ANDROID: Add document for kernel AutoFDO profile

Bug: 444197707
Test: none
Change-Id: If132c044ab68fee94f4277f95799c5bd5f032ac2
Signed-off-by: Yabin Cui <yabinc@google.com>

gki/aarch64/afdo/README.md

@@ -0,0 +1,81 @@
+# AutoFDO profiles for Android common kernels
+
+This directory contains AutoFDO profiles for Android common kernels. These profiles are used to
+optimize kernel builds, improving performance for specific architectures and kernel versions.
+
+## Profile Availability
+
+The AutoFDO profile (kernel.afdo) for vmlinux is updated regularly for the following kernel
+branches:
+
+* [android15-6.6](https://android.googlesource.com/kernel/common/+/refs/heads/android15-6.6/android/gki/aarch64/afdo)
+* [android16-6.12](https://android.googlesource.com/kernel/common/+/refs/heads/android16-6.12/gki/aarch64/afdo/)
+
+
+## Performance improvements
+
+When applying these AutoFDO profiles to the android15-6.6 and android16-6.12 kernels, we observed
+the following performance improvements during testing on a Pixel 6 device.
+
+
+| Benchmark             | Improvement |
+| --------------------- | ----------- |
+| Boot time             | 2-3%        |
+| Cold App launch time  | 3-4%        |
+| Binder-rpc            | 8-9%        |
+| Binder-addints        | 12-25%      |
+| Hwbinder              | 12-18%      |
+| Bionic (syscall_mmap) | 6%          |
+
+
+## Steps to reproduce the profile
+
+A kernel profile is generated by running app crawling and app launching for top 100 apps from Google
+Play Store. While running, we collect ETM data for the kernel, which records executed instruction
+stream. Finally, we merge and convert ETM data to one AutoFDO profile.
+
+### 1. Build a kernel image and flash it on an Android device
+  * The source code and test device used to generate each profile are described above.
+  * We use a Pixel device. But using other real devices should get a similar profile.
+
+### 2. Run app crawling and app launching for top 100 apps
+  * Add a gmail account on the test device. Because app crawler can use the account to automatically
+    login some of the apps.
+  * We run [App Crawler](https://developer.android.com/studio/test/other-testing-tools/app-crawler)
+    for one app for 3 minutes, and run it twice.
+  * We run app launching for one app for 3 seconds, and run it 15 times. After each running, the
+    app is killed and cache is cleared. So we get profile for cold app startups.
+
+### 3. Generate the AutoFDO profile
+
+First, collect performance data using simpleperf with Coresight ETM or ARM ETE while running app
+crawling and app launching scenarios. Following is an example recording kernel activity for 180
+seconds. For a complete guide, refer to [Record ETM data for the kernel](https://android.googlesource.com/platform/system/extras/+/refs/heads/android16-release/simpleperf/doc/collect_etm_data_for_autofdo.md#A-complete-example_kernel).
+
+```sh
+(device) / $ simpleperf record -e cs-etm:k -a --duration 180 -z -o perf.data
+```
+
+Next, convert the ETM data to an AutoFDO profile using `simpleperf inject` and `create_llvm_prof`
+on the host machine. For a complete guide, refer to [Convert ETM data to AutoFDO Profile on Host](https://android.googlesource.com/platform/system/extras/+/refs/heads/android16-release/simpleperf/doc/collect_etm_data_for_autofdo.md#A-complete-example_kernel).
+
+Note that `create_llvm_prof` recently enabled the symbol list by default, which can cause Clang
+to de-optimize any kernel function not listed in the profile. This is usually not we want in the
+kernel, unless the profile cover all critical paths. So we recommend adding `--prof_sym_list=false`
+to avoid de-optimization. This also prevents the following build error from section mismatch.
+
+```
+WARNING: modpost: vmlinux: section mismatch in reference: list_add+0x0 (section: .text.hot.list_add) -> dir_list (section: .init.data)
+```
+
+We use the following command to perform the conversion.
+
+```sh
+# Convert the AutoFDO profile to the LLVM profile format:
+# The --prof_sym_list=false flag is important for kernel profiles. Without it, clang
+# assumes any function not listed in the profile is cold. This can lead to unwanted
+# deoptimizations, even when -fprofile-sample-accurate is not enabled.
+(host) $ create_llvm_prof --profiler=text --binary={vmlinux_dir}/vmlinux \
+         --profile=kernel.autofdo --format=extbinary --use_fs_discriminator \
+         --out=kernel.afdo --prof_sym_list=false
+```