// Every call in a goroutine checks for stack overflow by
	// comparing the current stack pointer to gp->stackguard0.
	// Setting gp->stackguard0 to StackPreempt folds
	// preemption into the normal stack overflow check.
	gp.stackguard0 = stackPreempt

	// Request an async preemption of this P.
	if preemptMSupported && debug.asyncpreemptoff == 0 {
		pp.preempt = true
		preemptM(mp)
	}