/*
 * Copyright (c) 2018-2021, Andreas Kling <kling@serenityos.org>
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are met:
 *
 * 1. Redistributions of source code must retain the above copyright notice, this
 *    list of conditions and the following disclaimer.
 *
 * 2. Redistributions in binary form must reproduce the above copyright notice,
 *    this list of conditions and the following disclaimer in the documentation
 *    and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
 * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
 * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
 * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
 * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
 * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */

#include <AK/Debug.h>
#include <AK/QuickSort.h>
#include <AK/ScopeGuard.h>
#include <AK/TemporaryChange.h>
#include <AK/Time.h>
#include <Kernel/PerformanceEventBuffer.h>
#include <Kernel/Process.h>
#include <Kernel/RTC.h>
#include <Kernel/Scheduler.h>
#include <Kernel/Time/TimeManagement.h>
#include <Kernel/TimerQueue.h>

//#define LOG_EVERY_CONTEXT_SWITCH
//#define SCHEDULER_DEBUG
//#define SCHEDULER_RUNNABLE_DEBUG

namespace Kernel {

class SchedulerPerProcessorData {
    AK_MAKE_NONCOPYABLE(SchedulerPerProcessorData);
    AK_MAKE_NONMOVABLE(SchedulerPerProcessorData);

public:
    SchedulerPerProcessorData() = default;

    WeakPtr<Thread> m_pending_beneficiary;
    const char* m_pending_donate_reason { nullptr };
    bool m_in_scheduler { true };
};

SchedulerData* g_scheduler_data;
RecursiveSpinLock g_scheduler_lock;

void Scheduler::init_thread(Thread& thread)
{
    ASSERT(g_scheduler_data);
    g_scheduler_data->m_nonrunnable_threads.append(thread);
}

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

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

void Scheduler::start()
{
    ASSERT_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();
    processor.set_scheduler_data(*new SchedulerPerProcessorData());
    ASSERT(processor.is_initialized());
    auto& idle_thread = *processor.idle_thread();
    ASSERT(processor.current_thread() == &idle_thread);
    ASSERT(processor.idle_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);
    ASSERT(idle_thread.affinity() == (1u << processor.id()));
    processor.initialize_context_switching(idle_thread);
    ASSERT_NOT_REACHED();
}

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

    auto current_thread = Thread::current();

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

    ScopedSpinLock lock(g_scheduler_lock);

    if (current_thread->should_die() && current_thread->state() == Thread::Running) {
        // Rather than immediately killing threads, yanking the kernel stack
        // away from them (which can lead to e.g. reference leaks), we always
        // allow Thread::wait_on to return. This allows the kernel stack to
        // clean up and eventually we'll get here shortly before transitioning
        // back to user mode (from Processor::exit_trap). At this point we
        // no longer want to schedule this thread. We can't wait until
        // Scheduler::enter_current because we don't want to allow it to
        // transition back to user mode.

        if constexpr (debug_scheduler)
            dbgln("Scheduler[{}]: Thread {} is dying", Processor::current().id(), *current_thread);

        current_thread->set_state(Thread::Dying);
    }

    if constexpr (debug_scheduler_runnable) {
        dbgln("Scheduler[{}j]: Non-runnables:", Processor::current().id());
        Scheduler::for_each_nonrunnable([&](Thread& thread) -> IterationDecision {
            if (thread.state() == Thread::Dying) {
                dbgln("  {:12} {} @ {:04x}:{:08x} Finalizable: {}",
                    thread.state_string(),
                    thread,
                    thread.tss().cs,
                    thread.tss().eip,
                    thread.is_finalizable());
            } else {
                dbgln("  {:12} {} @ {:04x}:{:08x}",
                    thread.state_string(),
                    thread,
                    thread.tss().cs,
                    thread.tss().eip);
            }

            return IterationDecision::Continue;
        });

        dbgln("Scheduler[{}j]: Runnables:", Processor::current().id());
        Scheduler::for_each_runnable([](Thread& thread) -> IterationDecision {
            dbgln("  {:3}/{:2} {:12} @ {:04x}:{:08x}",
                thread.effective_priority(),
                thread.priority(),
                thread.state_string(),
                thread.tss().cs,
                thread.tss().eip);

            return IterationDecision::Continue;
        });
    }

    Thread* thread_to_schedule = nullptr;

    auto pending_beneficiary = scheduler_data.m_pending_beneficiary.strong_ref();
    Vector<Thread*, 128> sorted_runnables;
    for_each_runnable([&](auto& thread) {
        if ((thread.affinity() & (1u << Processor::current().id())) == 0)
            return IterationDecision::Continue;
        if (thread.state() == Thread::Running && &thread != current_thread)
            return IterationDecision::Continue;
        sorted_runnables.append(&thread);
        if (&thread == pending_beneficiary) {
            thread_to_schedule = &thread;
            return IterationDecision::Break;
        }
        return IterationDecision::Continue;
    });

    if (thread_to_schedule) {
        // The thread we're supposed to donate to still exists
        const char* reason = scheduler_data.m_pending_donate_reason;
        scheduler_data.m_pending_beneficiary = nullptr;
        scheduler_data.m_pending_donate_reason = nullptr;

