ci: create reusable workflow for nix kernel builds

Adds a reusable workflow `build-kernel.yml` to cover the build-kernel
job using the new Nix based build process on the dedicated runner.

All of the `build-kernel` jobs are identical except for their git repo
and git branch name. Factor these out into a reusable workflow to reduce
code duplication.

This also removes any suffixes from the cache, which might (unlikely)
increase hit rates. Each build was suffixing their kernels separately
even though the build was identical, which was slightly wasteful. It is
unlikely any of these repos/branches have identical states though.

Test plan:
- Ran the CI and waited for build-kernel-nix to succeed in each workflow
  (added a temporary `push:` condition to make sure they all build).
  Cancelled the rest as this built kernel isn't used yet so there's no
  point causing extra queuing.

.github/workflows/bpf-next-test.yml

@@ -6,6 +6,12 @@ on:
       - cron: "0 */6 * * *"
 
 jobs:
+  build-kernel-nix:
+    uses: ./.github/workflows/build-kernel.yml
+    with:
+      git-repo: "https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git"
+      branch: for-next
+
   build-kernel:
     runs-on: ubuntu-24.04
     steps:

.github/workflows/build-kernel.yml

@@ -0,0 +1,69 @@
+on:
+  workflow_call:
+    inputs:
+      git-repo:
+        required: true
+        type: string
+      branch:
+        required: true
+        type: string
+
+jobs:
+  build-kernel:
+    runs-on: [ "self-hosted", "linux", "x64" ]
+    steps:
+      # Uncomment these if reverting to GitHub hosted runners
+      # - name: Install Nix
+      #   uses: DeterminateSystems/nix-installer-action@main
+
+      # Make very basic dependencies available in PATH
+      - name: Install basic dependencies
+        run: |
+          nix profile install nixpkgs#{git,openssl,gawk,gnutar,zstd}
+          echo "$HOME/.nix-profile/bin" >> $GITHUB_PATH
+
+      # get latest head commit of sched_ext for-next
+      - run: echo "SCHED_EXT_KERNEL_COMMIT=$(git ls-remote ${{ inputs.git-repo }} heads/${{ inputs.branch }} | awk '{print $1}')" >> $GITHUB_ENV
+
+      # check for cached kernel without downloading
+      - name: Cache Kernel
+        id: cache-kernel
+        uses: actions/cache@v4
+        with:
+          path: |
+            linux/arch/x86/boot/bzImage
+            linux/usr/include
+            linux/**/*.h
+          key: kernel-build-${{ env.SCHED_EXT_KERNEL_COMMIT }}-7
+          lookup-only: true
+
+      - if: ${{ steps.cache-kernel.outputs.cache-hit != 'true' }}
+        uses: actions/checkout@v4
+
+      - if: ${{ steps.cache-kernel.outputs.cache-hit != 'true' }}
+        uses: nicknovitski/nix-develop@v1
+        with:
+          arguments: ./.github/workflows#build-kernel
+
+      - if: ${{ steps.cache-kernel.outputs.cache-hit != 'true' }}
+        name: Clone Kernel
+        uses: cytopia/[email protected]
+        with:
+          retries: 10
+          pause: 18
+          command: git clone --single-branch -b ${{ inputs.branch }} --depth 1 ${{ inputs.git-repo }} linux
+
+      - if: ${{ steps.cache-kernel.outputs.cache-hit != 'true' }}
+        name: Select correct commit for cache hash
+        run: |
+          cd linux
+          git switch --detach ${{ env.SCHED_EXT_KERNEL_COMMIT }}
+          git log -1 --pretty=format:"%h %ad %s" --date=short
+
+      - if: ${{ steps.cache-kernel.outputs.cache-hit != 'true' }}
+      # Build a minimal kernel (with sched-ext enabled) using virtme-ng
+        run: cd linux && virtme-ng -v --build --config ../.github/workflows/sched-ext.config
+
+      - if: ${{ steps.cache-kernel.outputs.cache-hit != 'true' }}
+      # Generate kernel headers
+        run: cd linux && make headers

