arm64: prevent regressions in compressed kernel image size when upgrading to binutils 2.27
diff mbox series

Message ID 20171026202907.91852-1-ndesaulniers@google.com
State New, archived
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Series
  • arm64: prevent regressions in compressed kernel image size when upgrading to binutils 2.27
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Commit Message

Nick Desaulniers Oct. 26, 2017, 8:29 p.m. UTC
Upon upgrading to binutils 2.27, we found that our lz4 compressed kernel
images were significantly larger, resulting is 10ms boot time regressions.

As noted by Rahul:
"aarch64 binaries uses RELA relocations, where each relocation entry
includes an addend value. This is similar to x86_64.  On x86_64, the
addend values are also stored at the relocation offset for relative
relocations. This is an optimization: in the case where code does not
need to be relocated, the loader can simply skip processing relative
relocations.  In binutils-2.25, both bfd and gold linkers did this for
x86_64, but only the gold linker did this for aarch64.  The kernel build
here is using the bfd linker, which stored zeroes at the relocation
offsets for relative relocations.  Since a set of zeroes compresses
better than a set of non-zero addend values, this behavior was resulting
in much better lz4 compression.

The bfd linker in binutils-2.27 is now storing the actual addend values
at the relocation offsets. The behavior is now consistent with what it
does for x86_64 and what gold linker does for both architectures.  The
change happened in this upstream commit:
https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=1f56df9d0d5ad89806c24e71f296576d82344613
Since a bunch of zeroes got replaced by non-zero addend values, we see
the side effect of lz4 compressed image being a bit bigger.

To get the old behavior from the bfd linker, "--no-apply-dynamic-relocs"
flag can be used:
$ LDFLAGS="--no-apply-dynamic-relocs" ./build/build.sh
With this flag, the compressed image size is back to what it was with
binutils-2.25.

If the kernel is using ASLR, there aren't additional runtime costs to
--no-apply-dynamic-relocs, as the relocations will need to be applied
again anyway after the kernel is relocated to a random address.

If the kernel is not using ASLR, then presumably the current default
behavior of the linker is better. Since the static linker performed the
dynamic relocs, and the kernel is not moved to a different address at
load time, it can skip applying the relocations all over again."

Some measurements:

$ ld -v
GNU ld (binutils-2.25-f3d35cf6) 2.25.51.20141117
                    ^
$ ls -l vmlinux
-rwxr-x--- 1 ndesaulniers eng 300652760 Oct 26 11:57 vmlinux
$ ls -l Image.lz4-dtb
-rw-r----- 1 ndesaulniers eng 16932627 Oct 26 11:57 Image.lz4-dtb

$ ld -v
GNU ld (binutils-2.27-53dd00a1) 2.27.0.20170315
                    ^
pre patch:
$ ls -l vmlinux
-rwxr-x--- 1 ndesaulniers eng 300376208 Oct 26 11:43 vmlinux
$ ls -l Image.lz4-dtb
-rw-r----- 1 ndesaulniers eng 18159474 Oct 26 11:43 Image.lz4-dtb

post patch:
$ ls -l vmlinux
-rwxr-x--- 1 ndesaulniers eng 300376208 Oct 26 12:06 vmlinux
$ ls -l Image.lz4-dtb
-rw-r----- 1 ndesaulniers eng 16932466 Oct 26 12:06 Image.lz4-dtb

10ms boot time savings isn't anything to get excited about, but users of
arm64+lz4+bfd-2.27 should not have to pay a penalty for no runtime
improvement.

Reported-by: Gopinath Elanchezhian <gelanchezhian@google.com>
Reported-by: Sindhuri Pentyala <spentyala@google.com>
Reported-by: Wei Wang <wvw@google.com>
Suggested-by: Rahul Chaudhry <rahulchaudhry@google.com>
Suggested-by: Siqi Lin <siqilin@google.com>
Suggested-by: Stephen Hines <srhines@google.com>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
---
* Question to reviewers: should I be using $(and ..., ...) here rather
  than double equals block? grep turns up no hits in the kernel for
  an example.

 arch/arm64/Makefile | 6 ++++++
 1 file changed, 6 insertions(+)

