jos/kern/sched.c

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#include <inc/assert.h>
#include <inc/x86.h>
#include <kern/spinlock.h>
#include <kern/env.h>
#include <kern/pmap.h>
#include <kern/monitor.h>
void sched_halt(void);
// Choose a user environment to run and run it.
void
sched_yield(void)
{
struct Env *idle;
// Implement simple round-robin scheduling.
//
// Search through 'envs' for an ENV_RUNNABLE environment in
// circular fashion starting just after the env this CPU was
// last running. Switch to the first such environment found.
//
// If no envs are runnable, but the environment previously
// running on this CPU is still ENV_RUNNING, it's okay to
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// choose that environment. Make sure curenv is not null before
// dereferencing it.
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//
// Never choose an environment that's currently running on
// another CPU (env_status == ENV_RUNNING). If there are
// no runnable environments, simply drop through to the code
// below to halt the cpu.
// LAB 4: Your code here.
// sched_halt never returns
sched_halt();
}
// Halt this CPU when there is nothing to do. Wait until the
// timer interrupt wakes it up. This function never returns.
//
void
sched_halt(void)
{
int i;
// For debugging and testing purposes, if there are no runnable
// environments in the system, then drop into the kernel monitor.
for (i = 0; i < NENV; i++) {
if ((envs[i].env_status == ENV_RUNNABLE ||
envs[i].env_status == ENV_RUNNING ||
envs[i].env_status == ENV_DYING))
break;
}
if (i == NENV) {
cprintf("No runnable environments in the system!\n");
while (1)
monitor(NULL);
}
// Mark that no environment is running on this CPU
curenv = NULL;
lcr3(PADDR(kern_pgdir));
// Mark that this CPU is in the HALT state, so that when
// timer interupts come in, we know we should re-acquire the
// big kernel lock
xchg(&thiscpu->cpu_status, CPU_HALTED);
// Release the big kernel lock as if we were "leaving" the kernel
unlock_kernel();
// Reset stack pointer, enable interrupts and then halt.
asm volatile (
"movl $0, %%ebp\n"
"movl %0, %%esp\n"
"pushl $0\n"
"pushl $0\n"
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// LAB 4:
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// Uncomment the following line after completing exercise 13
//"sti\n"
"1:\n"
"hlt\n"
"jmp 1b\n"
: : "a" (thiscpu->cpu_ts.ts_esp0));
}