sched/fair: Spare idle load balancing on nohz_full CPUs

Although idle load balancing obviously only concerns idle CPUs, it can
be a disturbance on a busy nohz_full CPU. Indeed a CPU can only get rid
of an idle load balancing duty once a tick fires while it runs a task
and this can take a while on a nohz_full CPU.

We could fix that and escape the idle load balancing duty from the very
idle exit path but that would bring unecessary overhead. Lets just not
bother and leave that job to housekeeping CPUs (those outside nohz_full
range). The nohz_full CPUs simply don't want any disturbance.
Signed-off-by: NFrederic Weisbecker <fweisbec@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1497838322-10913-4-git-send-email-fweisbec@gmail.comSigned-off-by: NIngo Molnar <mingo@kernel.org>

kernel/sched/fair.c

@@ -8683,6 +8683,10 @@ void nohz_balance_enter_idle(int cpu)
 	if (!cpu_active(cpu))
 		return;
 
+	/* Spare idle load balancing on CPUs that don't want to be disturbed: */
+	if (!is_housekeeping_cpu(cpu))
+		return;
+
 	if (test_bit(NOHZ_TICK_STOPPED, nohz_flags(cpu)))
 		return;