Scheduler.cpp

/*
 * Copyright (c) 2018-2021, Andreas Kling <kling@serenityos.org>
 *
 * SPDX-License-Identifier: BSD-2-Clause
 */

#include <AK/ScopeGuard.h>
#include <AK/Singleton.h>
#include <AK/Time.h>
#include <Kernel/Arch/x86/InterruptDisabler.h>
#include <Kernel/Debug.h>
#include <Kernel/Panic.h>
#include <Kernel/PerformanceManager.h>
#include <Kernel/Process.h>
#include <Kernel/RTC.h>
#include <Kernel/Scheduler.h>
#include <Kernel/Sections.h>
#include <Kernel/Time/TimeManagement.h>

// Remove this once SMP is stable and can be enabled by default
#define SCHEDULE_ON_ALL_PROCESSORS 0

namespace Kernel {

RecursiveSpinlock g_scheduler_lock;

static u32 time_slice_for(const Thread& thread)
{
    // One time slice unit == 4ms (assuming 250 ticks/second)
    if (thread.is_idle_thread())
        return 1;
    return 2;
}

READONLY_AFTER_INIT Thread* g_finalizer;
READONLY_AFTER_INIT WaitQueue* g_finalizer_wait_queue;
Atomic<bool> g_finalizer_has_work { false };
READONLY_AFTER_INIT static Process* s_colonel_process;

struct ThreadReadyQueue {
    IntrusiveList<&Thread::m_ready_queue_node> thread_list;
};

struct ThreadReadyQueues {
    u32 mask {};
    static constexpr size_t count = sizeof(mask) * 8;
    Array<ThreadReadyQueue, count> queues;
};

static Singleton<SpinlockProtected<ThreadReadyQueues>> g_ready_queues;

static SpinlockProtected<TotalTimeScheduled> g_total_time_scheduled;

// The Scheduler::current_time function provides a current time for scheduling purposes,
// which may not necessarily relate to wall time
u64 (*Scheduler::current_time)();

static void dump_thread_list(bool = false);

static inline u32 thread_priority_to_priority_index(u32 thread_priority)
{
    // Converts the priority in the range of THREAD_PRIORITY_MIN...THREAD_PRIORITY_MAX
    // to a index into g_ready_queues where 0 is the highest priority bucket
    VERIFY(thread_priority >= THREAD_PRIORITY_MIN && thread_priority <= THREAD_PRIORITY_MAX);
    constexpr u32 thread_priority_count = THREAD_PRIORITY_MAX - THREAD_PRIORITY_MIN + 1;
    static_assert(thread_priority_count > 0);
    auto priority_bucket = ((thread_priority_count - (thread_priority - THREAD_PRIORITY_MIN)) / thread_priority_count) * (ThreadReadyQueues::count - 1);
    VERIFY(priority_bucket < ThreadReadyQueues::count);
    return priority_bucket;
}

Thread& Scheduler::pull_next_runnable_thread()
{
    auto affinity_mask = 1u << Processor::current_id();

    return g_ready_queues->with([&](auto& ready_queues) -> Thread& {
        auto priority_mask = ready_queues.mask;
        while (priority_mask != 0) {
            auto priority = __builtin_ffsl(priority_mask);
            VERIFY(priority > 0);
            auto& ready_queue = ready_queues.queues[--priority];
            for (auto& thread : ready_queue.thread_list) {
                VERIFY(thread.m_runnable_priority == (int)priority);
                if (thread.is_active())
                    continue;
                if (!(thread.affinity() & affinity_mask))
                    continue;
                thread.m_runnable_priority = -1;
                ready_queue.thread_list.remove(thread);
                if (ready_queue.thread_list.is_empty())
                    ready_queues.mask &= ~(1u << priority);
                // Mark it as active because we are using this thread. This is similar
                // to comparing it with Processor::current_thread, but when there are
                // multiple processors there's no easy way to check whether the thread
                // is actually still needed. This prevents accidental finalization when
                // a thread is no longer in Running state, but running on another core.

