Android kernel PGO with LLVM

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1

Hello,

I'm working on Android kernel profile-guided optimization to figure out
whether pgo can improve the performance of Android kernel. I have tried
to added '-fprofile-generate' option to build Android kernel(msm-4.19).
But error occurs during building: "undefined reference to
'__llvm_profile_instrument_target'".
I have tried to add libclang_rt.profile-aarch64-android.a as a library
when building Android kernel, but the error still shows up. I think the
kernel may not be able to call libraries outside the kernel source tree,
so I was porting pgo-related files(compiler-rt/lib/profile/) from
compiler-rt to Android kernel source tree.

Do you know why did this error occurs? And am I doing the right thing?
Is it possible to improve the performance of Android kernel with llvm
pgo mechanism?

Thanks,
Yi Ma

2

The entry point is for value profiling and should be defined in InstrProfilingValue.o in the archive library. Can you check the symbol table of the .a file?

David

3

Hi David,

Thank you for replying! I have checked the symbol of libclang_rt.profile-

aarch64-android.a file. This archive library contains InstrProfilingValue.o

as well as the symbol of ‘__llvm_profile_instrument_target’ that i need(see

$ nm libclang_rt.profile-aarch64-android.a | grep llvm0000000000000000 T llv - Pastebin.com ). So I have tried to add this archive

library as a link-time library when building the Android kernel. But the

error still remains. I think the Android kernel may not be able to call

libraries outside the kernel source tree when linking and running. So I’m

porting pgo-related files (compiler-rt/lib/profile/) from compiler-rt to

Android kernel source tree. Is this the right and feasible way?

Yi Ma

4

For kernel PGO, you will probably need to invoke the buffer API to dump the profile data: __llvm_profile_write_buffer. Before that call, try to get the buffer size with __llvm_profile_get_size_for_buffer() and allocate memory with that size.

Note the buffer API will ignore value profile data, so you should also disable value profiling in build with --mllvm -disable-vp option to avoid the unsats at link time.

David