Implement Preemptive Scheduling

app/src/main/cpp/skyline/kernel/scheduler.cpp

@@ -1,18 +1,30 @@
 // SPDX-License-Identifier: MPL-2.0
 // Copyright © 2020 Skyline Team and Contributors (https://github.com/skyline-emu/)
 
+#include <unistd.h>
 #include <common/signal.h>
 #include "types/KThread.h"
 #include "scheduler.h"
 
 namespace skyline::kernel {
-    Scheduler::CoreContext::CoreContext(u8 id) : id(id) {}
+    Scheduler::CoreContext::CoreContext(u8 id, u8 preemptionPriority) : id(id), preemptionPriority(preemptionPriority) {}
 
     Scheduler::Scheduler(const DeviceState &state) : state(state) {}
 
+    void Scheduler::SignalHandler(int signal, siginfo *info, ucontext *ctx, void **tls) {
+        if (*tls) {
+            const auto &state{*reinterpret_cast<nce::ThreadContext *>(*tls)->state};
+            state.scheduler->Rotate(false);
+            state.scheduler->WaitSchedule();
+            YieldPending = false;
+        } else {
+            YieldPending = true;
+        }
+    }
+
     Scheduler::CoreContext &Scheduler::LoadBalance() {
         auto &thread{state.thread};
-        auto currentCore{&cores.at(thread->coreId)};
+        auto *currentCore{&cores.at(thread->coreId)};
 
         if (!currentCore->queue.empty() && thread->affinityMask.count() != 1) {
             // Select core where the current thread will be scheduled the earliest based off average timeslice durations for resident threads
@@ -71,15 +83,26 @@ namespace skyline::kernel {
         auto &thread{state.thread};
         auto &core{loadBalance ? LoadBalance() : cores.at(thread->coreId)};
 
+        signal::SetSignalHandler({YieldSignal}, SignalHandler);
+
+        if (!thread->preemptionTimer) {
+            struct sigevent event{
+                .sigev_signo = YieldSignal,
+                .sigev_notify = SIGEV_THREAD_ID,
+                .sigev_notify_thread_id = gettid(),
+            };
+            timer_create(CLOCK_THREAD_CPUTIME_ID, &event, &*thread->preemptionTimer);
+        }
+
         std::unique_lock lock(core.mutex);
         auto nextThread{std::find_if(core.queue.begin(), core.queue.end(), [&](const std::shared_ptr<type::KThread> &it) { return it->priority > thread->priority; })};
         if (nextThread == core.queue.begin() && nextThread != core.queue.end()) {
-            throw exception("Migration Interrupt Required");
+            core.queue.front()->SendSignal(YieldSignal);
+            core.queue.insert(std::next(core.queue.begin()), thread);
         } else {
             core.queue.insert(nextThread, thread);
+            core.mutateCondition.notify_all(); // We only want to trigger the conditional variable if the current thread isn't going to be scheduled next
         }
-
-        core.mutateCondition.notify_all();
     }
 
     void Scheduler::WaitSchedule() {
@@ -88,7 +111,7 @@ namespace skyline::kernel {
 
         std::shared_lock lock(core->mutex);
         if (thread->affinityMask.count() > 1) {
-            std::chrono::milliseconds loadBalanceThreshold{1}; //!< The amount of time that needs to pass unscheduled for a thread to attempt load balancing
+            std::chrono::milliseconds loadBalanceThreshold{PreemptiveTimeslice * 2}; //!< The amount of time that needs to pass unscheduled for a thread to attempt load balancing
             while (!core->mutateCondition.wait_for(lock, loadBalanceThreshold, [&]() { return core->queue.front() == thread; })) {
                 lock.unlock();
                 LoadBalance();
@@ -106,10 +129,16 @@ namespace skyline::kernel {
             core->mutateCondition.wait(lock, [&]() { return core->queue.front() == thread; });
         }
 
+        if (thread->priority == core->preemptionPriority) {
+            struct itimerspec spec{.it_value = {.tv_nsec = std::chrono::duration_cast<std::chrono::nanoseconds>(PreemptiveTimeslice).count()}};
+            timer_settime(*thread->preemptionTimer, 0, &spec, nullptr);
+            thread->isPreempted = true;
+        }
+
         thread->timesliceStart = util::GetTimeTicks();
     }
 
