Jungfraujoch/common/ThreadAffinity.cpp

// SPDX-FileCopyrightText: 2026 Filip Leonarski, Paul Scherrer Institute <filip.leonarski@psi.ch>
// SPDX-License-Identifier: GPL-3.0-only

#include "ThreadAffinity.h"

#ifdef __linux__

#include <cstdio>
#include <filesystem>
#include <fstream>
#include <sstream>

#include <pthread.h>
#include <sched.h>

namespace {
    // The process's CPUs, taken before any thread has been pinned: static initialisation runs on the
    // main thread before main().
    cpu_set_t StartMask() {
        cpu_set_t mask;
        CPU_ZERO(&mask);
        if (sched_getaffinity(0, sizeof(mask), &mask) != 0)
            for (int c = 0; c < CPU_SETSIZE; c++)
                CPU_SET(c, &mask);
        return mask;
    }
    const cpu_set_t start_mask = StartMask();

    int NumaNodeCount() {
        int n = 0;
        std::error_code ec;
        for (const auto &e : std::filesystem::directory_iterator("/sys/devices/system/node", ec)) {
            const std::string name = e.path().filename().string();
            if (name.rfind("node", 0) == 0 && name.size() > 4
                && name.find_first_not_of("0123456789", 4) == std::string::npos)
                n++;
        }
        return n;
    }

    // "0-23,48-71" -> the CPUs it names.
    bool ParseCpuList(const std::string &list, cpu_set_t &out) {
        CPU_ZERO(&out);
        std::stringstream ss(list);
        std::string range;
        bool any = false;
        while (std::getline(ss, range, ',')) {
            int lo, hi;
            if (std::sscanf(range.c_str(), "%d-%d", &lo, &hi) == 2) {
            } else if (std::sscanf(range.c_str(), "%d", &lo) == 1) {
                hi = lo;
            } else
                continue;
            for (int c = lo; c <= hi && c < CPU_SETSIZE; c++) {
                CPU_SET(c, &out);
                any = true;
            }
        }
        return any;
    }
}

int NumaNodeOfPciDevice(const std::string &pci_bus_id) {
    if (NumaNodeCount() < 2)
        return -1;
    unsigned domain, bus, device, function;
    if (std::sscanf(pci_bus_id.c_str(), "%x:%x:%x.%x", &domain, &bus, &device, &function) != 4)
        return -1;
    char path[128];
    std::snprintf(path, sizeof(path), "/sys/bus/pci/devices/%04x:%02x:%02x.%x/numa_node",
                  domain, bus, device, function);
    std::ifstream f(path);
    int node = -1;
    if (!(f >> node))
        return -1;
    return node;
}

void PinThreadToNumaNode(int node) {
    if (node < 0)
        return;
    std::ifstream f("/sys/devices/system/node/node" + std::to_string(node) + "/cpulist");
    std::string list;
    cpu_set_t node_cpus;
    if (!std::getline(f, list) || !ParseCpuList(list, node_cpus))
        return;
    cpu_set_t mask;
    CPU_AND(&mask, &node_cpus, &start_mask);
    if (CPU_COUNT(&mask) == 0)
        return;
    pthread_setaffinity_np(pthread_self(), sizeof(mask), &mask);
}

void RestoreThreadAffinity() {
    pthread_setaffinity_np(pthread_self(), sizeof(start_mask), &start_mask);
}

#else

int NumaNodeOfPciDevice(const std::string &) { return -1; }
void PinThreadToNumaNode(int) {}
void RestoreThreadAffinity() {}

#endif