Kernel: Change preemption condition

Instead of keeping track of the current time and rescheduling when
interval has passed, keep track of the next expected reschedule time.
This prevents theoretically missing every second pre-emption when
scheduler's timer is interrupting at same rate as the interval.

kernel/include/kernel/Scheduler.h

@@ -101,7 +101,7 @@ namespace Kernel
 		InterruptStack* m_interrupt_stack { nullptr };
 		InterruptRegisters* m_interrupt_registers { nullptr };
 
-		uint64_t m_last_reschedule_ns { 0 };
+		uint64_t m_next_reschedule_ns { 0 };
 		uint64_t m_last_load_balance_ns { 0 };
 
 		struct ThreadInfo

kernel/kernel/Scheduler.cpp

@@ -139,6 +139,8 @@ namespace Kernel
 		if (Processor::count() > 1)
 			Processor::set_smp_enabled();
 
+		m_next_reschedule_ns = SystemTimer::get().ns_since_boot();
+
 		return {};
 	}
 
@@ -348,13 +350,10 @@ namespace Kernel
 
 		wake_up_sleeping_threads();
 
+		if (SystemTimer::get().ns_since_boot() >= m_next_reschedule_ns)
 		{
-			const uint64_t current_ns = SystemTimer::get().ns_since_boot();
-			if (current_ns >= m_last_reschedule_ns + s_reschedule_interval_ns)
-			{
-				m_last_reschedule_ns = current_ns;
-				Processor::yield();
-			}
+			m_next_reschedule_ns += s_reschedule_interval_ns;
+			Processor::yield();
 		}
 	}