                // We need to mark it active here so that this thread won't be
                // scheduled on another core if it were to be queued before actually
                // switching to it.
                // FIXME: Figure out a better way maybe?
                thread.set_active(true);
                return thread;
            }
            priority_mask &= ~(1u << priority);
        }
        return *Processor::idle_thread();
    });
}

Thread* Scheduler::peek_next_runnable_thread()
{
    auto affinity_mask = 1u << Processor::current_id();

    return g_ready_queues->with([&](auto& ready_queues) -> Thread* {
        auto priority_mask = ready_queues.mask;
        while (priority_mask != 0) {
            auto priority = __builtin_ffsl(priority_mask);
            VERIFY(priority > 0);
            auto& ready_queue = ready_queues.queues[--priority];
            for (auto& thread : ready_queue.thread_list) {
                VERIFY(thread.m_runnable_priority == (int)priority);
                if (thread.is_active())
                    continue;
                if (!(thread.affinity() & affinity_mask))
                    continue;
                return &thread;
            }
            priority_mask &= ~(1u << priority);
        }

        // Unlike in pull_next_runnable_thread() we don't want to fall back to
        // the idle thread. We just want to see if we have any other thread ready
        // to be scheduled.
        return nullptr;
    });
}

bool Scheduler::dequeue_runnable_thread(Thread& thread, bool check_affinity)
{
    if (thread.is_idle_thread())
        return true;

    return g_ready_queues->with([&](auto& ready_queues) {
        auto priority = thread.m_runnable_priority;
        if (priority < 0) {
            VERIFY(!thread.m_ready_queue_node.is_in_list());
            return false;
        }

        if (check_affinity && !(thread.affinity() & (1 << Processor::current_id())))
            return false;

        VERIFY(ready_queues.mask & (1u << priority));
        auto& ready_queue = ready_queues.queues[priority];
        thread.m_runnable_priority = -1;
        ready_queue.thread_list.remove(thread);
        if (ready_queue.thread_list.is_empty())
            ready_queues.mask &= ~(1u << priority);
        return true;
    });
}

void Scheduler::enqueue_runnable_thread(Thread& thread)
{
    VERIFY(g_scheduler_lock.is_locked_by_current_processor());
    if (thread.is_idle_thread())
        return;
    auto priority = thread_priority_to_priority_index(thread.priority());

    g_ready_queues->with([&](auto& ready_queues) {
        VERIFY(thread.m_runnable_priority < 0);
        thread.m_runnable_priority = (int)priority;
        VERIFY(!thread.m_ready_queue_node.is_in_list());
        auto& ready_queue = ready_queues.queues[priority];
        bool was_empty = ready_queue.thread_list.is_empty();
        ready_queue.thread_list.append(thread);
        if (was_empty)
            ready_queues.mask |= (1u << priority);
    });
}

UNMAP_AFTER_INIT void Scheduler::start()
{
    VERIFY_INTERRUPTS_DISABLED();

    // We need to acquire our scheduler lock, which will be released
    // by the idle thread once control transferred there
    g_scheduler_lock.lock();

    auto& processor = Processor::current();
    VERIFY(processor.is_initialized());
    auto& idle_thread = *Processor::idle_thread();
    VERIFY(processor.current_thread() == &idle_thread);
    idle_thread.set_ticks_left(time_slice_for(idle_thread));
    idle_thread.did_schedule();
    idle_thread.set_initialized(true);
    processor.init_context(idle_thread, false);
    idle_thread.set_state(Thread::Running);
    VERIFY(idle_thread.affinity() == (1u << processor.id()));
    processor.initialize_context_switching(idle_thread);
    VERIFY_NOT_REACHED();
}

bool Scheduler::pick_next()
{
    VERIFY_INTERRUPTS_DISABLED();

    // Set the in_scheduler flag before acquiring the spinlock. This
    // prevents a recursive call into Scheduler::invoke_async upon
    // leaving the scheduler lock.
    ScopedCritical critical;
    Processor::set_current_in_scheduler(true);
    ScopeGuard guard(
        []() {
            // We may be on a different processor after we got switched
            // back to this thread!
            VERIFY(Processor::current_in_scheduler());
            Processor::set_current_in_scheduler(false);
        });

    SpinlockLocker lock(g_scheduler_lock);

    if constexpr (SCHEDULER_RUNNABLE_DEBUG) {
        dump_thread_list();
    }

    auto& thread_to_schedule = pull_next_runnable_thread();
    if constexpr (SCHEDULER_DEBUG) {
        dbgln("Scheduler[{}]: Switch to {} @ {:#04x}:{:p}",
            Processor::current_id(),
            thread_to_schedule,
            thread_to_schedule.regs().cs, thread_to_schedule.regs().ip());
    }