-    void Scheduler::Rotate() {
+    void Scheduler::Rotate(bool cooperative) {
         auto &thread{state.thread};
         auto &core{cores.at(thread->coreId)};
 
@@ -122,6 +151,14 @@ namespace skyline::kernel {
 
             core.mutateCondition.notify_all();
         }
+
+        if (cooperative && thread->isPreempted) {
+            // If a preemptive thread did a cooperative yield then we need to disarm the preemptive timer
+            struct itimerspec spec{};
+            timer_settime(*thread->preemptionTimer, 0, &spec, nullptr);
+        }
+
+        thread->isPreempted = false;
     }
 
     void Scheduler::RemoveThread() {
@@ -130,5 +167,13 @@ namespace skyline::kernel {
 
         std::unique_lock lock(core.mutex);
         core.queue.erase(std::remove(core.queue.begin(), core.queue.end(), thread), core.queue.end());
+
+        if (thread->isPreempted) {
+            struct itimerspec spec{};
+            timer_settime(*thread->preemptionTimer, 0, &spec, nullptr);
+            thread->isPreempted = false;
+        }
+
+        YieldPending = false;
     }
 }

app/src/main/cpp/skyline/kernel/scheduler.h

@@ -8,7 +8,7 @@
 
 namespace skyline {
     namespace constant {
-        constexpr u8 CoreCount{4}; // The amount of cores an HOS process can be scheduled onto (User applications can only be on the first 3 cores, the last one is reserved for the system)
+        constexpr u8 CoreCount{4}; //!< The amount of cores an HOS process can be scheduled onto (User applications can only be on the first 3 cores, the last one is reserved for the system)
     }
 
     namespace kernel {
@@ -44,22 +44,32 @@ namespace skyline {
 
             struct CoreContext {
                 u8 id;
+                u8 preemptionPriority; //!< The priority at which this core becomes preemptive as opposed to cooperative
                 std::shared_mutex mutex; //!< Synchronizes all operations on the queue
                 std::condition_variable_any mutateCondition; //!< A conditional variable which is signalled on every mutation of the queue
                 std::deque<std::shared_ptr<type::KThread>> queue; //!< A queue of threads which are running or to be run on this core
 
-                explicit CoreContext(u8 id);
+                CoreContext(u8 id, u8 preemptionPriority);
             };
 
-            std::array<CoreContext, constant::CoreCount> cores{CoreContext(0), CoreContext(1), CoreContext(2), CoreContext(3)};
+            std::array<CoreContext, constant::CoreCount> cores{CoreContext(0, 59), CoreContext(1, 59), CoreContext(2, 59), CoreContext(3, 63)};
 
           public:
+            static constexpr std::chrono::milliseconds PreemptiveTimeslice{10}; //!< The duration of time a preemptive thread can run before yielding
+            inline static int YieldSignal{SIGRTMIN}; //!< The signal used to cause a yield in running threads
+            inline static thread_local bool YieldPending{}; //!< A flag denoting if a yield is pending on this thread, it's checked at SVC exit
+
             Scheduler(const DeviceState &state);
 
+            /**
+             * @brief A signal handler designed to cause a non-cooperative yield for preemption and higher priority threads being inserted
+             */
+            static void SignalHandler(int signal, siginfo *info, ucontext *ctx, void **tls);
+
             /**
              * @brief Checks all cores and migrates the calling thread to the core where the calling thread should be scheduled the earliest
              * @return A reference to the CoreContext of the core which the calling thread is running on after load balancing
-             * @note This doesn't insert the thread into the migrated process's queue after load-balancing
+             * @note This doesn't insert the thread into the migrated process's queue after load balancing
              */
             CoreContext& LoadBalance();
 
@@ -77,8 +87,9 @@ namespace skyline {
 
             /**
              * @brief Rotates the calling thread's resident core queue, if it is at the front of it
+             * @param cooperative If this was triggered by a cooperative yield as opposed to a preemptive one
              */
-            void Rotate();
+            void Rotate(bool cooperative = true);
 
