sched/fair: Defer calling scaling functions

Do not call the scaling functions in case time goes backwards or the
last update of the sched_avg structure has happened less than 1024ns
ago.

Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Juri Lelli <Juri.Lelli@arm.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: daniel.lezcano@linaro.org <daniel.lezcano@linaro.org>
Cc: mturquette@baylibre.com <mturquette@baylibre.com>
Cc: pang.xunlei@zte.com.cn <pang.xunlei@zte.com.cn>
Cc: rjw@rjwysocki.net <rjw@rjwysocki.net>
Cc: sgurrappadi@nvidia.com <sgurrappadi@nvidia.com>
Cc: vincent.guittot@linaro.org <vincent.guittot@linaro.org>
Cc: yuyang.du@intel.com <yuyang.du@intel.com>
Link: http://lkml.kernel.org/r/55EDA2E9.8040900@arm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/sched/fair.c

@@ -2552,8 +2552,7 @@ __update_load_avg(u64 now, int cpu, struct sched_avg *sa,
 	u64 delta, scaled_delta, periods;
 	u32 contrib;
 	unsigned int delta_w, scaled_delta_w, decayed = 0;
-	unsigned long scale_freq = arch_scale_freq_capacity(NULL, cpu);
-	unsigned long scale_cpu = arch_scale_cpu_capacity(NULL, cpu);
+	unsigned long scale_freq, scale_cpu;
 
 	delta = now - sa->last_update_time;
 	/*
@@ -2574,6 +2573,9 @@ __update_load_avg(u64 now, int cpu, struct sched_avg *sa,
 		return 0;
 	sa->last_update_time = now;
 
+	scale_freq = arch_scale_freq_capacity(NULL, cpu);
+	scale_cpu = arch_scale_cpu_capacity(NULL, cpu);
+
 	/* delta_w is the amount already accumulated against our next period */
 	delta_w = sa->period_contrib;
 	if (delta + delta_w >= 1024) {