perf(config): cut container-start latency — HZ=1000, voluntary preemption, PSI default-off (ABX-498)

[AprilNEA]

Supersedes #14 (HZ=1000) and #15 (voluntary-preemption experiment). Consolidates the three validated container-start kernel-config wins from the ABX-498 investigation into one release.

Changes (both arches, identical)

symbol	from	to	why
CONFIG_HZ	100	1000	container start is a chain of jiffy-rounded scheduler waits; 10 ms→1 ms rounding
preemption	PREEMPT_LAZY	PREEMPT_VOLUNTARY	lazy⇒PREEMPT_RCU makes rcu_barrier() on veth/netns teardown wait ~74 ms/veth; +25% ctx-switch
CONFIG_PSI_DEFAULT_DISABLED	—	=y	PSI accounting on the wakeup fast path costs ~0.85 s/build; keep compiled in (psi=1 re-enables)

Build-time assertions updated in both the host and docker build paths: HZ_1000 and PSI_DEFAULT_DISABLED join the olddefconfig-survival for sym loop; the PREEMPT_LAZY conditional assertion is replaced by a hard PREEMPT_VOLUNTARY assertion (a future kernel dropping the voluntary choice now fails loudly instead of silently re-selecting lazy).

Validation (ABX-498 four-kernel A/B, same host, docker run --rm alpine true ×10)

kernel	veth del	same-core switch	run e2e
6.18 lazy HZ100 (shipping)	74 ms	8.6 µs	496 ms
6.18 lazy HZ1000 (#14)	10 ms	8.6 µs	378 ms
6.18 voluntary HZ1000 (#15)	~0 ms	6.8 µs	371 ms
+ psi=0 (this PR, compile-time)	~0 ms	3.5 µs	—

With all three: docker run 496→246 ms, simple cold build 8.1→1.31 s (Colima 173 ms / 0.99 s). Guest-internal container start reaches parity; the residual is a moby exit-lock phase race (upstream, container-lifetime <30 ms only).

Balloon safety (PSI default-off)

CONFIG_PSI=y stays, so the capability is compiled in. The guest agent's balloon pressure watch (PsiTrigger::open) already treats a failed /proc/pressure/memory write (EOPNOTSUPP when PSI is default-disabled) as "stay on the meminfo/refault sampling path" — a designed, tested degrade. The balloon keeps functioning; memory-pressure detection moves from event-driven to 1 s polling while shrunk. psi=1 on the cmdline restores the trigger.

Consumed by arcbox after a kernel release → boot-assets bundle bump → assets.lock.

[linear-code]

ABX-498

[greptile-apps]

Greptile Summary

Updates both supported architecture configurations to reduce container-start latency:

• Selects a 1000 Hz timer frequency and voluntary preemption.
• Keeps PSI compiled in but disables it by default.
• Adds matching post-olddefconfig assertions to native and Docker build paths.

Confidence Score: 5/5

The PR appears safe to merge.

No blocking failure remains within the scope of the available previous review threads.

T-Rex Logs

What T-Rex did

• Reviewed the pre-change state where PSI support was unresolved due to CONFIG_PSI_DEFAULT_DISABLED not being set.
• Validated the post-change state where the fragment now resolves all five requested symbols and the build assertions pass with exit code 0.
• Confirmed shell syntax validation passed for the build script, the injection script, and the generated harness.

_{Ran code and verified through T-Rex}

Important Files Changed

Filename	Overview
configs/arcbox-arm64.config	Applies the HZ, preemption, and PSI defaults to the arm64 kernel configuration.
configs/arcbox-x86_64.config	Applies the same kernel configuration changes to x86_64.
scripts/build-kernel.sh	Verifies that the required configuration choices survive olddefconfig in both build paths.

_{Reviews (2): Last reviewed commit: "perf(config): disable PSI by default (ke..." | Re-trigger Greptile}