From 98bbefa622333430ccf66c21f3a83da570d48e83 Mon Sep 17 00:00:00 2001
From: Archivist <archivist@noreply.nekoit.xyz>
Date: Thu, 4 Nov 2021 14:24:21 +0000
Subject: [PATCH] Add 'less_minimal_scheduler.cpp'

---
 less_minimal_scheduler.cpp | 469 +++++++++++++++++++++++++++++++++++++
 1 file changed, 469 insertions(+)
 create mode 100644 less_minimal_scheduler.cpp

diff --git a/less_minimal_scheduler.cpp b/less_minimal_scheduler.cpp
new file mode 100644
index 0000000..10f82fe
--- /dev/null
+++ b/less_minimal_scheduler.cpp
@@ -0,0 +1,469 @@
+// Build with: clang++ --std=c++20 -g -O1 schedtest.cpp
+
+#include <array>
+#include <span>
+#include <memory>
+#include <list>
+#include <functional>
+#include <iostream>
+#include <map>
+#include <chrono>
+#include <exception>
+#include <atomic>
+#include <set>
+#include <vector>
+#include <optional>
+
+#if defined(__x86_64__)
+    #if defined(__GNUG__)
+        constexpr bool is_x86_64_linux = true;
+    #else
+        #if defined(__clang__)
+            constexpr bool is_x86_64_linux = true;
+        #else
+            constexpr bool is_x86_64_linux = false;
+        #endif
+    #endif
+#else
+    constexpr bool is_x86_64_linux = false;
+#endif
+
+static_assert(is_x86_64_linux, "This code probably only works on x86_64 GNU Extensions C++");
+
+struct platform_data {
+    platform_data() = default;
+    
+    platform_data(std::span<char> stack_str)
+    : stack_ptr(&*(stack_str.end()-16))
+    , base_ptr(stack_ptr)
+    {}
+
+    uint64_t rbx, r12, r13, r14, r15;
+
+    void* stack_ptr;
+    void* base_ptr;
+
+    void pull() __attribute__((always_inline))
+    {
+        __asm__ __volatile__(
+            "movq %%rsp, %0\n"
+            "movq %%rbp, %1\n" 
+            "movq %%rbx, %2\n" 
+            "movq %%r12, %3\n" 
+            "movq %%r13, %4\n" 
+            "movq %%r14, %5\n" 
+            "movq %%r15, %6\n" 
+            : "=m"(stack_ptr)
+            , "=m"(base_ptr)
+            , "=m"(rbx)
+            , "=m"(r12)
+            , "=m"(r13)
+            , "=m"(r14)
+            , "=m"(r15)
+        );
+    }
+
+    void* push(void* location)  __attribute__((always_inline))
+    {
+        volatile void* volatile tmp = static_cast<char*>(stack_ptr) - sizeof(void*);
+        *static_cast<volatile void* volatile * volatile>(tmp) = location;
+        __asm__ __volatile__(
+            "movq %1, %%rsp\n"
+            "movq %2, %%rbp\n"
+            "movq %3, %%rbx\n"
+            "movq %4, %%r12\n"
+            "movq %5, %%r13\n"
+            "movq %6, %%r14\n"
+            "movq %7, %%r15\n"
+            "popq %0\n"
+            : "+r"(location)
+            : "m"(tmp)
+            , "m"(base_ptr)
+            , "m"(rbx)
+            , "m"(r12)
+            , "m"(r13)
+            , "m"(r14)
+            , "m"(r15)
+            : "memory"
+        );
+        return location;
+    }
+};
+
+enum class process_status {
+    inactive = 0,
+    running = 1,
+    waiting = 2,
+    finished = 3,
+    zombie = 4
+};
+
+struct process {
+    static int64_t counter;
+
+    char* stack;
+    size_t sz;
+    platform_data scheduling_swapper;
+    process_status state = process_status::inactive;
+    std::function<void()> fn;
+    int64_t t_id;
+
+    process(std::function<void()> _fn, size_t _sz = 16384)
+    : stack(new char[_sz])
+    , sz(_sz)
+    , scheduling_swapper(std::span<char>(stack, sz))
+    , fn(_fn)
+    , t_id(counter++)
+    {}
+
+    process(char* stack_base)
+    : stack(stack_base)
+    , sz(0)
+    , t_id(counter++)
+    {}
+
+    process(const process&) = delete;
+
+    ~process() {
+        if(sz) delete[] stack;
+    }
+};
+int64_t process::counter = 0;
+
+
+__attribute__((noinline)) struct system* spawner (struct system* sys);
+
+struct system {
+    static system sys;
+
+    std::list<std::unique_ptr<process>>
+        running,
+        waiting,
+        naughty;
+    
+    std::unique_ptr<process> previous;
+    std::unique_ptr<process> current;
+    
+    std::unique_ptr<process> one() {
+        auto v = std::move(running.back());
+        running.pop_back();
+        return v;
+    }
+
+    void rid(std::unique_ptr<process> current) {
+        switch(current->state) {
+            case process_status::inactive:
+            case process_status::running:
+                running.push_front(std::move(current));
+                break;
+            case process_status::finished:
+                clean(std::move(current));