Comments

Ard Biesheuvel Oct. 26, 2017, 8:41 p.m. UTC | #1
On 26 October 2017 at 21:29, Nick Desaulniers <ndesaulniers@google.com> wrote:
> Upon upgrading to binutils 2.27, we found that our lz4 compressed kernel
> images were significantly larger, resulting is 10ms boot time regressions.
>
> As noted by Rahul:
> "aarch64 binaries uses RELA relocations, where each relocation entry
> includes an addend value. This is similar to x86_64.  On x86_64, the
> addend values are also stored at the relocation offset for relative
> relocations. This is an optimization: in the case where code does not
> need to be relocated, the loader can simply skip processing relative
> relocations.  In binutils-2.25, both bfd and gold linkers did this for
> x86_64, but only the gold linker did this for aarch64.  The kernel build
> here is using the bfd linker, which stored zeroes at the relocation
> offsets for relative relocations.  Since a set of zeroes compresses
> better than a set of non-zero addend values, this behavior was resulting
> in much better lz4 compression.
>
> The bfd linker in binutils-2.27 is now storing the actual addend values
> at the relocation offsets. The behavior is now consistent with what it
> does for x86_64 and what gold linker does for both architectures.  The
> change happened in this upstream commit:
> https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=1f56df9d0d5ad89806c24e71f296576d82344613
> Since a bunch of zeroes got replaced by non-zero addend values, we see
> the side effect of lz4 compressed image being a bit bigger.
>
> To get the old behavior from the bfd linker, "--no-apply-dynamic-relocs"
> flag can be used:
> $ LDFLAGS="--no-apply-dynamic-relocs" ./build/build.sh
> With this flag, the compressed image size is back to what it was with
> binutils-2.25.
>
> If the kernel is using ASLR, there aren't additional runtime costs to
> --no-apply-dynamic-relocs, as the relocations will need to be applied
> again anyway after the kernel is relocated to a random address.
>
> If the kernel is not using ASLR, then presumably the current default
> behavior of the linker is better. Since the static linker performed the
> dynamic relocs, and the kernel is not moved to a different address at
> load time, it can skip applying the relocations all over again."
>
> Some measurements:
>
> $ ld -v
> GNU ld (binutils-2.25-f3d35cf6) 2.25.51.20141117
>                     ^
> $ ls -l vmlinux
> -rwxr-x--- 1 ndesaulniers eng 300652760 Oct 26 11:57 vmlinux
> $ ls -l Image.lz4-dtb
> -rw-r----- 1 ndesaulniers eng 16932627 Oct 26 11:57 Image.lz4-dtb
>
> $ ld -v
> GNU ld (binutils-2.27-53dd00a1) 2.27.0.20170315
>                     ^
> pre patch:
> $ ls -l vmlinux
> -rwxr-x--- 1 ndesaulniers eng 300376208 Oct 26 11:43 vmlinux
> $ ls -l Image.lz4-dtb
> -rw-r----- 1 ndesaulniers eng 18159474 Oct 26 11:43 Image.lz4-dtb
>
> post patch:
> $ ls -l vmlinux
> -rwxr-x--- 1 ndesaulniers eng 300376208 Oct 26 12:06 vmlinux
> $ ls -l Image.lz4-dtb
> -rw-r----- 1 ndesaulniers eng 16932466 Oct 26 12:06 Image.lz4-dtb
>
> 10ms boot time savings isn't anything to get excited about, but users of
> arm64+lz4+bfd-2.27 should not have to pay a penalty for no runtime
> improvement.
>
> Reported-by: Gopinath Elanchezhian <gelanchezhian@google.com>
> Reported-by: Sindhuri Pentyala <spentyala@google.com>
> Reported-by: Wei Wang <wvw@google.com>
> Suggested-by: Rahul Chaudhry <rahulchaudhry@google.com>
> Suggested-by: Siqi Lin <siqilin@google.com>
> Suggested-by: Stephen Hines <srhines@google.com>
> Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
> ---
> * Question to reviewers: should I be using $(and ..., ...) here rather
>   than double equals block? grep turns up no hits in the kernel for
>   an example.
>
>  arch/arm64/Makefile | 6 ++++++
>  1 file changed, 6 insertions(+)
>
> diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
> index 939b310913cf..eed3b8bdc089 100644
> --- a/arch/arm64/Makefile
> +++ b/arch/arm64/Makefile
> @@ -18,6 +18,12 @@ ifneq ($(CONFIG_RELOCATABLE),)
>  LDFLAGS_vmlinux                += -pie -shared -Bsymbolic
>  endif
>
> +ifeq ($(CONFIG_KERNEL_LZ4), y)
> +ifeq ($(CONFIG_RANDOMIZE_BASE), y)
> +LDFLAGS_vmlinux                += $(call ld-option, --no-apply-dynamic-relocs)
> +endif
> +endif
> +