             /**
              * @brief Removes the calling thread from it's resident core queue

app/src/main/cpp/skyline/kernel/svc.cpp

@@ -299,7 +299,7 @@ namespace skyline::kernel::svc {
             state.scheduler->WaitSchedule();
         } else if (in == yieldWithoutCoreMigration || in == yieldWithCoreMigration || in == yieldToAnyThread) {
             // Core Migration doesn't affect us as threads schedule and load balance themselves
-            state.logger->Debug("svcSleepThread: Yielding thread ({})", in);
+            state.logger->Debug("svcSleepThread: Cooperative Yield");
             state.scheduler->Rotate();
             state.scheduler->WaitSchedule();
         }

app/src/main/cpp/skyline/kernel/types/KThread.cpp

@@ -7,6 +7,7 @@
 #include <nce.h>
 #include <os.h>
 #include "KProcess.h"
+#include "KThread.h"
 
 namespace skyline::kernel::type {
     KThread::KThread(const DeviceState &state, KHandle handle, KProcess *parent, size_t id, void *entry, u64 argument, void *stackTop, i8 priority, i8 idealCore) : handle(handle), parent(parent), id(id), entry(entry), entryArgument(argument), stackTop(stackTop), idealCore(idealCore), coreId(idealCore), KSyncObject(state, KType::KThread) {
@@ -23,6 +24,9 @@ namespace skyline::kernel::type {
         }
         if (thread.joinable())
             thread.join();
+
+        if (preemptionTimer)
+            timer_delete(*preemptionTimer);
     }
 
     void KThread::StartThread() {
@@ -188,6 +192,12 @@ namespace skyline::kernel::type {
         }
     }
 
+    void KThread::SendSignal(int signal) {
+        std::lock_guard lock(mutex);
+        if (running)
+            pthread_kill(pthread, signal);
+    }
+
     void KThread::UpdatePriority(i8 priority) {
         this->priority = priority;
     }

app/src/main/cpp/skyline/kernel/types/KThread.h

@@ -44,6 +44,8 @@ namespace skyline {
             CoreMask affinityMask{}; //!< A mask of CPU cores this thread is allowed to run on
             u64 timesliceStart{}; //!< Start of the scheduler timeslice
             u64 averageTimeslice{}; //!< A weighted average of the timeslice duration for this thread
+            std::optional<timer_t> preemptionTimer{}; //!< A kernel timer used for preemption interrupts
+            bool isPreempted{}; //!< If the preemption timer has been armed and will fire
 
             KThread(const DeviceState &state, KHandle handle, KProcess *parent, size_t id, void *entry, u64 argument, void *stackTop, i8 priority, i8 idealCore);
 
@@ -61,6 +63,11 @@ namespace skyline {
              */
             void Kill(bool join);
 
+            /**
+             * @brief Sends a host OS signal to the thread which is running this KThread
+             */
+            void SendSignal(int signal);
+
             void UpdatePriority(i8 priority);
         };
     }

app/src/main/cpp/skyline/nce.cpp

@@ -24,6 +24,12 @@ namespace skyline::nce {
             } else {
                 throw exception("Unimplemented SVC 0x{:X}", svc);
             }
+
+            if (kernel::Scheduler::YieldPending) {
+                state.scheduler->Rotate(false);
+                state.scheduler->WaitSchedule();
+                kernel::Scheduler::YieldPending = false;
+            }
         } catch (const signal::SignalException &e) {
             if (e.signal != SIGINT) {
                 state.logger->Error("{} (SVC: 0x{:X})", e.what(), svc);
@@ -46,7 +52,7 @@ namespace skyline::nce {
     }
 
     void NCE::SignalHandler(int signal, siginfo *info, ucontext *ctx, void **tls) {
-        if (*tls) {
+        if (*tls) { // If TLS was restored then this occurred in guest code
             auto &mctx{ctx->uc_mcontext};
             const auto &state{*reinterpret_cast<ThreadContext *>(*tls)->state};
             if (signal != SIGINT) {
@@ -95,7 +101,7 @@ namespace skyline::nce {
             mctx.regs[1] = true;
 
             *tls = nullptr;
-        } else {
+        } else { // If TLS wasn't restored then this occurred in host code
             signal::ExceptionalSignalHandler(signal, info, ctx); //!< Delegate throwing a host exception to the exceptional signal handler
         }
     }

app/src/main/cpp/skyline/vfs/npdm.cpp

@@ -31,8 +31,8 @@ namespace skyline::vfs {
             .magic = MetaMagic,
         };
         threadInfo = {
-            .coreMask = 0b1110,
-            .priority = {0, 63},
+            .coreMask = 0b0111,
+            .priority = {0, 59},
         };
     }