Tickless scheduler step 3: Correct for tick lengths.

Collect the error in the tick rate and correct the current tick to be a
close approximation of the value that we'd have with a ticky scheduler.

sdk/core/scheduler/timer.h

@@ -28,7 +28,9 @@ namespace
 
 	class Timer final : private TimerCore
 	{
-		inline static uint64_t lastTickTime = 0;
+		inline static uint64_t lastTickTime         = 0;
+		inline static uint32_t accumulatedTickError = 0;
+
 		public:
 		static void interrupt_setup()
 		{
@@ -41,25 +43,40 @@ namespace
 
 		static void update()
 		{
-
 			if (Thread::waitingList == nullptr)
 			{
 				clear();
 			}
 			else
 			{
-				setnext(TIMERCYCLES_PER_TICK * (Thread::waitingList->expiryTime - Thread::ticksSinceBoot));
+				setnext(
+				  TIMERCYCLES_PER_TICK *
+				  (Thread::waitingList->expiryTime - Thread::ticksSinceBoot));
 			}
 		}
 
 		static void expiretimers()
 		{
 			// TODO: Should be reading the timer's time, not the core's time.
 			// They are currently the same value, but that's not guaranteed.
-			uint64_t now = rdcycle64();
+			uint64_t now     = rdcycle64();
 			uint32_t elapsed = now - lastTickTime;
+			int32_t  error   = elapsed % TIMERCYCLES_PER_TICK;
+			if (elapsed < TIMERCYCLES_PER_TICK)
+			{
+				error = TIMERCYCLES_PER_TICK - error;
+			}
+			accumulatedTickError += error;
+			int32_t errorDirection = accumulatedTickError < 0 ? -1 : 1;
+			int32_t absoluteError  = accumulatedTickError * errorDirection;
+			if (absoluteError >= TIMERCYCLES_PER_TICK)
+			{
+				Thread::ticksSinceBoot += errorDirection;
+				accumulatedTickError += TIMERCYCLES_PER_TICK * -errorDirection;
+			}
 			lastTickTime = now;
-			Thread::ticksSinceBoot += std::max(1U, elapsed / TIMERCYCLES_PER_TICK);
+			Thread::ticksSinceBoot +=
+			  std::max(1U, elapsed / TIMERCYCLES_PER_TICK);
 			if (Thread::waitingList == nullptr)
 			{
 				return;