[llvm] [BOLT] Add CustomOffset flag for optimizing kernel (PR #98153)

[via llvm-commits]

lifengxiang1025 wrote:

Kernel and perf version:
````
# os release : 5.14.0
# perf version : 5.14.g7d2a07b76933
````
One line of perf script's output:
````
swapper     0 [038] 179384.938151:   29186224   cycles:  ffffffff866497af poll_idle+0x7f (/lib/modules/5.14.0/build/vmlinux)
````
The symbol `poll_idle`'s address matches `/proc/kallsyms`:
````
# cat /proc/kallsyms | grep poll_idle
ffffffff86649730 t poll_idle
````
doesn't match `/lib/modules/5.14.0/build/vmlinux`:
````
# nm /lib/modules/5.14.0/build/vmlinux | grep poll_idle
ffffffff81a49730 t poll_idle

# addr2line -e /lib/modules/5.14.0/build/vmlinux -f ffffffff866497af
??
??:0
````
Make sure this `/lib/modules/5.14.0/build/vmlinux` is sampled by perf:
````
# file /lib/modules/5.14.0/build/vmlinux
/lib/modules/5.14.0/build/vmlinux: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), statically linked, BuildID[sha1]=29346d1237f67c8b2fdeaa079a2b471bb42de737, not stripped

# /root/linux/tools/perf/perf buildid-list -i perf-5.14.data  | grep vmlinux
29346d1237f67c8b2fdeaa079a2b471bb42de737 /lib/modules/5.14.0/build/vmlinux
````



https://github.com/llvm/llvm-project/pull/98153