kern/sched.c

   1 #include <inc/assert.h>
   2
   3 #include <kern/env.h>
   4 #include <kern/pmap.h>
   5 #include <kern/monitor.h>
   6
   7
   8 // Choose a user environment to run and run it.
   9 void
  10 sched_yield(void)
  11 {
  12         // Implement simple round-robin scheduling.
  13         // Search through 'envs' for a runnable environment,
  14         // in circular fashion starting after the previously running env,
  15         // and switch to the first such environment found.
  16         // It's OK to choose the previously running env if no other env
  17         // is runnable.
  18         // But never choose envs[0], the idle environment,
  19         // unless NOTHING else is runnable.
  20         // LAB 4: Your code here.
  21         int i;
  22
  23         // if there are no previous running environment,
  24         // set this to the envs, i.e. envs[0]
  25         if (curenv == NULL)
  26                 curenv = envs;
  27
  28         for (i = curenv - envs + 1; i < NENV; i++)
  29                 if (envs[i].env_status == ENV_RUNNABLE)
  30                         env_run(&envs[i]);
  31
  32         for (i = 1; i <= curenv - envs; i++)
  33                 if (envs[i].env_status == ENV_RUNNABLE)
  34                         env_run(&envs[i]);
  35
  36         // Run the special idle environment when nothing else is runnable.
  37         if (envs[0].env_status == ENV_RUNNABLE)
  38                 env_run(&envs[0]);
  39         else {
  40                 cprintf("Destroyed all environments - nothing more to do!\n");
  41                 while (1)
  42                         monitor(NULL);
  43         }
  44 }