If the programmer puts an explicit call to sched_yield() in the code that T1 is running, then the scheduler will look to see if there is another runnable thread (T2) of the same priority (there can’t be a higher priority runnable thread).
When a programmer adds a direct call to `sched_yield()` into T1's code, the scheduler checks for another runnable thread with the same priority level. If such a thread is found, it will be given the opportunity to run.  However, if there is a higher-priority runnable thread already running, then `sched_yield()` won't be executed.