Using the global variable is unsafe in syscall.c. Interrupt can be occurred, then context switching can be occured. It means that synchronization method must be applied to the global variable.
But it is bad to performance, so using function parameter instead of global variable is both good to safety and performace.

thread_set_nice
thread_update_priority
thread_wake
thread_aging

Above functions don't invoke thread_yield or intr_yield_on_return.
It can occur bugs...
So if bugs due to yield issue are occured, don't confuse but just think about yield issue.

[Project1 Threads]

All pass without mlfqs-nice-10

[Project2 Userprog]

All pass.

[Project3 VM]

pass before page-linear (not include page-linear)
Not working after page-linear...:(

mlfqs-nice-10 fail....

I don't why....:(