From f5d130f832ac257e50457de9ed7bcb50ffa51398 Mon Sep 17 00:00:00 2001
From: Gleb Koval <gleb@koval.net>
Date: Thu, 10 Oct 2024 11:28:35 +0100
Subject: [PATCH] Implement non-busy timer_sleep (task 0)

---
 src/devices/timer.c | 51 +++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 49 insertions(+), 2 deletions(-)

diff --git a/src/devices/timer.c b/src/devices/timer.c
index 08ffe61..4bb602e 100644
--- a/src/devices/timer.c
+++ b/src/devices/timer.c
@@ -3,6 +3,7 @@
 #include <inttypes.h>
 #include <round.h>
 #include <stdio.h>
+#include <list.h>
 #include "devices/pit.h"
 #include "threads/interrupt.h"
 #include "threads/synch.h"
@@ -17,6 +18,15 @@
 #error TIMER_FREQ <= 1000 recommended
 #endif
 
+struct asleep_thread
+{
+  int64_t end_at;             /* Number of timer ticks to stop sleeping at. */
+  struct semaphore semaphore; /* Semaphore used to block the thread. */
+  struct list_elem elem;      /* List element. */
+};
+/* List of threads that are sleeping. */
+static struct list sleeping_threads;
+
 /* Number of timer ticks since OS booted. */
 static int64_t ticks;
 
@@ -29,6 +39,9 @@ static bool too_many_loops (unsigned loops);
 static void busy_wait (int64_t loops);
 static void real_time_sleep (int64_t num, int32_t denom);
 static void real_time_delay (int64_t num, int32_t denom);
+static bool sleeping_threads_less (const struct list_elem *a,
+                                   const struct list_elem *b,
+                                   void *aux UNUSED);
 
 /* Sets up the timer to interrupt TIMER_FREQ times per second,
    and registers the corresponding interrupt. */
@@ -36,6 +49,7 @@ void
 timer_init (void) 
 {
   pit_configure_channel (0, 2, TIMER_FREQ);
+  list_init (&sleeping_threads);
   intr_register_ext (0x20, timer_interrupt, "8254 Timer");
 }
 
@@ -89,11 +103,25 @@ timer_elapsed (int64_t then)
 void
 timer_sleep (int64_t ticks) 
 {
+  enum intr_level old_level;
   int64_t start = timer_ticks ();
 
   ASSERT (intr_get_level () == INTR_ON);
-  while (timer_elapsed (start) < ticks) 
-    thread_yield ();
+  if (ticks < 0)
+    return;
+
+  struct asleep_thread st;
+  st.end_at = start + ticks;
+  sema_init (&st.semaphore, 0);
+
+  old_level = intr_disable ();
+  list_insert_ordered (&sleeping_threads, &st.elem, &sleeping_threads_less,
+                       NULL);
+  intr_set_level (old_level);
+  sema_down (&st.semaphore);
+  old_level = intr_disable ();
+  list_remove (&st.elem);
+  intr_set_level (old_level);
 }
 
 /* Sleeps for approximately MS milliseconds.  Interrupts must be
@@ -171,6 +199,15 @@ static void
 timer_interrupt (struct intr_frame *args UNUSED)
 {
   ticks++;
+  for (struct list_elem *e = list_begin (&sleeping_threads);
+       e != list_end (&sleeping_threads); e = list_next (e))
+    {
+      struct asleep_thread *st = list_entry (e, struct asleep_thread, elem);
+      if (ticks >= st->end_at)
+        sema_up (&st->semaphore);
+      else
+        break;
+    }
   thread_tick ();
 }
 
@@ -244,3 +281,13 @@ real_time_delay (int64_t num, int32_t denom)
   ASSERT (denom % 1000 == 0);
   busy_wait (loops_per_tick * num / 1000 * TIMER_FREQ / (denom / 1000)); 
 }
+
+/* list_less_func for sleeping_threads list */
+bool
+sleeping_threads_less (const struct list_elem *a, const struct list_elem *b,
+                       void *aux UNUSED)
+{
+  struct asleep_thread *sta = list_entry (a, struct asleep_thread, elem);
+  struct asleep_thread *stb = list_entry (b, struct asleep_thread, elem);
+  return sta->end_at < stb->end_at;
+}