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Build linux-tkg profile optimization with AutoFDO, Propeller, and BOLT

Accurate, representative profiles are required. Google reports gains of around 10% in microbenchmarks and 5% in larger real-world workloads.

Requirements

  • Kernel 6.13 or newer
  • Clang/LLVM 19 or newer (llvm-profgen and llvm-profdata)
  • llvm-propeller installed for Propeller
  • llvm-bolt and perf2bolt installed for BOLT
  • BOLT currently supports x86-64 kernels 7.1 and 7.2 in linux-tkg
  • perf installed and working with the running kernel; AMD profiling requires perf built with libpfm support

Notes

  • Run the documented commands from the root of the linux-tkg source tree.
  • The helper generate-profile.sh automatically records the appropriate perf event and saves the profile in ~/.config/frogminer, creating the directory if needed.
  • Keep all build inputs unchanged across kernel builds, including _compileroptlevel, _processor_opt, _lto_mode, _cpusched, the remaining configuration, sources, patches, and compiler version.
  • Each profile option points to its profile; _llvm_propeller_profile is the prefix of the two files created in step 4, not a separate file.

References:


1. Build AutoFDO Pass 1

Set in customization.cfg:

_compiler="llvm"
_lto_mode="thin"
_llvm_autofdo="pass1"

Automatic: _llvm_propeller="false", _debugdisable="false", _STRIP="false"; ignores _llvm_autofdo_profile and _llvm_propeller_profile

Build with the normal command for your distribution:

# Arch
makepkg -si

# Other supported distributions
./install.sh install

Boot the new kernel.


2. Create the AutoFDO profile

Run the helper as your normal user:

./optimization/generate-profile.sh autofdo

The default recording lasts 600 seconds. Pass a different duration in seconds:

./optimization/generate-profile.sh autofdo 1800

Keep the system under representative load for the complete recording.

For example, spend about 600 seconds each:

  • Playing games
  • Compiling or running benchmarks
  • Using normal desktop, network, and file workloads

The sleep command only sets the duration; perf records system-wide activity.

Output:

~/.config/frogminer/tkg.afdo

3. Build AutoFDO Pass 2 + Propeller Pass 1

For AutoFDO-only, set _llvm_propeller="false" and use this as the final build.

To build AutoFDO + Propeller, set in customization.cfg:

_llvm_autofdo="pass2"
_llvm_autofdo_profile="~/.config/frogminer/tkg.afdo"

_llvm_propeller="pass1"

Automatic: _debugdisable="false", _STRIP="false"; ignores _llvm_propeller_profile

Build with the normal command for your distribution:

# Arch
makepkg -si

# Other supported distributions
./install.sh install

Boot the new kernel.


4. Create the Propeller profiles

After booting the kernel built in step 3, run:

./optimization/generate-profile.sh propeller

The default recording lasts 600 seconds. To record for 1800 seconds:

./optimization/generate-profile.sh propeller 1800

Use the same representative workloads during the complete Propeller recording.

Output:

~/.config/frogminer/tkg-propeller_cc_profile.txt
~/.config/frogminer/tkg-propeller_ld_profile.txt

The helper creates both files using the default prefix ~/.config/frogminer/tkg-propeller.


5. Build the final kernel

Set:

_llvm_autofdo="pass2"
_llvm_autofdo_profile="~/.config/frogminer/tkg.afdo"
_llvm_propeller="pass2"
_llvm_propeller_profile="~/.config/frogminer/tkg-propeller"

# optional; allowed again
_STRIP="true"

pass2 means that the configured profiles must be used:

  • If the AutoFDO profile is missing, both AutoFDO and Propeller are disabled.
  • If only a Propeller profile is missing, AutoFDO remains enabled and Propeller is disabled.

Build and install the kernel once more. The final build must report both the AutoFDO and Propeller profiles.

Build with the normal command for your distribution:

# Arch
makepkg -si

# Other supported distributions
./install.sh install

Boot the new AutoFDO + Propeller build kernel! 🚀


Optional BOLT extension

BOLT optimizes the linked vmlinux after AutoFDO and Propeller. It needs one profiling build and one final build.

6. Build BOLT Pass 1

Keep the final compiler profile settings from step 5 and add:

_llvm_bolt="pass1"

_debugdisable="false" and _STRIP="false" are forced automatically. Build, install, and boot this kernel.

Do not rebuild or change its configuration before creating the BOLT profile. The helper needs the exact unstripped vmlinux from this running kernel. The default path is:

/lib/modules/$(uname -r)/build/vmlinux

7. Create the BOLT profile

./optimization/generate-profile.sh bolt

The default recording lasts 600 seconds. For 1800 seconds at a lower sampling frequency:

BOLT_FREQUENCY=1000 ./optimization/generate-profile.sh bolt 1800

Output:

~/.config/frogminer/tkg.fdata

If the matching vmlinux is stored elsewhere, specify it explicitly:

VMLINUX=/path/to/pass1/vmlinux ./optimization/generate-profile.sh bolt

8. Build BOLT Pass 2

Keep every other build setting unchanged and change only:

_llvm_bolt="pass2"
_llvm_bolt_profile="~/.config/frogminer/tkg.fdata"

Build and install once more. The build must report the BOLT profile and BOLT vmlinux. Boot the final kernel.