.github/workflows/caching-build.yml

@@ -27,63 +27,10 @@ jobs:
       - run: git diff --exit-code
 
   build-kernel-nix:
-    runs-on: [ "self-hosted", "linux", "x64" ]
-    steps:
-      # Uncomment these if reverting to GitHub hosted runners
-      # - name: Install Nix
-      #   uses: DeterminateSystems/nix-installer-action@main
-
-      # Make very basic dependencies available in PATH
-      - name: Install basic dependencies
-        run: |
-          nix profile install nixpkgs#{git,openssl,gawk,gnutar,zstd}
-          echo "$HOME/.nix-profile/bin" >> $GITHUB_PATH
-
-      # get latest head commit of sched_ext for-next
-      - run: echo "SCHED_EXT_KERNEL_COMMIT=$(git ls-remote https://git.kernel.org/pub/scm/linux/kernel/git/tj/sched_ext.git heads/for-6.13 | awk '{print $1}')" >> $GITHUB_ENV
-
-      # check for cached kernel without downloading
-      - name: Cache Kernel
-        id: cache-kernel
-        uses: actions/cache@v4
-        with:
-          path: |
-            linux/arch/x86/boot/bzImage
-            linux/usr/include
-            linux/**/*.h
-          key: kernel-build-${{ env.SCHED_EXT_KERNEL_COMMIT }}-7
-          lookup-only: true
-
-      - if: ${{ steps.cache-kernel.outputs.cache-hit != 'true' }}
-        uses: actions/checkout@v4
-
-      - if: ${{ steps.cache-kernel.outputs.cache-hit != 'true' }}
-        uses: nicknovitski/nix-develop@v1
-        with:
-          arguments: ./.github/workflows#build-kernel
-
-      - if: ${{ steps.cache-kernel.outputs.cache-hit != 'true' }}
-        name: Clone Kernel
-        uses: cytopia/[email protected]
-        with:
-          retries: 10
-          pause: 18
-          command: git clone --single-branch -b for-6.13 --depth 1 https://git.kernel.org/pub/scm/linux/kernel/git/tj/sched_ext.git linux
-
-      - if: ${{ steps.cache-kernel.outputs.cache-hit != 'true' }}
-        name: Select correct commit for cache hash
-        run: |
-          cd linux
-          git switch --detach ${{ env.SCHED_EXT_KERNEL_COMMIT }}
-          git log -1 --pretty=format:"%h %ad %s" --date=short
-
-      - if: ${{ steps.cache-kernel.outputs.cache-hit != 'true' }}
-      # Build a minimal kernel (with sched-ext enabled) using virtme-ng
-        run: cd linux && virtme-ng -v --build --config ../.github/workflows/sched-ext.config
-
-      - if: ${{ steps.cache-kernel.outputs.cache-hit != 'true' }}
-      # Generate kernel headers
-        run: cd linux && make headers
+    uses: ./.github/workflows/build-kernel.yml
+    with:
+      git-repo: "https://git.kernel.org/pub/scm/linux/kernel/git/tj/sched_ext.git"
+      branch: for-6.13
 
   build-kernel:
     runs-on: ubuntu-24.04

.github/workflows/for-next-test.yml

@@ -6,6 +6,12 @@ on:
       - cron: "0 */6 * * *"
 
 jobs:
+  build-kernel-nix:
+    uses: ./.github/workflows/build-kernel.yml
+    with:
+      git-repo: "https://git.kernel.org/pub/scm/linux/kernel/git/tj/sched_ext.git"
+      branch: for-next
+
   lint:
     runs-on: ubuntu-24.04
     steps:

.github/workflows/stable.yml

@@ -6,6 +6,12 @@ on:
     - cron: "0 */6 * * *"
 
 jobs:
+  build-kernel-nix:
+    uses: ./.github/workflows/build-kernel.yml
+    with:
+      git-repo: "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git"
+      branch: linux-rolling-stable
+
   build-kernel:
     runs-on: ubuntu-24.04
     steps: