feat(sched): arm the scheduler preemption-rate gate; ADR-0022 Accepted (prereq 2, PR-2) - #218
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…d (prereq 2, PR-2) Flips scripts/test_qos_sched.py from report-only to a real gate. It asserts the PR-1-calibrated scalar — preemptions per 1000 guest ticks — within [100, 600], on top of the non-vacuous liveness floor. The metric is a delta over GUEST ticks, so the band is host-CPU-invariant: a slow CI runner reads the same rate (it just takes longer wall-clock to fill the window). - FLOOR = 100 (= q5-median 200 x 0.5) catches a COARSENED round-robin cadence (the scheduler getting more sluggish — ADR-0022's regression direction). - CEIL = 600 (= q5-median x 3) is a loose thrash cap that does NOT redden on the intended future fix (a SMALLER quantum raises the rate). Revert-confirmed on WSL (feedback-anti-vacuous-boundary-gates — PR not done until RED observed): default SCHED_QUANTUM_TICKS=5 -> gate GREEN (rate 199.9); set 20 -> rate 49.9 < floor -> gate RED with "round-robin cadence has COARSENED"; restore 5 -> GREEN. (q=80 grinds the windowed harness itself, so 20 is the practical lever, as PR-1's calibration established.) Completes ADR-0022's second prerequisite, so the ADR flips to Accepted: the deferral decision and both prerequisites (latency gate + scheduler baseline) are done. The latency fix itself (the I/O-priority-boost epic or a measured quantum reduction) stays a deliberately deferred, now-unblocked future epic. No kernel change (docs + harness + gate comments only). py-compile + YAML green. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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What
PR-2 of ADR-0022 prerequisite 2 — arms the scheduler baseline gate on the scalar PR-1's calibration selected, and flips ADR-0022 to Accepted.
scripts/test_qos_sched.pyflips from report-only to a real gate: it asserts preemptions per 1000 guest ticks in[100, 600](the calibrated metric, ~200 at the default quantum) on top of the non-vacuous liveness floor. Everything asserted is a delta over guest ticks, so the band is host-CPU-invariant — a slow CI runner reads the same rate.Revert-confirmed (anti-vacuous)
On WSL, byte-identical protocol:
SCHED_QUANTUM_TICKSThe gate reddens on exactly the regression the ADR guards (a coarsened quantum). q=80 grinds the windowed harness itself, so 20 is the practical lever — as PR-1's calibration established.
ADR status
Completes ADR-0022's second prerequisite, so the ADR flips to Accepted: the deferral decision and both prerequisites (latency gate + scheduler baseline) are done. The latency fix itself (the I/O-priority-boost epic, or a measured quantum reduction) stays a deliberately deferred, now-unblocked future epic — it now has both a proven latency gate and a proven scheduler baseline to be measured against.
Verification
No kernel change (docs + harness + gate comments only).
make ci-smoke-schedpasses at the default quantum (199.9 ∈ [100,600]); py-compile + YAML green.🤖 Generated with Claude Code