    // We need to leave our first critical section before switching context,
    // but since we're still holding the scheduler lock we're still in a critical section
    critical.leave();

    thread_to_schedule.set_ticks_left(time_slice_for(thread_to_schedule));
    return context_switch(&thread_to_schedule);
}

bool Scheduler::yield()
{
    InterruptDisabler disabler;

    auto current_thread = Thread::current();
    dbgln_if(SCHEDULER_DEBUG, "Scheduler[{}]: yielding thread {} in_irq={}", Processor::current_id(), *current_thread, Processor::current_in_irq());
    VERIFY(current_thread != nullptr);
    if (Processor::current_in_irq() || Processor::in_critical()) {
        // If we're handling an IRQ we can't switch context, or we're in
        // a critical section where we don't want to switch contexts, then
        // delay until exiting the trap or critical section
        Processor::current().invoke_scheduler_async();
        return false;
    }

    if (!Scheduler::pick_next())
        return false;

    if constexpr (SCHEDULER_DEBUG)
        dbgln("Scheduler[{}]: yield returns to thread {} in_irq={}", Processor::current_id(), *current_thread, Processor::current_in_irq());
    return true;
}

bool Scheduler::context_switch(Thread* thread)
{
    if (Memory::s_mm_lock.is_locked_by_current_processor()) {
        PANIC("In context switch while holding Memory::s_mm_lock");
    }

    thread->did_schedule();

    auto from_thread = Thread::current();
    if (from_thread == thread)
        return false;

    if (from_thread) {
        // If the last process hasn't blocked (still marked as running),
        // mark it as runnable for the next round.
        if (from_thread->state() == Thread::Running)
            from_thread->set_state(Thread::Runnable);

#ifdef LOG_EVERY_CONTEXT_SWITCH
        const auto msg = "Scheduler[{}]: {} -> {} [prio={}] {:#04x}:{:p}";

        dbgln(msg,
            Processor::current_id(), from_thread->tid().value(),
            thread->tid().value(), thread->priority(), thread->regs().cs, thread->regs().ip());
#endif
    }

    auto& proc = Processor::current();
    if (!thread->is_initialized()) {
        proc.init_context(*thread, false);
        thread->set_initialized(true);
    }
    thread->set_state(Thread::Running);

    PerformanceManager::add_context_switch_perf_event(*from_thread, *thread);

    proc.switch_context(from_thread, thread);

    // NOTE: from_thread at this point reflects the thread we were
    // switched from, and thread reflects Thread::current()
    enter_current(*from_thread, false);
    VERIFY(thread == Thread::current());

    if (thread->process().is_user_process() && thread->previous_mode() != Thread::PreviousMode::KernelMode && thread->current_trap()) {
        auto& regs = thread->get_register_dump_from_stack();
        auto iopl = get_iopl_from_eflags(regs.flags());
        if (iopl != 0) {
            PANIC("Switched to thread {} with non-zero IOPL={}", Thread::current()->tid().value(), iopl);
        }
    }

    return true;
}

void Scheduler::enter_current(Thread& prev_thread, bool is_first)
{
    VERIFY(g_scheduler_lock.is_locked_by_current_processor());

    // We already recorded the scheduled time when entering the trap, so this merely accounts for the kernel time since then
    auto scheduler_time = Scheduler::current_time();
    prev_thread.update_time_scheduled(scheduler_time, true, true);
    auto* current_thread = Thread::current();
    current_thread->update_time_scheduled(scheduler_time, true, false);

    prev_thread.set_active(false);
    if (prev_thread.state() == Thread::Dying) {
        // If the thread we switched from is marked as dying, then notify
        // the finalizer. Note that as soon as we leave the scheduler lock
        // the finalizer may free from_thread!
        notify_finalizer();
    } else if (!is_first) {
        // Check if we have any signals we should deliver (even if we don't
        // end up switching to another thread).
        if (!current_thread->is_in_block() && current_thread->previous_mode() != Thread::PreviousMode::KernelMode && current_thread->current_trap()) {
            SpinlockLocker lock(current_thread->get_lock());
            if (current_thread->state() == Thread::Running && current_thread->pending_signals_for_state()) {
                current_thread->dispatch_one_pending_signal();
            }
        }
    }
}

void Scheduler::leave_on_first_switch(u32 flags)
{
    // This is called when a thread is switched into for the first time.
    // At this point, enter_current has already be called, but because
    // Scheduler::context_switch is not in the call stack we need to
    // clean up and release locks manually here
    g_scheduler_lock.unlock(flags);

