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A bunch of random code samples
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PublicBin/less_minimal_scheduler.cpp

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// 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;
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;
}