Implement round-robin scheduler

Makefile

@@ -42,6 +42,7 @@ plic:
 process:
 	$(CC) -c src/process/syscall.c $(CFLAGS) -o syscall.o
 	$(CC) -c src/process/process.c $(CFLAGS) -o process.o
+	$(CC) -c src/process/sched.c $(CFLAGS) -o sched.o
 
 kmain:
 	$(CC) -c src/kmain.c $(CFLAGS) -o kmain.o

src/kmain.c

@@ -8,6 +8,7 @@
 #include "plic/cpu.h"
 #include "plic/plic.h"
 #include "process/process.h"
+#include "process/sched.h"
 
 extern const size_t INIT_START;
 extern const size_t INIT_END;
@@ -55,12 +56,24 @@ void kmain(void) {
   PLIC_ENABLE(PLIC_UART);
   PLIC_SET_PRIO(PLIC_UART, 1);
 
+  kprintf("Initializing the process scheduler ...\n");
+  sched_init();
+
+  kprintf("Adding a second and third process to test our scheduler ...\n");
+  sched_enqueue(init_process);
+  sched_enqueue(init_process);
+
   kprintf("Starting our first process ...\n");
-  struct process *pid1 = create_process(init_process);
-  ASSERT(pid1 != NULL,
-	 "kmain(): failed to allocate structure for init process\n");
-  switch_to_user((size_t)pid1->frame, pid1->pc,
-		 SATP_FROM(MODE_SV39, pid1->pid,
-			   (size_t)pid1->root >> PAGE_ORDER));
+  struct process *process = sched_schedule();
+  ASSERT(process != NULL,
+	 "kmain(): process structure returned from scheduler was unexpectedly NULL\n");
+  kprintf("Our first process has PID = %d\n", process->pid);
+
+  kprintf("Issuing our first context switch timer ...\n");
+  set_timer_interrupt_delay_us(1 * US_PER_SECOND);
+
+  switch_to_user((size_t)process->frame, process->pc,
+		 SATP_FROM(MODE_SV39, process->pid,
+			   (size_t)process->root >> PAGE_ORDER));
   PANIC("kmain(): failed to start our first process!\n");
 }

src/plic/trap_handler.c

@@ -6,6 +6,10 @@
 #include "plic.h"
 #include "../syscon/syscon.h"
 #include "../process/syscall.h"
+#include "../process/sched.h"
+#include "../process/process.h"
+#include "../mm/sv39.h"
+#include "../mm/page.h"
 
 // Handle only the following interrupts for now:
 //
@@ -24,7 +28,17 @@ size_t m_mode_trap_handler(size_t epc, size_t tval, size_t cause, size_t hart,
     switch (exception_code) {
     case 7:
       // Timer interrupt
-      set_timer_interrupt_delay_us(1 * US_PER_SECOND);
+      {
+	set_timer_interrupt_delay_us(1 * US_PER_SECOND);
+	struct process *process = sched_schedule();
+	ASSERT(process != NULL,
+	       "m_mode_trap_handler(): unexpected got NULL when attempting to schedule next process\n");
+	kprintf("Context switch: scheduling next process with PID = %d\n",
+		process->pid);
+	switch_to_user((size_t)process->frame, process->pc,
+		       SATP_FROM(MODE_SV39, process->pid,
+				 (size_t)process->root >> PAGE_ORDER));
+      }
       break;
     case 11:
       // External interrupt (UART)

src/process/sched.c

@@ -0,0 +1,40 @@
+#include <stddef.h>
+#include "sched.h"
+#include "../common/common.h"
+#include "process.h"
+#include "../mm/kmem.h"
+
+static struct process_ll *PROCESSES = NULL;
+
+void sched_init(void) {
+  ASSERT(PROCESSES == NULL,
+	 "sched_init(): should only be called once at system startup\n");
+  sched_enqueue(init_process);
+}
+
+void sched_enqueue(void (*func)(void)) {
+  struct process_ll *nd = kmalloc(sizeof(struct process_ll));
+  ASSERT(nd != NULL,
+	 "sched_enqueue(): failed to allocate linked list node for new process\n");
+  nd->process = create_process(func);
+  if (PROCESSES == NULL) {
+    nd->prev = nd;
+    nd->next = nd;
+    PROCESSES = nd;
+  } else {
+    nd->prev = PROCESSES->prev;
+    nd->next = PROCESSES;
+    PROCESSES->prev->next = nd;
+    PROCESSES->prev = nd;
+  }
+}
+
+struct process *sched_schedule(void) {
+  ASSERT(PROCESSES != NULL,
+	 "sched_schedule(): cannot schedule a process from an empty process list - did you call sched_init()?\n");
+  struct process *process = PROCESSES->process;
+  ASSERT(process != NULL,
+	 "sched_schedule(): process structure from head of process list was unexpectedly NULL\n");
+  PROCESSES = PROCESSES->next;
+  return process;
+}

src/process/sched.h

@@ -0,0 +1,14 @@
+#ifndef SCHED_H
+#define SCHED_H
+
+struct process_ll {
+  struct process *process;
+  struct process_ll *prev;
+  struct process_ll *next;
+};
+
+void sched_init(void);
+void sched_enqueue(void (*)(void));
+struct process *sched_schedule(void);
+
+#endif