Idle task boosting is a nono in general. There is one exception, when
NOHZ is active:

The idle task calls get_next_timer_interrupt() and holds the timer
wheel base->lock on the CPU and another CPU wants to access the timer
(probably to cancel it). We can safely ignore the boosting request, as
the idle CPU runs this code with interrupts disabled and will complete
the lock protected section without being interrupted. So there is no
real need to boost.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>