    VERIFY(Processor::current_in_scheduler());
    Processor::set_current_in_scheduler(false);
}

void Scheduler::prepare_after_exec()
{
    // This is called after exec() when doing a context "switch" into
    // the new process. This is called from Processor::assume_context
    VERIFY(g_scheduler_lock.is_locked_by_current_processor());

    VERIFY(!Processor::current_in_scheduler());
    Processor::set_current_in_scheduler(true);
}

void Scheduler::prepare_for_idle_loop()
{
    // This is called when the CPU finished setting up the idle loop
    // and is about to run it. We need to acquire he scheduler lock
    VERIFY(!g_scheduler_lock.is_locked_by_current_processor());
    g_scheduler_lock.lock();

    VERIFY(!Processor::current_in_scheduler());
    Processor::set_current_in_scheduler(true);
}

Process* Scheduler::colonel()
{
    VERIFY(s_colonel_process);
    return s_colonel_process;
}

static u64 current_time_tsc()
{
    return read_tsc();
}

static u64 current_time_monotonic()
{
    // We always need a precise timestamp here, we cannot rely on a coarse timestamp
    return (u64)TimeManagement::the().monotonic_time(TimePrecision::Precise).to_nanoseconds();
}

UNMAP_AFTER_INIT void Scheduler::initialize()
{
    VERIFY(Processor::is_initialized()); // sanity check

    // Figure out a good scheduling time source
    if (Processor::current().has_feature(CPUFeature::TSC)) {
        // TODO: only use if TSC is running at a constant frequency?
        current_time = current_time_tsc;
    } else {
        // TODO: Using HPET is rather slow, can we use any other time source that may be faster?
        current_time = current_time_monotonic;
    }

    RefPtr<Thread> idle_thread;
    g_finalizer_wait_queue = new WaitQueue;

    g_finalizer_has_work.store(false, AK::MemoryOrder::memory_order_release);
    s_colonel_process = Process::create_kernel_process(idle_thread, KString::must_create("colonel"), idle_loop, nullptr, 1, Process::RegisterProcess::No).leak_ref();
    VERIFY(s_colonel_process);
    VERIFY(idle_thread);
    idle_thread->set_priority(THREAD_PRIORITY_MIN);
    idle_thread->set_name(KString::must_create("idle thread #0"));

    set_idle_thread(idle_thread);
}

UNMAP_AFTER_INIT void Scheduler::set_idle_thread(Thread* idle_thread)
{
    idle_thread->set_idle_thread();
    Processor::current().set_idle_thread(*idle_thread);
    Processor::set_current_thread(*idle_thread);
}

UNMAP_AFTER_INIT Thread* Scheduler::create_ap_idle_thread(u32 cpu)
{
    VERIFY(cpu != 0);
    // This function is called on the bsp, but creates an idle thread for another AP
    VERIFY(Processor::is_bootstrap_processor());

    VERIFY(s_colonel_process);
    Thread* idle_thread = s_colonel_process->create_kernel_thread(idle_loop, nullptr, THREAD_PRIORITY_MIN, KString::must_create(String::formatted("idle thread #{}", cpu)), 1 << cpu, false);
    VERIFY(idle_thread);
    return idle_thread;
}

void Scheduler::add_time_scheduled(u64 time_to_add, bool is_kernel)
{
    g_total_time_scheduled.with([&](auto& total_time_scheduled) {
        total_time_scheduled.total += time_to_add;
        if (is_kernel)
            total_time_scheduled.total_kernel += time_to_add;
    });
}

void Scheduler::timer_tick(const RegisterState& regs)
{
    VERIFY_INTERRUPTS_DISABLED();
    VERIFY(Processor::current_in_irq());

    auto current_thread = Processor::current_thread();
    if (!current_thread)
        return;

    // Sanity checks
    VERIFY(current_thread->current_trap());
    VERIFY(current_thread->current_trap()->regs == &regs);