        // 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();

        dbgln<debug_scheduler>("Processing pending donate to {} reason={}", *thread_to_schedule, reason);
        return donate_to_and_switch(thread_to_schedule, reason);
    }

    // Either we're not donating or the beneficiary disappeared.
    // Either way clear any pending information
    scheduler_data.m_pending_beneficiary = nullptr;
    scheduler_data.m_pending_donate_reason = nullptr;

    quick_sort(sorted_runnables, [](auto& a, auto& b) { return a->effective_priority() >= b->effective_priority(); });

    for (auto* thread : sorted_runnables) {
        if (thread->process().exec_tid() && thread->process().exec_tid() != thread->tid())
            continue;

        ASSERT(thread->state() == Thread::Runnable || thread->state() == Thread::Running);

        if (!thread_to_schedule) {
            thread->m_extra_priority = 0;
            thread_to_schedule = thread;
        } else {
            thread->m_extra_priority++;
        }
    }

    if (!thread_to_schedule)
        thread_to_schedule = Processor::current().idle_thread();

    if constexpr (debug_scheduler) {
        dbgln("Scheduler[{}]: Switch to {} @ {:04x}:{:08x}",
            Processor::current().id(),
            *thread_to_schedule,
            thread_to_schedule->tss().cs, thread_to_schedule->tss().eip);
    }

    // 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& proc = Processor::current();
    auto& scheduler_data = proc.get_scheduler_data();

    // Clear any pending beneficiary
    scheduler_data.m_pending_beneficiary = nullptr;
    scheduler_data.m_pending_donate_reason = nullptr;

    auto current_thread = Thread::current();
    dbgln<debug_scheduler>("Scheduler[{}]: yielding thread {} in_irq={}", proc.id(), *current_thread, proc.in_irq());
    ASSERT(current_thread != nullptr);
    if (proc.in_irq() || proc.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
        proc.invoke_scheduler_async();
        return false;
    }

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

    if constexpr (debug_scheduler)
        dbgln("Scheduler[{}]: yield returns to thread {} in_irq={}", Processor::current().id(), *current_thread, Processor::current().in_irq());
    return true;
}

bool Scheduler::donate_to_and_switch(Thread* beneficiary, [[maybe_unused]] const char* reason)
{
    ASSERT(g_scheduler_lock.own_lock());

    auto& proc = Processor::current();
    ASSERT(proc.in_critical() == 1);

    unsigned ticks_left = Thread::current()->ticks_left();
    if (!beneficiary || beneficiary->state() != Thread::Runnable || ticks_left <= 1)
        return Scheduler::yield();

    unsigned ticks_to_donate = min(ticks_left - 1, time_slice_for(*beneficiary));
    dbgln<debug_scheduler>("Scheduler[{}]: Donating {} ticks to {}, reason={}", proc.id(), ticks_to_donate, *beneficiary, reason);
    beneficiary->set_ticks_left(ticks_to_donate);

    return Scheduler::context_switch(beneficiary);
}

bool Scheduler::donate_to(RefPtr<Thread>& beneficiary, const char* reason)
{
    ASSERT(beneficiary);

    if (beneficiary == Thread::current())
        return Scheduler::yield();

    // Set the m_in_scheduler flag before acquiring the spinlock. This
    // prevents a recursive call into Scheduler::invoke_async upon
    // leaving the scheduler lock.
    ScopedCritical critical;
    auto& proc = Processor::current();
    auto& scheduler_data = proc.get_scheduler_data();
    scheduler_data.m_in_scheduler = true;
    ScopeGuard guard(
        []() {
            // We may be on a different processor after we got switched
            // back to this thread!
            auto& scheduler_data = Processor::current().get_scheduler_data();
            ASSERT(scheduler_data.m_in_scheduler);
            scheduler_data.m_in_scheduler = false;
        });

    ASSERT(!proc.in_irq());

    if (proc.in_critical() > 1) {
        scheduler_data.m_pending_beneficiary = beneficiary; // Save the beneficiary
        scheduler_data.m_pending_donate_reason = reason;
        proc.invoke_scheduler_async();
        return false;
    }

    ScopedSpinLock lock(g_scheduler_lock);