Since we will need to support bfd ld < 2.27 for a while to come, and
given that we cannot test in the code whether the relocation targets
are seeded with the correct values, I propose we simply drop the outer
ifeq here, and  stick with the old behavior unconditionally. Once
we're ready to drop support for <2.27 binutils, we can revisit this if
desired.

Also, you should be using CONFIG_RELOCATABLE not CONFIG_RANDOMIZE_BASE,.

>  ifeq ($(CONFIG_ARM64_ERRATUM_843419),y)
>    ifeq ($(call ld-option, --fix-cortex-a53-843419),)
>  $(warning ld does not support --fix-cortex-a53-843419; kernel may be susceptible to erratum)
> --
> 2.15.0.rc2.357.g7e34df9404-goog
>
>
> _______________________________________________
> linux-arm-kernel mailing list
> linux-arm-kernel@lists.infradead.org
> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
Nick Desaulniers Oct. 26, 2017, 8:55 p.m. UTC | #2
On Thu, Oct 26, 2017 at 1:41 PM, Ard Biesheuvel
<ard.biesheuvel@linaro.org> wrote:
> Since we will need to support bfd ld < 2.27 for a while to come, and
> given that we cannot test in the code whether the relocation targets
> are seeded with the correct values, I propose we simply drop the outer
> ifeq here, and  stick with the old behavior unconditionally. Once
> we're ready to drop support for <2.27 binutils, we can revisit this if
> desired.

Ard, thanks for the quick review!

--no-apply-dynamic-relocs was added in binutils 2.27, so ld-option
should support 2.27 and prior:

$ aarch64-linux-android/bin/ld -v
GNU ld (binutils-2.27-53dd00a1) 2.27.0.20170315
$ aarch64-linux-android/bin/ld -E --no-apply-dynamic-relocs temp.o
$ echo $?
0

$ aarch64-linux-android/bin/ld -v
GNU ld (binutils-2.25-f3d35cf6) 2.25.51.20141117
$ aarch64-linux-android/bin/ld -E --no-apply-dynamic-relocs temp.o
./prebuilts/gcc/linux-x86/aarch64/aarch64-linux-android-4.9/aarch64-linux-android/bin/ld:
unrecognized option '--no-apply-dynamic-relocs'
$ echo $?
1

ld-option will catch that.

> are seeded with the correct values, I propose we simply drop the outer

I haven't verified this with other compression schemes, but my
teammate Wei just reported this benefits gzip as well.  What do you
think?

> Also, you should be using CONFIG_RELOCATABLE not CONFIG_RANDOMIZE_BASE,.

Sure thing, will send v2 once you clarify the outer conditional and if
ld-option helps us take this patch now.
Nick Desaulniers Oct. 26, 2017, 9:06 p.m. UTC | #3
+ folks in Suggested-by/Reported by lines, since git send-email seems
to only pull in folks on Signed-off-by line :(

https://lkml.org/lkml/2017/10/26/590
Siqi Lin Oct. 26, 2017, 9:17 p.m. UTC | #4
On Thu, Oct 26, 2017 at 2:06 PM, Nick Desaulniers
<ndesaulniers@google.com> wrote:
>
> + folks in Suggested-by/Reported by lines, since git send-email seems
> to only pull in folks on Signed-off-by line :(
>
> https://lkml.org/lkml/2017/10/26/590

I'm OK with sticking with the <2.27 binutils behavior. The gzip data is:

binutils 2.25:
Image 41467904
Image.gz 13395151
binutils 2.27:
Image 41467392
Image.gz 14114953

gzipped kernel increased by 0.69 MiB.

