runbound 0.4.17

// SPDX-License-Identifier: AGPL-3.0-or-later
// Copyright (C) 2024-2026 RedLemonBe — https://github.com/redlemonbe/Runbound
// CPU topology helpers — physical-core discovery and per-thread affinity pinning.
// On non-Linux targets all functions compile but pin_to_cpu() is a no-op.

/// Parse a kernel CPU-list string (e.g. "0,4" or "0-3,8-11") into CPU IDs.
fn parse_cpu_list(s: &str) -> Vec<usize> {
    let mut cpus = Vec::new();
    for part in s.trim().split(',') {
        if let Some((a, b)) = part.split_once('-') {
            if let (Ok(lo), Ok(hi)) = (a.trim().parse::<usize>(), b.trim().parse::<usize>()) {
                cpus.extend(lo..=hi);
            }
        } else if let Ok(n) = part.trim().parse::<usize>() {
            cpus.push(n);
        }
    }
    cpus
}

/// Returns one logical CPU ID per physical core, SMT/HyperThreading siblings excluded.
///
/// Reads `thread_siblings_list` for each online CPU. A CPU is a physical-core
/// representative if and only if it is the lowest-numbered CPU in its sibling group.
/// This works identically on Intel HT and AMD SMT (Threadripper PRO / EPYC).
///
/// Example — 4C/8T CPU:
///   cpu0: siblings = [0,4] → min=0 == cpu_id → keep
///   cpu4: siblings = [0,4] → min=0 ≠ cpu_id → skip (SMT sibling of cpu0)
///
/// Falls back to all available logical CPUs when `/sys` is unavailable
/// (containers, non-Linux, permission errors).
pub fn physical_cores() -> Vec<usize> {
    let mut physical = Vec::new();
    for cpu_id in 0..4096 {
        let path = format!(
            "/sys/devices/system/cpu/cpu{cpu_id}/topology/thread_siblings_list"
        );
        match std::fs::read_to_string(&path) {
            Ok(s) => {
                let siblings = parse_cpu_list(&s);
                let min = siblings.iter().copied().min().unwrap_or(cpu_id);
                if min == cpu_id {
                    physical.push(cpu_id);
                }
            }
            Err(_) => break,
        }
    }
    if physical.is_empty() {
        let n = std::thread::available_parallelism().map(|n| n.get()).unwrap_or(1);
        (0..n).collect()
    } else {
        physical
    }
}

/// Pin the calling thread to `cpu_id` using `sched_setaffinity(2)`.
/// Silent no-op on failure or on non-Linux targets.
pub fn pin_to_cpu(cpu_id: usize) {
    #[cfg(target_os = "linux")]
    unsafe {
        let mut set = std::mem::zeroed::<libc::cpu_set_t>();
        libc::CPU_SET(cpu_id, &mut set);
        // Errors (EPERM in containers, invalid cpu_id) are silently ignored.
        libc::sched_setaffinity(0, std::mem::size_of::<libc::cpu_set_t>(), &set);
    }
    #[cfg(not(target_os = "linux"))]
    let _ = cpu_id;
}