M6.5 Phase 3: Scheduling Contracts

Phase 3 validates scheduling-related Linux contracts: nice values, process priority, sched_yield, sched_getaffinity, and basic fork scheduling fairness.

Tests implemented

nice_values — Tests setpriority/getpriority round-trip for nice values 0→5→10→19. The test only increases nice (lower priority) since Linux denies nice decrease for unprivileged users (EPERM).

sched_yield — Validates that sched_yield() returns 0 and sched_getaffinity() returns at least 1 CPU.

sched_fairness — Forks a child, waits for it via waitpid(), verifies the child ran and exited with the expected status. This is intentionally minimal: proper CFS weight testing is timing-sensitive under QEMU TCG and prone to false failures.

getpriority — Already passing from Phase 1.5.

Bug fix: sched_getaffinity return value

sched_getaffinity was returning 0 instead of the number of bytes written. musl uses this return value to determine how many bits to scan in the cpu_set_t mask. Returning 0 made CPU_COUNT() always return 0.

#![allow(unused)]
fn main() {
// Before:
Ok(0)
// After:
Ok(size as isize)
}

Known gaps

• MAP_SHARED + fork: Kevlar's fork deep-copies all pages, including MAP_SHARED mappings. This breaks shared-memory IPC between parent and child. A proper fix needs VMA flags tracking (MAP_SHARED vs MAP_PRIVATE) and page-table-level sharing during fork. Tracked for future work.

• Preemption latency test: Skipped for now — requires setitimer and SIGALRM delivery (Phase 4 scope).

• CFS weights: No test for proportional CPU time distribution based on nice values. The scheduler stores nice but doesn't use it for scheduling decisions yet.

Results

Full suite: 13/14 PASS (sa_restart needs setitimer).