Scheduler scales poorly

### Prepare

Apply the following patch to hermit-rs:

```patch
From 2359c5885a057ea40620409e70a7e23ba897a35f Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Martin=20Kr=C3=B6ning?=

Date: Tue, 16 Jun 2026 08:18:13 +0000
Subject: [PATCH] feat: focus on laplace

---
examples/demo/src/laplace.rs | 4 +---
examples/demo/src/main.rs | 16 +++++++---------
2 files changed, 8 insertions(+), 12 deletions(-)

diff --git a/examples/demo/src/laplace.rs b/examples/demo/src/laplace.rs
index 7112320..3e48db4 100644
--- a/examples/demo/src/laplace.rs
+++ b/examples/demo/src/laplace.rs
@@ -7,7 +7,7 @@ use std::{mem, vec};

use rayon::prelude::*;

-const SIZE: usize = if cfg!(debug_assertions) { 16 } else { 64 };
+const SIZE: usize = 0x1000;
const ITERATIONS: usize = 1000;

pub fn laplace() {
@@ -20,8 +20,6 @@ pub fn laplace() {
let residual = compute(&mut matrix, SIZE, SIZE, ITERATIONS);
let elapsed = now.elapsed();
eprintln!("{ITERATIONS} iterations: {elapsed:?} (residual: {residual})");
-
- assert!(residual < 0.001);
}

fn matrix_setup(size_x: usize, size_y: usize) -> vec::Vec {
diff --git a/examples/demo/src/main.rs b/examples/demo/src/main.rs
index 4239096..51e290e 100644
--- a/examples/demo/src/main.rs
+++ b/examples/demo/src/main.rs
@@ -11,16 +11,14 @@ mod pi;
mod thread;

fn main() -> io::Result<()> {
- hello();
- print_env();
- arithmetic();
- thread::sleep();
- thread::spawn()?;
- fs::fs()?;
- pi::pi();
- matmul::matmul();
+ let rayon_num_threads = env::var("RAYON_NUM_THREADS").unwrap();
+ println!("RAYON_NUM_THREADS = {rayon_num_threads}");
+
+ laplace::laplace();
+ laplace::laplace();
+ laplace::laplace();
+ laplace::laplace();
laplace::laplace();
- mandelbrot::mandelbrot();
Ok(())
}

--
2.53.0
```

### Run on Linux (Host)

```bash
RAYON_NUM_THREADS=16 cargo run --package rusty_demo --release
```

### Run on Hermit

```bash
cargo build --package rusty_demo --release --target x86_64-unknown-hermit -Zbuild-std=std,panic_abort

sudo qemu-system-x86_64 \
-accel kvm \
-cpu host \
-display none \
-serial stdio \
-kernel kernel/hermit-loader-x86_64 \
-initrd target/x86_64-unknown-hermit/release/rusty_demo \
-smp 16 \
-m 1G \
-append "env=RAYON_NUM_THREADS=16"
```

### Results

Run on an AMD EPYC 9354P 32-Core Processor:

| OS | Available Parallelism | `RAYON_NUM_THREADS` | Time |
| ------------ | ---------------------:| -------------------:| -----:|
| Linux (Host) | 32 | 1 | 24s |
| Linux (Host) | 32 | 2 | 13.5s |
| Linux (Host) | 32 | 4 | 9.0s |
| Linux (Host) | 32 | 8 | 7.6s |
| Linux (Host) | 32 | 16 | 6.2s |
| Hermit (VM) | 16 | 1 | 11.5s |
| Hermit (VM) | 16 | 2 | 5.8s |
| Hermit (VM) | 16 | 4 | 5.2s |
| Hermit (VM) | 16 | 8 | 6.9s |
| Hermit (VM) | 16 | 16 | 12.5s |

### Issue

The results are surprising, to say the least. Hermit being faster with a single thread is nice and suspicious. Still, Hermit should not get slower when running on more cores. Not sure if this is a futex issue, scheduler bug, or a symptom of the scheduler not doing work stealing yet.