+                break;
+            case process_status::zombie:
+                naughty.push_front(std::move(current));
+                break;
+            case process_status::waiting:
+                waiting.push_front(std::move(current));
+                break;
+        }
+    }
+
+    void clean(std::unique_ptr<process>) {}
+
+    void yield_to(std::unique_ptr<process> target) noexcept {
+        current->scheduling_swapper.pull();
+        sys.rid(std::move(current));
+        current = std::move(target);
+        current->scheduling_swapper.push(this);
+        spawner(&sys);
+    }
+
+
+    void yield() noexcept {
+        current->scheduling_swapper.pull();
+        sys.rid(std::move(current));
+        current = one();
+        current->scheduling_swapper.push(this);
+        spawner(&sys);
+    }
+
+    template<typename fn>
+    void steal_and_yield(fn func) noexcept {
+        current->scheduling_swapper.pull();
+        func(std::move(current));
+        current = one();
+        current->scheduling_swapper.push(this);
+        spawner(&sys);
+    }
+};
+
+
+// Needs to return the system one way or another
+__attribute__((noinline)) struct system* spawner (struct system* sys) {
+    auto& proc = *system::sys.current;
+    if(proc.state == process_status::inactive) {
+        proc.state = process_status::running;
+        proc.fn();
+        proc.state = process_status::finished;
+        sys->current->scheduling_swapper.pull();
+        sys->yield();
+    }
+    return sys;
+}
+
+struct system system::sys;
+
+/*********************       ***********************/
+
+class dirty_bottleneck {
+    std::atomic_bool flag;
+
+    [[nodiscard]] bool try_lock() {
+        bool f = false;
+        bool t = true;
+        return flag.compare_exchange_strong(f,t,std::memory_order::acquire);
+    }
+
+    [[nodiscard]] bool try_unlock() {
+        bool f = false;
+        bool t = true;
+        return flag.compare_exchange_strong(t,f,std::memory_order::release);
+    }
+public:
+    dirty_bottleneck() = default;
+    dirty_bottleneck(dirty_bottleneck&) = delete;
+    dirty_bottleneck(dirty_bottleneck&&) = delete;
+
+    void lock() {
+        while(not try_lock());
+    }
+
+    void unlock() {
+        if(!try_unlock()) throw std::runtime_error("Unlocking failed in dirty_bottleneck: potential double unlocking issue");
+    }
+};
+
+/*********************       ***********************/
+
+template<typename T>
+class lock_guard {
+    T& ref;
+public:
+    lock_guard(T& _ref)
+    : ref(_ref)
+    {
+        ref.lock();
+    }
+
+    ~lock_guard() {
+        ref.unlock();
+    }
+};
+
+/*********************       ***********************/
+
+using mutex_handle = size_t;
+
+enum class thread_state {
+    locking,
+    waiting,
+    unlocking
+};
+
+enum class mutex_state {
+    remove = 0,
+    create = 1
+};
+
+using namespace std::chrono_literals;
+
+void mutex_state_update(mutex_handle, mutex_state);
+void signal_locking(thread_state state, mutex_handle mtx, int64_t thrd);
+
+class fast_bottleneck {
+    
+    /** This is a secret tool that will help us later **/
+    static std::atomic<size_t> counter;
+    const mutex_handle handle;
+    dirty_bottleneck trigger_lock;
+    std::list<std::unique_ptr<process>> waiting; 
+    
+    std::atomic_bool flag;
+
+    [[nodiscard]] bool try_lock() {
+        bool f = false;
+        bool t = true;
+        return flag.compare_exchange_strong(f,t,std::memory_order::acquire);
+    }
+
+    [[nodiscard]] bool try_unlock() {
+        bool f = false;
+        bool t = true;
+        return flag.compare_exchange_strong(t,f,std::memory_order::release);
+    }
+public:
+    fast_bottleneck()
+    : flag()
+    , handle(counter.fetch_add(1)) //< We have to initialize that
+    {
+        mutex_state_update(handle, mutex_state::create);
+    }
+    fast_bottleneck(fast_bottleneck&) = delete;
+    fast_bottleneck(fast_bottleneck&&) = delete;
+    fast_bottleneck& operator=(fast_bottleneck&) = delete;
+    fast_bottleneck& operator=(fast_bottleneck&&) = delete;
+    
+    ~fast_bottleneck() {
+        mutex_state_update(handle, mutex_state::remove);
+    }
+
+
+    void lock() {
+        /// The exponential backing variables
+        constexpr std::chrono::milliseconds max{1};
+        std::chrono::nanoseconds wait{256};
+        while(not try_lock()) {
+            /// The implementation of our little trick when waiting
