Kernel: Add a basic thread boosting mechanism

This patch introduces a syscall:

    int set_thread_boost(int tid, int amount)

You can use this to add a permanent boost value to the effective thread
priority of any thread with your UID (or any thread in the system if
you are the superuser.)

This is quite crude, but opens up some interesting opportunities. :^)

Kernel/Process.cpp

@@ -3906,3 +3906,19 @@ int Process::sys$futex(const Syscall::SC_futex_params* params)
 
     return 0;
 }
+
+int Process::sys$set_thread_boost(int tid, int amount)
+{
+    if (amount < 0 || amount > 20)
+        return -EINVAL;
+    InterruptDisabler disabler;
+    auto* thread = Thread::from_tid(tid);
+    if (!thread)
+        return -ESRCH;
+    if (thread->state() == Thread::State::Dead || thread->state() == Thread::State::Dying)
+        return -ESRCH;
+    if (!is_superuser() && thread->process().uid() != euid())
+        return -EPERM;
+    thread->set_priority_boost(amount);
+    return 0;
+}

Kernel/Process.h

@@ -234,6 +234,7 @@ public:
     int sys$profiling_disable(pid_t);
     void* sys$get_kernel_info_page();
     int sys$futex(const Syscall::SC_futex_params*);
+    int sys$set_thread_boost(int tid, int amount);
 
     static void initialize();
 

Kernel/Syscall.h

@@ -151,7 +151,8 @@ typedef u32 socklen_t;
     __ENUMERATE_SYSCALL(profiling_enable)           \
     __ENUMERATE_SYSCALL(profiling_disable)          \
     __ENUMERATE_SYSCALL(get_kernel_info_page)       \
-    __ENUMERATE_SYSCALL(futex)
+    __ENUMERATE_SYSCALL(futex)                      \
+    __ENUMERATE_SYSCALL(set_thread_boost)
 
 namespace Syscall {
 

Kernel/Thread.cpp

@@ -786,3 +786,15 @@ void Thread::wake_from_queue()
     ASSERT(state() == State::Queued);
     set_state(State::Runnable);
 }
+
+Thread* Thread::from_tid(int tid)
+{
+    ASSERT_INTERRUPTS_DISABLED();
+    Thread* found_thread = nullptr;
+    Thread::for_each([&](auto& thread) {
+        if (thread.tid() == tid)
+            found_thread = &thread;
+        return IterationDecision::Continue;
+    });
+    return found_thread;
+}

Kernel/Thread.h

@@ -49,6 +49,7 @@ public:
     explicit Thread(Process&);
     ~Thread();
 
+    static Thread* from_tid(int);
     static void initialize();
     static void finalize_dying_threads();
 
@@ -61,7 +62,10 @@ public:
     void set_priority(u32 p) { m_priority = p; }
     u32 priority() const { return m_priority; }
 
-    u32 effective_priority() const { return m_priority + m_extra_priority; }
+    void set_priority_boost(u32 boost) { m_priority_boost = boost; }
+    u32 priority_boost() const { return m_priority_boost; }
+
+    u32 effective_priority() const { return m_priority + m_priority_boost + m_extra_priority; }
 
     void set_joinable(bool j) { m_is_joinable = j; }
     bool is_joinable() const { return m_is_joinable; }
@@ -452,6 +456,7 @@ private:
     String m_name;
     u32 m_priority { THREAD_PRIORITY_NORMAL };
     u32 m_extra_priority { 0 };
+    u32 m_priority_boost { 0 };
     bool m_has_used_fpu { false };
     bool m_dump_backtrace_on_finalization { false };
     bool m_should_die { false };

Libraries/LibC/serenity.cpp

@@ -28,6 +28,13 @@ int profiling_disable(pid_t pid)
     __RETURN_WITH_ERRNO(rc, rc, -1);
 }
 
+int set_thread_boost(int tid, int amount)
+{
+    int rc = syscall(SC_set_thread_boost, tid, amount);
+    __RETURN_WITH_ERRNO(rc, rc, -1);
+}
+
+
 int futex(int32_t* userspace_address, int futex_op, int32_t value, const struct timespec* timeout)
 {
     Syscall::SC_futex_params params { userspace_address, futex_op, value, timeout };

Libraries/LibC/serenity.h

@@ -50,6 +50,8 @@ int profiling_disable(pid_t);
 #define THREAD_PRIORITY_HIGH 50
 #define THREAD_PRIORITY_MAX 99
 
+int set_thread_boost(int tid, int amount);
+
 #define FUTEX_WAIT 1
 #define FUTEX_WAKE 2