Re-sort ready_list when priorities update + PRI_UPDATE_FREQ

- PRI_UPDATE_FREQ is a separate value from TIME_SLICE, they just
  happen to be set the same.

src/threads/thread.c

@@ -55,6 +55,7 @@ static fp32_t load_avg = { 0 }; /* System load average. */
 
 /* Scheduling. */
 #define TIME_SLICE 4            /* # of timer ticks to give each thread. */
+#define PRI_UPDATE_FREQ 4       /* # of timer ticks to update priorities. */
 static unsigned thread_ticks;   /* # of timer ticks since last yield. */
 
 /* If false (default), use round-robin scheduler.
@@ -172,9 +173,12 @@ thread_tick (void)
           load_avg = fp_add (old_coeff, new_coeff);
 
           thread_foreach (update_recent_cpu, NULL);
+          /* Priorities have been updated, need to re-sort. */
+          list_sort (&ready_list, priority_more, NULL);
         }
 
-      if (ticks % TIME_SLICE == 0) // recent_cpu was updated, update priority.
+      /* Recent cpu was updated, update priority. */
+      if (ticks % PRI_UPDATE_FREQ == 0)
         t->priority = calculate_bsd_priority (t->recent_cpu, t->nice);
     }