The one special case I see is !CONFIG_RELOCATABLE and compression is
used, where there's a tradeoff between compressed image size and the
benefit of dynamic relocs.
Nick Desaulniers Oct. 26, 2017, 9:23 p.m. UTC | #5
On Thu, Oct 26, 2017 at 2:17 PM, Siqi Lin <siqilin@google.com> wrote:
> I'm OK with sticking with the <2.27 binutils behavior. The gzip data is:

That's what this patch does; goes back to the <2.27 behavior for 2.27+.

> binutils 2.25:
> Image 41467904
> Image.gz 13395151
> binutils 2.27:
> Image 41467392
> Image.gz 14114953
>
> gzipped kernel increased by 0.69 MiB.

That's without this patch applied?  With it applied, what are the
stats (for gzip)?

> The one special case I see is !CONFIG_RELOCATABLE and compression is
> used, where there's a tradeoff between compressed image size and the
> benefit of dynamic relocs.

if !CONFIG_RELOCATABLE, then this patch (well v2 which will use
CONFIG_RELOCATABLE rather than CONFIG_RANDOMIZE_BASE) doesn't do
anything.
Nick Desaulniers Oct. 26, 2017, 9:41 p.m. UTC | #6
> On Thu, Oct 26, 2017 at 1:41 PM, Ard Biesheuvel
<ard.biesheuvel@linaro.org> wrote:>are seeded with the correct values,
I propose we simply drop the outer

> On Thu, Oct 26, 2017 at 2:17 PM, Siqi Lin <siqilin@google.com> wrote:
>> I'm OK with sticking with the <2.27 binutils behavior. The gzip data is:

> ... Nick:
> That's what this patch does; goes back to the <2.27 behavior for 2.27+.

Sorry, rereading this thread, it sounds like neither of you were
disagreeing with me?  Will post v2.
Siqi Lin Oct. 26, 2017, 9:51 p.m. UTC | #7
On Thu, Oct 26, 2017 at 2:23 PM, Nick Desaulniers
<ndesaulniers@google.com> wrote:
>
> On Thu, Oct 26, 2017 at 2:17 PM, Siqi Lin <siqilin@google.com> wrote:
> > I'm OK with sticking with the <2.27 binutils behavior. The gzip data is:
>
> That's what this patch does; goes back to the <2.27 behavior for 2.27+.
>
> > binutils 2.25:
> > Image 41467904
> > Image.gz 13395151
> > binutils 2.27:
> > Image 41467392
> > Image.gz 14114953
> >
> > gzipped kernel increased by 0.69 MiB.
>
> That's without this patch applied?  With it applied, what are the
> stats (for gzip)?
>

binutils 2.27 with this patch (with --no-apply-dynamic-relocs):
Image 41535488
Image.gz 13404067

binutils 2.27 without this patch (without --no-apply-dynamic-relocs):
Image 41535488
Image.gz 14125516

The 2.27 gzipped size with this patch is about the same as 2.25.

> > The one special case I see is !CONFIG_RELOCATABLE and compression is
> > used, where there's a tradeoff between compressed image size and the
> > benefit of dynamic relocs.
>
> if !CONFIG_RELOCATABLE, then this patch (well v2 which will use
> CONFIG_RELOCATABLE rather than CONFIG_RANDOMIZE_BASE) doesn't do
> anything.

Patch
diff mbox series

diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
index 939b310913cf..eed3b8bdc089 100644
--- a/arch/arm64/Makefile
+++ b/arch/arm64/Makefile
@@ -18,6 +18,12 @@  ifneq ($(CONFIG_RELOCATABLE),)
 LDFLAGS_vmlinux		+= -pie -shared -Bsymbolic
 endif
 
+ifeq ($(CONFIG_KERNEL_LZ4), y)
+ifeq ($(CONFIG_RANDOMIZE_BASE), y)
+LDFLAGS_vmlinux		+= $(call ld-option, --no-apply-dynamic-relocs)
+endif
+endif
+
 ifeq ($(CONFIG_ARM64_ERRATUM_843419),y)
   ifeq ($(call ld-option, --fix-cortex-a53-843419),)
 $(warning ld does not support --fix-cortex-a53-843419; kernel may be susceptible to erratum)