The Design and Implementation of the Anykernel and Rump Kernels

87
void
schedule_cpu()
{
struct lwp *lwp = curlwp;
/* 1: fastpath */
cpu = lwp->prevcpu;
if (atomic_cas(cpu->prevlwp, lwp, CPU_BUSY) == lwp)
return;
/* 2: slowpath */
mutex_enter(cpu->mutex);
for (;;) {
/* 3: signal we want the CPU */
old = atomic_swap(cpu->prevlwp, CPU_WANTED);
if (old != CPU_BUSY && old != CPU_WANTED) {
membar();
if (atomic_cas(cpu->prevlwp, CPU_WANTED, CPU_BUSY) == CPU_WANTED) {
break;
}
}
newcpu = migrate(lwp, cpu);
if (newcpu != cpu) {
continue;
}
}
/* 4: wait for CPU */
cpu->wanted++;
cv_wait(cpu->cv, cpu->mutex);
cpu->wanted--;
}
mutex_exit(cpu->mutex);
return;
Figure 3.6: CPU scheduling algorithm in pseudocode. See the text for a
detailed description.