#if !SCHEDULE_ON_ALL_PROCESSORS
    if (!Processor::is_bootstrap_processor())
        return; // TODO: This prevents scheduling on other CPUs!
#endif

    if (current_thread->process().is_kernel_process()) {
        // Because the previous mode when entering/exiting kernel threads never changes
        // we never update the time scheduled. So we need to update it manually on the
        // timer interrupt
        current_thread->update_time_scheduled(current_time(), true, false);
    }

    if (current_thread->previous_mode() == Thread::PreviousMode::UserMode && current_thread->should_die() && !current_thread->is_blocked()) {
        SpinlockLocker scheduler_lock(g_scheduler_lock);
        dbgln_if(SCHEDULER_DEBUG, "Scheduler[{}]: Terminating user mode thread {}", Processor::current_id(), *current_thread);
        current_thread->set_state(Thread::Dying);
        Processor::current().invoke_scheduler_async();
        return;
    }

    if (current_thread->tick())
        return;

    if (!current_thread->is_idle_thread() && !peek_next_runnable_thread()) {
        // If no other thread is ready to be scheduled we don't need to
        // switch to the idle thread. Just give the current thread another
        // time slice and let it run!
        current_thread->set_ticks_left(time_slice_for(*current_thread));
        current_thread->did_schedule();
        dbgln_if(SCHEDULER_DEBUG, "Scheduler[{}]: No other threads ready, give {} another timeslice", Processor::current_id(), *current_thread);
        return;
    }

    VERIFY_INTERRUPTS_DISABLED();
    VERIFY(Processor::current_in_irq());
    Processor::current().invoke_scheduler_async();
}

void Scheduler::invoke_async()
{
    VERIFY_INTERRUPTS_DISABLED();
    VERIFY(!Processor::current_in_irq());

    // Since this function is called when leaving critical sections (such
    // as a Spinlock), we need to check if we're not already doing this
    // to prevent recursion
    if (!Processor::current_in_scheduler())
        pick_next();
}

void Scheduler::notify_finalizer()
{
    if (g_finalizer_has_work.exchange(true, AK::MemoryOrder::memory_order_acq_rel) == false)
        g_finalizer_wait_queue->wake_all();
}

void Scheduler::idle_loop(void*)
{
    auto& proc = Processor::current();
    dbgln("Scheduler[{}]: idle loop running", proc.id());
    VERIFY(are_interrupts_enabled());

    for (;;) {
        proc.idle_begin();
        asm("hlt");

        proc.idle_end();
        VERIFY_INTERRUPTS_ENABLED();
#if SCHEDULE_ON_ALL_PROCESSORS
        yield();
#else
        if (Processor::current_id() == 0)
            yield();
#endif
    }
}

void Scheduler::dump_scheduler_state(bool with_stack_traces)
{
    dump_thread_list(with_stack_traces);
}

bool Scheduler::is_initialized()
{
    // The scheduler is initialized iff the idle thread exists
    return Processor::idle_thread() != nullptr;
}

TotalTimeScheduled Scheduler::get_total_time_scheduled()
{
    return g_total_time_scheduled.with([&](auto& total_time_scheduled) { return total_time_scheduled; });
}

void dump_thread_list(bool with_stack_traces)
{
    dbgln("Scheduler thread list for processor {}:", Processor::current_id());

    auto get_cs = [](Thread& thread) -> u16 {
        if (!thread.current_trap())
            return thread.regs().cs;
        return thread.get_register_dump_from_stack().cs;
    };

    auto get_eip = [](Thread& thread) -> u32 {
        if (!thread.current_trap())
            return thread.regs().ip();
        return thread.get_register_dump_from_stack().ip();
    };

    Thread::for_each([&](Thread& thread) {
        switch (thread.state()) {
        case Thread::Dying:
            dmesgln("  {:14} {:30} @ {:04x}:{:08x} Finalizable: {}, (nsched: {})",
                thread.state_string(),
                thread,
                get_cs(thread),
                get_eip(thread),
                thread.is_finalizable(),
                thread.times_scheduled());
            break;
        default:
            dmesgln("  {:14} Pr:{:2} {:30} @ {:04x}:{:08x} (nsched: {})",
                thread.state_string(),
                thread.priority(),
                thread,
                get_cs(thread),
                get_eip(thread),
                thread.times_scheduled());
            break;
        }
        if (with_stack_traces)
            dbgln("{}", thread.backtrace());
        return IterationDecision::Continue;
    });
}

}