On Thu, Sep 22, 2022 at 10:01 PM Denis Nikitin <denik@xxxxxxxxxxxx> wrote:
Hi Mark,
On Thu, Sep 22, 2022 at 3:38 AM Marc Zyngier <maz@xxxxxxxxxx> wrote:
>
> I was really hoping that you'd just drop the flags from the CFLAGS
> instead of removing the generated section. Something like:
>
> diff --git a/arch/arm64/kvm/hyp/nvhe/Makefile b/arch/arm64/kvm/hyp/nvhe/Makefile
> index b5c5119c7396..e5b2d43925b4 100644
> --- a/arch/arm64/kvm/hyp/nvhe/Makefile
> +++ b/arch/arm64/kvm/hyp/nvhe/Makefile
> @@ -88,7 +88,7 @@ quiet_cmd_hypcopy = HYPCOPY $@
>
> # Remove ftrace, Shadow Call Stack, and CFI CFLAGS.
> # This is equivalent to the 'notrace', '__noscs', and '__nocfi' annotations.
> -KBUILD_CFLAGS := $(filter-out $(CC_FLAGS_FTRACE) $(CC_FLAGS_SCS) $(CC_FLAGS_CFI), $(KBUILD_CFLAGS))
> +KBUILD_CFLAGS := $(filter-out $(CC_FLAGS_FTRACE) $(CC_FLAGS_SCS) $(CC_FLAGS_CFI) -fprofile-sample-use, $(KBUILD_CFLAGS))
>
> # KVM nVHE code is run at a different exception code with a different map, so
> # compiler instrumentation that inserts callbacks or checks into the code may
Sorry, I moved on with a different approach and didn't explain the rationale.
Like you mentioned before, the flag `-fprofile-sample-use` does not appear
in the kernel. And it looks confusing when the flag is disabled or filtered out
here. This was the first reason.
The root cause of the build failure wasn't the compiler profile guided
optimization but the extra metadata in SHT_REL section which llvm injected
into kvm_nvhe.tmp.o for further link optimization.
If we remove the .llvm.call-graph-profile section we fix the build and avoid
potential problems with relocations optimized by the linker. The profile
guided optimization will still be applied by the compiler.
Let me know what you think about it.
>
> However, I even failed to reproduce your problem using LLVM 14 as
> packaged by Debian (if that matters, I'm using an arm64 build
> machine). I build the kernel with:
>
> $ make LLVM=1 KCFLAGS=-fprofile-sample-use -j8 vmlinux
>
> and the offending object only contains the following sections:
>
Just some comments based on my ChromeOS build experience.
fprofile-sample-use needs the profile file name argument to read the pgo data from
i.e. -fprofile-sample-use=/path/to/gcov.profile.
Since the path to filename can change, it makes filtering out more difficult.
It is certainly possible to find and filter the exact argument by some string search of KCFLAGS.
But passing -fno-profile-sample-use is easier and less error prone which I believe the previous patch version tried to do.
> arch/arm64/kvm/hyp/nvhe/kvm_nvhe.tmp.o: file format elf64-littleaarch64
>
> Sections:
> Idx Name Size VMA LMA File off Algn
> 0 .hyp.idmap.text 00000ae4 0000000000000000 0000000000000000 00000800 2**11
> CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
> 1 .hyp.text 0000e988 0000000000000000 0000000000000000 00001800 2**11
> CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
> 2 .hyp.data..ro_after_init 00000820 0000000000000000 0000000000000000 00010188 2**3
> CONTENTS, ALLOC, LOAD, DATA
> 3 .hyp.rodata 00002e70 0000000000000000 0000000000000000 000109a8 2**3
> CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA
> 4 .hyp.data..percpu 00001ee0 0000000000000000 0000000000000000 00013820 2**4
> CONTENTS, ALLOC, LOAD, DATA
> 5 .hyp.bss 00001158 0000000000000000 0000000000000000 00015700 2**3
> ALLOC
> 6 .comment 0000001f 0000000000000000 0000000000000000 00017830 2**0
> CONTENTS, READONLY
> 7 .llvm_addrsig 000000b8 0000000000000000 0000000000000000 0001784f 2**0
> CONTENTS, READONLY, EXCLUDE
> 8 .altinstructions 00001284 0000000000000000 0000000000000000 00015700 2**0
> CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA
> 9 __jump_table 00000960 0000000000000000 0000000000000000 00016988 2**3
> CONTENTS, ALLOC, LOAD, RELOC, DATA
> 10 __bug_table 0000051c 0000000000000000 0000000000000000 000172e8 2**2
> CONTENTS, ALLOC, LOAD, RELOC, DATA
> 11 __kvm_ex_table 00000028 0000000000000000 0000000000000000 00017808 2**3
> CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA
> 12 .note.GNU-stack 00000000 0000000000000000 0000000000000000 00027370 2**0
> CONTENTS, READONLY
>
> So what am I missing to trigger this issue? Does it rely on something
> like PGO, which is not upstream yet? A bit of handholding would be
> much appreciated.
Right, it relies on the PGO profile.
On ChromeOS we collect the sample PGO profile from Arm devices with
enabled CoreSight/ETM. You can find more details on ETM at
https://www.kernel.org/doc/Documentation/trace/coresight/coresight.rst.
https://github.com/Linaro/OpenCSD/blob/master/decoder/tests/auto-fdo/autofdo.md
contains information about the pipeline of collecting, processing, and applying
the profile.
Generally the difficult part is in collecting a good matching profile for the workload.
So I think this patch is better than previous since it still keeps the compiler optimization for the hot code paths
in the file but removes the problematic section.
Thanks,
Manoj
>
> Thanks,
>
> M.
>
> --
> Without deviation from the norm, progress is not possible.
Thanks,
Denis
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