perf(serverless): defer CPU probe off worker import path

[deanq]

Summary

rp_debugger ran cpuinfo.get_cpu_info() at module level. rp_job imports rp_debugger unconditionally, so every serverless worker paid the CPU probe at startup, even though the value is only used when a job sets rp_debugger: true.

• PROCESSOR constant replaced by a cached get_processor_info(), resolved on first debugger output
• import cpuinfo moved inside it
• Missing brand_raw still falls back to "Unable to get processor info."

The module-level PROCESSOR name is removed; nothing in the SDK referenced it outside the old test.

Impact

On x86 the probe is much more expensive than on macOS: py-cpuinfo measures clock speed in a child process around a fixed sleep(1) (CPUID.get_raw_hz). On arm64 it skips that path.

Measured on a real RTX 4090 serverless worker (AMD EPYC 7B13, 128 CPUs):

	Time
cpuinfo.get_cpu_info()	1169 / 1171 / 1172 ms
Generated handler loaded -> first worker-start fitness check, 2.0.0rc1	1459 / 1469 ms
Same span with the lazy probe bundled	536 ms

"cpuinfo" in sys.modules was False inside the handler with the lazy probe. Locally, import runpod.serverless: ~240ms -> ~162ms (macOS arm64, warm, best of 3).

Test plan

• New: subprocess test that cpuinfo is not in sys.modules after importing rp_job (fails on main)
• New: probe runs once and is cached; fallback on missing brand_raw; debugger output includes processor
• make quality-check: 1446 passed, 91.8% coverage

Linear: SLS-808, SLS-814

[Copilot]

🟢 Approval recommended

The focused change preserves debugger output and fallback behavior, with targeted regression coverage and no unresolved findings.

No open issues 🎉

What changed in this PR

Defers CPU detection until debugger output is requested, reducing serverless worker startup overhead.

Changes:

• Replaces the eager CPU probe with a cached, lazy lookup.
• Preserves the missing-processor fallback.
• Adds regression tests for lazy loading, caching, and debugger output.

File	Description
tests/​test_serverless/​test_utils/​test_debugger.py	Tests processor caching, fallback, and output.
tests/​test_performance/​test_cold_start.py	Checks worker imports do not load cpuinfo.
runpod/​serverless/​utils/​rp_debugger.py	Defers and caches CPU detection.

_{Review effort: Balanced}