    // "Leave" the critical section before switching context. Since we
    // still hold the scheduler lock, we're not actually leaving it.
    // Processor::switch_context expects Processor::in_critical() to be 1
    critical.leave();
    donate_to_and_switch(beneficiary, reason);
    return false;
}

bool Scheduler::context_switch(Thread* thread)
{
    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
        dbgln("Scheduler[{}]: {} -> {} [prio={}] {:04x}:{:08x}", Processor::current().id(), from_thread->tid().value(), thread->tid().value(), thread->priority(), thread->tss().cs, thread->tss().eip);
#endif
    }

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

    // 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.
    thread->set_active(true);

    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);
    ASSERT(thread == Thread::current());

#if ARCH(I386)
    auto iopl = get_iopl_from_eflags(Thread::current()->get_register_dump_from_stack().eflags);
    if (thread->process().is_user_process() && iopl != 0) {
        dbgln("PANIC: Switched to thread {} with non-zero IOPL={}", Thread::current()->tid().value(), iopl);
        Processor::halt();
    }
#endif

    return true;
}

void Scheduler::enter_current(Thread& prev_thread, bool is_first)
{
    ASSERT(g_scheduler_lock.own_lock());
    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).
        auto current_thread = Thread::current();
        if (!current_thread->is_in_block()) {
            ScopedSpinLock 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);
    auto& scheduler_data = Processor::current().get_scheduler_data();
    ASSERT(scheduler_data.m_in_scheduler);
    scheduler_data.m_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
    ASSERT(g_scheduler_lock.own_lock());
    auto& scheduler_data = Processor::current().get_scheduler_data();
    ASSERT(!scheduler_data.m_in_scheduler);
    scheduler_data.m_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
    ASSERT(!g_scheduler_lock.own_lock());
    g_scheduler_lock.lock();
    auto& scheduler_data = Processor::current().get_scheduler_data();
    ASSERT(!scheduler_data.m_in_scheduler);
    scheduler_data.m_in_scheduler = true;
}

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

void Scheduler::initialize()
{
    ASSERT(&Processor::current() != nullptr); // sanity check

    RefPtr<Thread> idle_thread;
    g_scheduler_data = new SchedulerData;
    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, "colonel", idle_loop, nullptr, 1).leak_ref();
    ASSERT(s_colonel_process);
    ASSERT(idle_thread);
    idle_thread->set_priority(THREAD_PRIORITY_MIN);
    idle_thread->set_name(StringView("idle thread #0"));

    set_idle_thread(idle_thread);
}

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

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

    ASSERT(s_colonel_process);
    Thread* idle_thread = s_colonel_process->create_kernel_thread(idle_loop, nullptr, THREAD_PRIORITY_MIN, String::format("idle thread #%u", cpu), 1 << cpu, false);
    ASSERT(idle_thread);
    return idle_thread;
}

void Scheduler::timer_tick(const RegisterState& regs)
{
    ASSERT_INTERRUPTS_DISABLED();
    ASSERT(Processor::current().in_irq());

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

    bool is_bsp = Processor::current().id() == 0;
    if (!is_bsp)
        return; // TODO: This prevents scheduling on other CPUs!
    if (current_thread->process().is_profiling()) {
        ASSERT(current_thread->process().perf_events());
        auto& perf_events = *current_thread->process().perf_events();
        [[maybe_unused]] auto rc = perf_events.append_with_eip_and_ebp(regs.eip, regs.ebp, PERF_EVENT_SAMPLE, 0, 0);
    }

    if (current_thread->tick((regs.cs & 3) == 0))
        return;

    ASSERT_INTERRUPTS_DISABLED();
    ASSERT(Processor::current().in_irq());
    Processor::current().invoke_scheduler_async();
}

void Scheduler::invoke_async()
{
    ASSERT_INTERRUPTS_DISABLED();
    auto& proc = Processor::current();
    ASSERT(!proc.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 (!proc.get_scheduler_data().m_in_scheduler)
        pick_next();
}

void Scheduler::yield_from_critical()
{
    auto& proc = Processor::current();
    ASSERT(proc.in_critical());
    ASSERT(!proc.in_irq());

    yield(); // Flag a context switch

    u32 prev_flags;
    u32 prev_crit = Processor::current().clear_critical(prev_flags, false);

    // Note, we may now be on a different CPU!
    Processor::current().restore_critical(prev_crit, prev_flags);
}

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*)
{
    dbgln("Scheduler[{}]: idle loop running", Processor::current().id());
    ASSERT(are_interrupts_enabled());

    for (;;) {
        asm("hlt");

        if (Processor::current().id() == 0)
            yield();
    }
}

}