+            signal_locking(thread_state::waiting, handle, system::sys.current->t_id);
+
+            system::sys.steal_and_yield([&](std::unique_ptr<process> p){
+                lock_guard triggers(trigger_lock);
+                p->state = process_status::waiting;
+                waiting.push_front(std::move(p));
+            });
+            
+            /// The exponential backing
+            //std::this_thread::sleep_for(wait);
+            wait += wait < max ? std::chrono::nanoseconds(wait.count()/2) : 0ns;
+        }
+        /// The implementation of our little trick when locking
+        signal_locking(thread_state::locking, handle, system::sys.current->t_id);
+    }
+
+    void unlock() {
+        if(!try_unlock()) throw std::runtime_error("Unlocking failed in fast_bottleneck: potential double unlocking issue");
+        /// The implementation of our little trick when unlocking
+        signal_locking(thread_state::unlocking, handle, system::sys.current->t_id);
+        
+        {
+            lock_guard triggers(trigger_lock);
+            if(waiting.size()) {
+                system::sys.running.push_front(std::move(waiting.back()));
+                waiting.pop_back();
+            }
+        }
+    }
+};
+
+dirty_bottleneck checker_lock;
+dirty_bottleneck lister_lock;
+std::map<int64_t, std::vector<mutex_handle>> owned_locks;
+std::map<int64_t, std::optional<mutex_handle>> waiting_locks;
+std::set<mutex_handle> locks_that_exist;
+
+void mutex_state_update(mutex_handle mtx, mutex_state state) {
+    lock_guard lister(lister_lock);
+    switch(state) {
+        case mutex_state::create: {
+            locks_that_exist.insert(mtx);
+        }break;
+        case mutex_state::remove: {
+            locks_that_exist.erase(mtx);
+        }break;
+    }
+}
+
+bool build_dependency_graph (
+    const mutex_handle mtx,
+    const int64_t thrd, 
+    std::map<int64_t, std::vector<mutex_handle>>& owned_locks, 
+    std::map<int64_t, std::optional<mutex_handle>>& waiting_locks
+) {
+    std::map<mutex_handle, std::set<mutex_handle>> graph;
+    for(auto& elem : waiting_locks) {
+        if(elem.second.has_value()) {
+            for(auto& n : owned_locks[elem.first]) {
+                graph[n].insert(elem.second.value());
+            }
+        }
+    }
+
+    std::set<mutex_handle> nodes;
+    {
+        lock_guard lister(lister_lock);
+        nodes = locks_that_exist;
+    }
+
+    bool happened = true;
+
+    while(happened) {
+        happened = false;
+        for(auto& n : nodes) {
+            if(graph[n].size() == 0)
+            {
+                happened = true;
+                for(auto v : graph) {
+                    v.second.erase(n);
+                }
+                nodes.erase(n);
+                break;
+            }
+        }
+    }
+
+    return nodes.size();
+}
+
+void signal_locking(thread_state state, mutex_handle mtx, int64_t thrd) {
+    bool bad = false;
+    {
+        lock_guard checker(checker_lock);
+        switch(state) {
+            case thread_state::locking: {
+                waiting_locks[thrd].reset();
+                owned_locks[thrd].push_back(mtx);
+            } break;
+            case thread_state::unlocking: {
+                auto it = std::find(owned_locks[thrd].begin(), owned_locks[thrd].end(), mtx);
+                if(it != owned_locks[thrd].end()) {
+                    owned_locks[thrd].erase(it);
+                }
+            } break;
+            case thread_state::waiting: {
+                waiting_locks[thrd] = mtx;
+                bad = build_dependency_graph(mtx, thrd, owned_locks, waiting_locks);
+            } break;
+        }
+    }
+    if(bad) throw std::runtime_error("Deadlock detected");
+}
+
+std::atomic<size_t> fast_bottleneck::counter;
+
+/*********************       ***********************/
+
+fast_bottleneck A;
+fast_bottleneck B;
+
+int main() {
+    char c;
+    system::sys.current = std::make_unique<process>(&c);
+
+    std::cout << "1" << std::endl;
+    A.lock();
+    system::sys.current->state = process_status::running;
+    system::sys.yield_to(std::make_unique<process>([](){
+        A.lock();
+        std::cout << "A" << std::endl;
+        A.unlock();
+    }));
+    A.unlock();
+    system::sys.yield();
+    system::sys.yield_to(std::make_unique<process>([](){
+        A.lock();
+        std::cout << "B" << std::endl;
+        A.unlock();
+    }));
+    std::cout << "3" << std::endl;
+    return 0;
+}