schied/fair: Skip calculating @contrib without load

Because of the:

	if (!load)
		runnable = running = 0;

clause in ___update_load_sum(), all the actual users of @contrib in
accumulate_sum():

	if (load)
		sa->load_sum += load * contrib;
	if (runnable)
		sa->runnable_load_sum += runnable * contrib;
	if (running)
		sa->util_sum += contrib << SCHED_CAPACITY_SHIFT;

don't happen, and therefore we don't care what @contrib actually is and
calculating it is pointless.

If we count the times when @load equals zero and not as below:

	if (load) {
		load_is_not_zero_count++;
		contrib = __accumulate_pelt_segments(periods,
				1024 - sa->period_contrib,delta);
	} else
		load_is_zero_count++;

As we can see, load_is_zero_count is much bigger than
load_is_zero_count, and the gap is gradually widening:

	load_is_zero_count:            6016044 times
	load_is_not_zero_count:         244316 times
	19:50:43 up 1 min,  1 user,  load average: 0.09, 0.06, 0.02

	load_is_zero_count:            7956168 times
	load_is_not_zero_count:         261472 times
	19:51:42 up 2 min,  1 user,  load average: 0.03, 0.05, 0.01

	load_is_zero_count:           10199896 times
	load_is_not_zero_count:         278364 times
	19:52:51 up 3 min,  1 user,  load average: 0.06, 0.05, 0.01

	load_is_zero_count:           14333700 times
	load_is_not_zero_count:         318424 times
	19:54:53 up 5 min,  1 user,  load average: 0.01, 0.03, 0.00

Perhaps we can gain some performance advantage by saving these
unnecessary calculation.

Signed-off-by: Peng Wang <rocking@linux.alibaba.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Vincent Guittot < vincent.guittot@linaro.org>
Link: https://lkml.kernel.org/r/1576208740-35609-1-git-send-email-rocking@linux.alibaba.com

kernel/sched/pelt.c

@@ -129,8 +129,20 @@ accumulate_sum(u64 delta, struct sched_avg *sa,
 		 * Step 2
 		 */
 		delta %= 1024;
-		contrib = __accumulate_pelt_segments(periods,
-				1024 - sa->period_contrib, delta);
+		if (load) {
+			/*
+			 * This relies on the:
+			 *
+			 * if (!load)
+			 *	runnable = running = 0;
+			 *
+			 * clause from ___update_load_sum(); this results in
+			 * the below usage of @contrib to dissapear entirely,
+			 * so no point in calculating it.
+			 */
+			contrib = __accumulate_pelt_segments(periods,
+					1024 - sa->period_contrib, delta);
+		}
 	}
 	sa->period_contrib = delta;
 
@@ -205,7 +217,9 @@ ___update_load_sum(u64 now, struct sched_avg *sa,
 	 * This means that weight will be 0 but not running for a sched_entity
 	 * but also for a cfs_rq if the latter becomes idle. As an example,
 	 * this happens during idle_balance() which calls
-	 * update_blocked_averages()
+	 * update_blocked_averages().
+	 *
+	 * Also see the comment in accumulate_sum().
 	 */
 	if (!load)
 		runnable = running = 0;