cpufreq: Reduce cpufreq_update_util() overhead a bit

Use the observation that cpufreq_update_util() is only called
by the scheduler with rq->lock held, so the callers of
cpufreq_set_update_util_data() can use synchronize_sched()
instead of synchronize_rcu() to wait for cpufreq_update_util()
to complete.  Moreover, if they are updated to do that,
rcu_read_(un)lock() calls in cpufreq_update_util() might be
replaced with rcu_read_(un)lock_sched(), respectively, but
those aren't really necessary, because the scheduler calls
that function from RCU-sched read-side critical sections
already.

In addition to that, if cpufreq_set_update_util_data() checks
the func field in the struct update_util_data before setting
the per-CPU pointer to it, the data->func check may be dropped
from cpufreq_update_util() as well.

Make the above changes to reduce the overhead from
cpufreq_update_util() in the scheduler paths invoking it
and to make the cleanup after removing its callbacks less
heavy-weight somewhat.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>

drivers/cpufreq/intel_pstate.c

@@ -1168,7 +1168,7 @@ static void intel_pstate_stop_cpu(struct cpufreq_policy *policy)
 	pr_debug("intel_pstate: CPU %d exiting\n", cpu_num);
 
 	cpufreq_set_update_util_data(cpu_num, NULL);
-	synchronize_rcu();
+	synchronize_sched();
 
 	if (hwp_active)
 		return;
@@ -1426,7 +1426,7 @@ out:
 	for_each_online_cpu(cpu) {
 		if (all_cpu_data[cpu]) {
 			cpufreq_set_update_util_data(cpu, NULL);
-			synchronize_rcu();
+			synchronize_sched();
 			kfree(all_cpu_data[cpu]);
 		}
 	}