Test a New Scheduler

End-to-end workflow: define a scheduler, write tests, run them.

1. Define the scheduler

Use declare_scheduler! to register a scheduler in the KTSTR_SCHEDULERS distributed slice. The verifier sweep picks it up automatically.

use ktstr::declare_scheduler;
use ktstr::prelude::*;

declare_scheduler!(MY_SCHED, {
    name = "my_sched",
    binary = "scx_my_sched",
    topology = (1, 2, 4, 1),
    kernels = ["6.14", "6.15..=7.0"],
    sched_args = ["--exit-dump-len", "1048576"],
});

The macro generates pub static MY_SCHED: Scheduler plus a private linkme registration so cargo ktstr verifier discovers the scheduler automatically. Tests reference the bare MY_SCHED ident via #[ktstr_test(scheduler = MY_SCHED)].

See Scheduler Definitions for every supported field.

2. Write integration tests

Tests inherit the scheduler’s topology. Override with explicit llcs, cores, or threads when needed.

use ktstr::prelude::*;

#[ktstr_test(scheduler = MY_SCHED)]
fn basic_steady(ctx: &Ctx) -> Result<AssertResult> {
    // Inherits 1n2l4c1t from MY_SCHED
    scenarios::steady(ctx)
}

#[ktstr_test(scheduler = MY_SCHED, threads = 2)]
fn smt_steady(ctx: &Ctx) -> Result<AssertResult> {
    // Inherits llcs=2, cores=4; overrides threads to exercise SMT
    scenarios::steady(ctx)
}

3. Build a kernel

Build a kernel with sched_ext support:

cargo ktstr kernel build

See Getting Started: Build a kernel for version selection and local source builds.

4. Run

cargo ktstr test --kernel ../linux

5. Check BPF complexity (optional)

Collect per-program verifier statistics across the declared kernels and accepted topology presets:

# Use the kernel auto-discovered via KTSTR_KERNEL / cache.
cargo ktstr verifier

# Pin to a specific kernel build.
cargo ktstr verifier --kernel ../linux

# Sweep across multiple kernels. Each scheduler's
# `kernels = [...]` declaration acts as a per-scheduler filter on
# the operator-supplied set; an empty (or omitted) `kernels` field
# means the scheduler runs against every kernel in the sweep.
cargo ktstr verifier --kernel 6.14 --kernel 7.0

See BPF Verifier for output format, cycle collapse, and the cell-name → kernel matching contract.

6. Manage the kernel cache

Cached kernel images accumulate under $XDG_CACHE_HOME/ktstr/kernels/. Keep a handful of recent builds and drop the rest when disk pressure grows:

cargo ktstr kernel list                # inspect cache contents
cargo ktstr kernel clean --keep 3      # keep the 3 most recent images
cargo ktstr kernel clean --force       # remove everything (non-interactive)

7. Debug failures

Boot an interactive shell with the scheduler binary:

cargo ktstr shell -i ./target/debug/scx_my_sched

Inside the guest, run /include-files/scx_my_sched manually to inspect behavior. See cargo-ktstr shell for all flags.

See The #[ktstr_test] Macro for all available attributes and Scheduler Definitions for the full Scheduler type and the declare_scheduler! macro.

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