Man page 9 : yield_to

yield_to - yield the current processor to another thread in your thread group, or accelerate that thread toward the processor itAqs on.

bool __sched yield_to(struct task_struct *  p , bool  preempt );

p
    target task

preempt
    whether task preemption is allowed or not

ItAqs the callerAqs job to ensure that the target task struct canAqt go away on us before we can do any checks.

true (>0) if we indeed boosted the target task. false (0) if we failed to boost the target. -ESRCH if thereAqs no task to yield to.