[PATCH] sched/fair: Fix negative energy delta in find_energy_efficient_cpu()

[Pierre.Gondois@arm.com]

From: Pierre Gondois <Pierre.Gondois@arm.com>

find_energy_efficient_cpu() (feec()) searches the best energy CPU
to place a task on. To do so, compute_energy() estimates the energy
impact of placing the task on a CPU, based on CPU and task utilization
signals.

Utilization signals can be concurrently updated while evaluating a
perf_domain. In some cases, this leads to having a 'negative delta',
i.e. placing the task in the perf_domain is seen as an energy gain.
Thus, any further energy comparison is biased.

In case of a 'negative delta', return prev_cpu since:
1. a 'negative delta' happens in less than 0.5% of feec() calls,
   on a Juno with 6 CPUs (4 little, 2 big)
2. it is unlikely to have two consecutive 'negative delta' for
   a task, so if the first call fails, feec() will correctly
   place the task in the next feec() call
3. EAS current behavior tends to select prev_cpu if the task
   doesn't raise the OPP of its current perf_domain. prev_cpu
   is EAS's generic decision
4. prev_cpu should be preferred to returning an error code.
   In the latter case, select_idle_sibling() would do the placement,
   selecting a big (and not energy efficient) CPU. As 3., the task
   would potentially reside on the big CPU for a long time

The patch also:
a. groups the compute_energy() calls to lower the chances of having
   concurrent updates in between the calls
b. skips the base_energy_pd computation if no CPU is available in a
   perf_domain

Fixes: eb92692b2544d sched/fair: Speed-up energy-aware wake-up
Reported-by: Xuewen Yan <xuewen.yan@unisoc.com>
Suggested-by: Xuewen Yan <xuewen.yan@unisoc.com>
Signed-off-by: Pierre Gondois <Pierre.Gondois@arm.com>
---
 kernel/sched/fair.c | 69 +++++++++++++++++++++++++--------------------
 1 file changed, 39 insertions(+), 30 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 0dba0ebc3657..577482aa8919 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6594,8 +6594,8 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
 {
 	unsigned long prev_delta = ULONG_MAX, best_delta = ULONG_MAX;
 	struct root_domain *rd = cpu_rq(smp_processor_id())->rd;
+	int cpu, best_energy_cpu = prev_cpu, target = -1;
 	unsigned long cpu_cap, util, base_energy = 0;
-	int cpu, best_energy_cpu = prev_cpu;
 	struct sched_domain *sd;
 	struct perf_domain *pd;
 
@@ -6614,19 +6614,18 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
 	if (!sd)
 		goto fail;
 
+	target = prev_cpu;
+
 	sync_entity_load_avg(&p->se);
 	if (!task_util_est(p))
-		goto unlock;
+		goto fail;
 
 	for (; pd; pd = pd->next) {
 		unsigned long cur_delta, spare_cap, max_spare_cap = 0;
+		bool compute_prev_delta = false;
 		unsigned long base_energy_pd;
 		int max_spare_cap_cpu = -1;
 
-		/* Compute the 'base' energy of the pd, without @p */
-		base_energy_pd = compute_energy(p, -1, pd);
-		base_energy += base_energy_pd;
-
 		for_each_cpu_and(cpu, perf_domain_span(pd), sched_domain_span(sd)) {
 			if (!cpumask_test_cpu(cpu, p->cpus_ptr))
 				continue;
@@ -6647,26 +6646,41 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
 			if (!fits_capacity(util, cpu_cap))
 				continue;
 
-			/* Always use prev_cpu as a candidate. */
 			if (cpu == prev_cpu) {
-				prev_delta = compute_energy(p, prev_cpu, pd);
-				prev_delta -= base_energy_pd;
-				best_delta = min(best_delta, prev_delta);
-			}
-
-			/*
-			 * Find the CPU with the maximum spare capacity in
-			 * the performance domain
-			 */
-			if (spare_cap > max_spare_cap) {
+				/* Always use prev_cpu as a candidate. */
+				compute_prev_delta = true;
+			} else if (spare_cap > max_spare_cap) {
+				/*
+				 * Find the CPU with the maximum spare capacity
+				 * in the performance domain.
+				 */
 				max_spare_cap = spare_cap;
 				max_spare_cap_cpu = cpu;
 			}
 		}
 
+		if (max_spare_cap_cpu < 0 && !compute_prev_delta)
+			continue;
+
+		/* Compute the 'base' energy of the pd, without @p */
+		base_energy_pd = compute_energy(p, -1, pd);
+		base_energy += base_energy_pd;
+
+		if (compute_prev_delta) {
+			prev_delta = compute_energy(p, prev_cpu, pd);
+			/* Prevent negative deltas and select prev_cpu */
+			if (prev_delta < base_energy_pd)
+				goto fail;
+			prev_delta -= base_energy_pd;
+			best_delta = min(best_delta, prev_delta);
+		}
+
 		/* Evaluate the energy impact of using this CPU. */
-		if (max_spare_cap_cpu >= 0 && max_spare_cap_cpu != prev_cpu) {
+		if (max_spare_cap_cpu >= 0) {
 			cur_delta = compute_energy(p, max_spare_cap_cpu, pd);
+			/* Prevent negative deltas and select prev_cpu */
+			if (cur_delta < base_energy_pd)
+				goto fail;
 			cur_delta -= base_energy_pd;
 			if (cur_delta < best_delta) {
 				best_delta = cur_delta;
@@ -6674,25 +6688,20 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
 			}
 		}
 	}
-unlock:
-	rcu_read_unlock();
 
 	/*
-	 * Pick the best CPU if prev_cpu cannot be used, or if it saves at
-	 * least 6% of the energy used by prev_cpu.
+	 * Pick the best CPU if:
+	 *  - prev_cpu cannot be used, or
+	 *  - it saves at least 6% of the energy used by prev_cpu
 	 */
-	if (prev_delta == ULONG_MAX)
-		return best_energy_cpu;
-
-	if ((prev_delta - best_delta) > ((prev_delta + base_energy) >> 4))
-		return best_energy_cpu;
-
-	return prev_cpu;
+	if ((prev_delta == ULONG_MAX) ||
+		(prev_delta - best_delta) > ((prev_delta + base_energy) >> 4))
+		target = best_energy_cpu;
 
 fail:
 	rcu_read_unlock();
 
-	return -1;
+	return target;
 }
 
 /*
-- 
2.17.1