[BOLT][perf2bolt] BOLTing the Linux Kernel, perf2bolt stage fails

[MichaelDeets]

I'm trying to BOLT my Linux Kernel. I've run a quick perf, then, while attempting to convert into a usable BOLT profile using perf2bolt, it results in this error:

Linux kernel version is unknown

I attempted to circumvent by patching bolt/lib/Rewrite/LinuxKernelRewriter.cpp and removed the check, which then, after re-running perf2bolt, results in this error:

BOLT-ERROR: while running linux-kernel-rewriter in pre-CFG state: invalid target of static keys jump at 0xffffffff81321c9c : 0xffffffff8120556e

Which stumped me for a while, as this is my first attempt at using BOLT and I'm unfamiliar with most things regarding it. What worked for me, was setting the "CONFIG_JUMP_LABEL=n" option in my Linux Kernel's configuration. I'm also using LTO, which might've contribute to this problem. I'll upload my full kernel configuration for reference.

I just hadn't seen anything regarding "CONFIG_JUMP_LABEL=n" being a requirement, and couldn't find anyone else online with similar issues, hence this bug report. Apologies in advanced if I had missed an existing bug report regarding this problem. I'm happy to attach anything else required, please let me know!

For all my LLVM related packages, I have them installed using my package manager (Portage), but for BOLT I simply build using 20.1.4 source, and also attempted the latest git version, but no changes between them.

Steps that resulted in the error:

Using https://github.com/llvm/llvm-project/blob/main/bolt/docs/OptimizingLinux.md as the guide:

1: Built the Linux Kernel with various options, like LTO, AutoFDO, Propeller, etc. enabled, all built using LLVM/Clang 20.1.4

2: Performed a perf using the command perf record -a -e cycles -j any,k -F 5000 -- sleep 600

3: Ran the command perf2bolt -p perf.data -o perf.fdata, which returned the first error.

I repeated the same steps after patching this out, and received the second error.

Only after setting "CONFIG_JUMP_LABEL=n", rebuilding the kernel, then repeating the steps, would it result in the perf.fdata file.

I have included the output from perf2bolt on the second error, but I'm using a different vmlinux binary so it might appear somewhat off, but it results in the same error.

perf2bolt.log

config.txt

[llvmbot]

@llvm/issue-subscribers-bolt

Author: deets (MichaelDeets)

I'm trying to BOLT my Linux Kernel. I've run a quick perf, then, while attempting to convert into a usable BOLT profile using perf2bolt, it results in this error:

Linux kernel version is unknown

I attempted to circumvent by patching bolt/lib/Rewrite/LinuxKernelRewriter.cpp and removed the check, which then, after re-running perf2bolt, results in this error:

BOLT-ERROR: while running linux-kernel-rewriter in pre-CFG state: invalid target of static keys jump at 0xffffffff81321c9c : 0xffffffff8120556e

Which stumped me for a while, as this is my first attempt at using BOLT and I'm unfamiliar with most things regarding it. What worked for me, was setting the "CONFIG_JUMP_LABEL=n" option in my Linux Kernel's configuration. I'm also using LTO, which might've contribute to this problem. I'll upload my full kernel configuration for reference.

I just hadn't seen anything regarding "CONFIG_JUMP_LABEL=n" being a requirement, and couldn't find anyone else online with similar issues, hence this bug report. Apologies in advanced if I had missed an existing bug report regarding this problem. I'm happy to attach anything else required, please let me know!

For all my LLVM related packages, I have them installed using my package manager (Portage), but for BOLT I simply build using 20.1.4 source, and also attempted the latest git version, but no changes between them.

Steps that resulted in the error:

Using https://github.com/llvm/llvm-project/blob/main/bolt/docs/OptimizingLinux.md as the guide:

1: Built the Linux Kernel with various options, like LTO, AutoFDO, Propeller, etc. enabled, all built using LLVM/Clang 20.1.4

2: Performed a perf using the command perf record -a -e cycles -j any,k -F 5000 -- sleep 600

3: Ran the command perf2bolt -p perf.data -o perf.fdata, which returned the first error.

I repeated the same steps after patching this out, and received the second error.

Only after setting "CONFIG_JUMP_LABEL=n", rebuilding the kernel, then repeating the steps, would it result in the perf.fdata file.

I have included the output from perf2bolt on the second error, but I'm using a different vmlinux binary so it might appear somewhat off, but it results in the same error.

perf2bolt.log

kernel-config.txt

[MichaelDeets]

After updating to 6.15-rc6, I rebased all my patches and it now works. I believe it was some old stale patch I had floating around that caused this issue. Will reopen if I come across again.

EDIT: Spoke too soon. It's probably something wrong with my configuration. Will do more tests before reopening.