ci(workflows): let forks opt into the benchmark workflows

[gongwei-130]

Description

Problem

The five benchmark-* workflows gate their only job on the repository name:

if: github.repository == 'smg-project/smg'

In a fork that job is therefore permanently skipped, with no way to turn it on.
The default is right — a fork should not silently burn its runners on scheduled
benchmarks — but the absence of an escape hatch has a cost: a fork that does
want benchmark coverage has to patch the guard, and then re-resolve that patch on
every sync from upstream. It also fails quietly: the workflow shows up in the
Actions tab, gets dispatched, and reports skipped with no explanation, so it is
easy to keep maintaining a workflow that has never executed.

Solution

Add an opt-in alongside the existing check:

if: github.repository == 'smg-project/smg' || vars.SMG_RUN_BENCHMARKS == 'true'

With the variable unset, vars.SMG_RUN_BENCHMARKS is the empty string, '' == 'true' is false, and the guard reduces to the repository check — no behaviour
change for this repository, or for any fork that does not set it.

Same shape as #2323: upstream default preserved in the expression, fork opts in
with a repository variable and touches no workflow file.

Changes

• benchmark-manual-policy.yml, benchmark-radix-tree.yml,
  benchmark-request-processing.yml, benchmark-tokenizer.yml,
  benchmark-tool-parser.yml — one line each.
• CONTRIBUTING.md — a "Benchmark workflows" subsection under the
  "Running CI on your own runners" section added by ci(workflows): read self-hosted runner labels from repository variables #2323.

Deliberately not changed: release-*, nightly-* and stale. A fork should
not publish artifacts or manage this repository's issues, and unlike the
benchmarks there is no legitimate fork use case for them.

Test Plan

No source code is touched — five workflow lines and one Markdown subsection, so
cargo fmt / clippy / cargo test have nothing to act on. What actually needs
proving is that the guard still evaluates false without the variable and true with
it. I ran both halves on a fork (togethercomputer/together-smg, whose name
matches neither the old nor the new repository check):

Before — old guard, SMG_RUN_BENCHMARKS unset. workflow_dispatch of
benchmark-radix-tree.yml:

run=32905934159  status=completed  conclusion=skipped
  job "Radix Tree (Cache-Aware Routing)"  conclusion=skipped  runnerName=null

After — this commit's guard, SMG_RUN_BENCHMARKS=true, same dispatch:

run=32906808531  status=in_progress
  job "Radix Tree (Cache-Aware Routing)"  status=in_progress
  runnerName=ubuntu-22.04-64core-1003039476  labels=["ubuntu-22.04-64core"]

The job is no longer skipped, it is scheduled and running. I cancelled it after
confirming it had a runner rather than let the full ~2.5 h criterion +
throughput suite complete:

run=32906808531  status=completed  conclusion=cancelled

The "no behaviour change upstream" half is the before case above: that fork's
name does not match the repository check, and with the variable unset the job was
skipped exactly as it is today. On this repository the left operand is true, so
the || short-circuits and nothing changes regardless of the variable.

YAML parse check on the five changed files:

$ python3 -c "import yaml,glob; [yaml.safe_load(open(f)) for f in glob.glob('.github/workflows/benchmark-*.yml')]; print('yaml OK')"
yaml OK

Checklist

• cargo +nightly fmt passes — n/a, no Rust changed
• cargo clippy --all-targets --all-features -- -D warnings passes — n/a, no Rust changed
• (Optional) Documentation updated — new "Benchmark workflows" subsection in CONTRIBUTING.md

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---

📝 Walkthrough

Summary by CodeRabbit

• Chores

  • Benchmark workflows can now be enabled in forked repositories using the SMG_RUN_BENCHMARKS setting.
  • Benchmark runs remain enabled automatically in the primary repository.
  • CPU benchmarks require a configured CPU runner; other benchmarks use hosted runners.

• Documentation

  • Added guidance for configuring GPU container runners, including required tools, CUDA support, Docker access, and image digest pinning.
  • Clarified that release, nightly, and stale workflows remain disabled in forks unless otherwise configured.

Walkthrough

Five benchmark workflows now support opt-in execution through SMG_RUN_BENCHMARKS. CONTRIBUTING.md documents GPU container configuration, benchmark runner settings, and workflows that remain non-opt-in.

Changes

Benchmark workflow opt-in

Layer / File(s)	Summary
Benchmark workflow gating .github/workflows/benchmark-*.yml	The benchmark jobs run in smg-project/smg or when SMG_RUN_BENCHMARKS is set to true.
Contributor benchmark guidance CONTRIBUTING.md	The documentation describes GPU container requirements, SMG_RUN_BENCHMARKS, SMG_RUNNER_CPU, and the non-opt-in release, nightly, and stale workflows.

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: 🔵 Low · up to f23dc

The change enables forks to opt into benchmark workflows while preserving existing defaults, but the contributor guide still inaccurately describes runner configuration for all benchmark workflows and could mislead users setting them up. The PR is mergeable with explicit owner follow-up to correct that documentation.

Suggested reviewers: catherinesue, key4ng

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely summarizes the main change: allowing forks to opt into benchmark workflows.
Description check	✅ Passed	The description directly explains the benchmark workflow gating change, opt-in variable, documentation updates, and validation performed.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0…
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

Full details: Docstring Coverage

Explanation

No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0 files. (1 skipped: 1 unsupported.)

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch ci/benchmarks-opt-in-for-forks

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[claude]

Clean, low-risk CI change. The five benchmark workflow guards are identically extended, documentation is accurate, and the PR includes real fork test evidence for both sides of the condition. No issues found.

[coderabbitai]

Actionable comments posted: 1

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Inline comments:
In `@CONTRIBUTING.md`:
- Around line 166-168: Correct the benchmark runner guidance in CONTRIBUTING.md:
state that SMG_RUNNER_CPU applies only to the benchmark-radix-tree workflow,
while the other benchmark workflows use ubuntu-latest; do not imply it is
required for every benchmark job.

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Configuration used: Organization UI

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📥 Commits

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📒 Files selected for processing (6)

• .github/workflows/benchmark-manual-policy.yml
• .github/workflows/benchmark-radix-tree.yml
• .github/workflows/benchmark-request-processing.yml
• .github/workflows/benchmark-tokenizer.yml
• .github/workflows/benchmark-tool-parser.yml
• CONTRIBUTING.md

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[gongwei-130]

Good catch — that was wrong. Only benchmark-radix-tree runs on the self-hosted CPU pool (runs-on: ${{ vars.SMG_RUNNER_CPU || 'k8s-runner-cpu' }}); the other four are on ubuntu-latest:

benchmark-manual-policy.yml            runs-on: ubuntu-latest
benchmark-radix-tree.yml               runs-on: ${{ vars.SMG_RUNNER_CPU || 'k8s-runner-cpu' }}
benchmark-request-processing.yml       runs-on: ubuntu-latest
benchmark-tokenizer.yml                runs-on: ubuntu-latest
benchmark-tool-parser.yml              runs-on: ubuntu-latest

Reworded so the guidance is scoped to benchmark-radix-tree instead of implying every benchmark-* job needs SMG_RUNNER_CPU.

[gongwei-130]

Rebased onto main — #2325 landed and both PRs added a subsection to the same part of CONTRIBUTING.md. Resolution keeps both, ordered "Container-mode GPU runners" then "Benchmark workflows".

The follow-up commit that corrected the SMG_RUNNER_CPU wording got folded into the resolution, so this is back to a single commit. No change to the five workflow guards.