07e3e3 ("x86, build: allow LTO to be selected")
Thanks for the correction!
>
> would be more precise.
>
> > Cc: Ricardo Ribalda <ribalda@chromium.org>
> > Cc: Sami Tolvanen <samitolvanen@google.com>
> > Cc: kexec@lists.infradead.org
> > Cc: linux-kernel@vger.kernel.org
> > Cc: x86@kernel.org
> > Cc: llvm@lists.linux.dev
> > Signed-off-by: Song Liu <song@kernel.org>
> >
> > ---
> > AFAICT, x86 is the only arch that supports LTO and purgatory.
> >
> > Changes in v2:
> > 1. Use CC_FLAGS_LTO instead of hardcode -flto. (Nick Desaulniers)
> > ---
> > arch/x86/purgatory/Makefile | 4 ++++
> > 1 file changed, 4 insertions(+)
> >
> > diff --git a/arch/x86/purgatory/Makefile b/arch/x86/purgatory/Makefile
> > index c2a29be35c01..08aa0f25f12a 100644
> > --- a/arch/x86/purgatory/Makefile
> > +++ b/arch/x86/purgatory/Makefile
> > @@ -19,6 +19,10 @@ CFLAGS_sha256.o := -D__DISABLE_EXPORTS -D__NO_FORTIFY
> > # optimization flags.
> > KBUILD_CFLAGS := $(filter-out -fprofile-sample-use=% -fprofile-use=%,$(KBUILD_CFLAGS))
> >
> > +# When LTO is enabled, llvm emits many text sections, which is not supported
> > +# by kexec. Remove -flto=* flags.
>
> -flto* in LLVM implies -ffunction-sections, which creates a .text.<fn
> name> section per function definition to facilitate -Wl,--gc-sections.
>
> Overall the question here is "do we really need to optimize kexec?"
>
> If the answer is yes, then this patch AND 8652d44f466a are both
> pessimizing kexec (though having it work at all is strictly better
> than not at all). The best fix IMO would be to provide a linker
> script for this purgatory image that rejoins the text sections back
> into one .text. For example:
>
> commit eff8728fe698 ("vmlinux.lds.h: Add PGO and AutoFDO input sections")
>
> I assume people do care about the time to kexec, hence the raison
> d'etre for projects like kpatch.
AFAICT, optimizations like LTO and PGO can give a few % of
improvement, which is probably not important for kexec. The benefit
is in the order of seconds (or less?). The benefit of kpatch is that we
can keep the workload running while fixing the kernel bug. Based on
our experience at Meta, it may take hours to graceful shutdown the
application to run kexec. In this case, a few seconds of improvement
(via LTO/PGO purgatory) doesn't save us much.
Thanks,
Song
>
> I'm fine to sign off on this approach if we don't really care, or want
> to care more later, but we can do better here.
>
> > +KBUILD_CFLAGS := $(filter-out $(CC_FLAGS_LTO),$(KBUILD_CFLAGS))
> > +
> > # When linking purgatory.ro with -r unresolved symbols are not checked,
> > # also link a purgatory.chk binary without -r to check for unresolved symbols.
> > PURGATORY_LDFLAGS := -e purgatory_start -z nodefaultlib
> > --
> > 2.34.1
> >
>
>
> --
> Thanks,
> ~Nick Desaulniers
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