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

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
performance domain (pd). In some cases, this leads to having a
'negative delta', i.e. placing the task in the pd 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 pd. 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

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>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Lukasz Luba <lukasz.luba@arm.com>
Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Reviewed-by: Vincent Donnefort <vincent.donnefort@arm.com>
Link: https://lkml.kernel.org/r/20210504090743.9688-3-Pierre.Gondois@arm.com

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index b229d0c2380666735d64de2f591468dc24a8b9f5..c209f68aad612022cc39c0fec5ea08cada9b6a36 100644 (file)
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6661,15 +6661,15 @@ 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;
 
        rcu_read_lock();
        pd = rcu_dereference(rd->pd);
        if (!pd || READ_ONCE(rd->overutilized))
-               goto fail;
+               goto unlock;
 
        /*
         * Energy-aware wake-up happens on the lowest sched_domain starting
@@ -6679,7 +6679,9 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
        while (sd && !cpumask_test_cpu(prev_cpu, sched_domain_span(sd)))
                sd = sd->parent;
        if (!sd)
-               goto fail;
+               goto unlock;
+
+       target = prev_cpu;
 
        sync_entity_load_avg(&p->se);
        if (!task_util_est(p))
@@ -6734,6 +6736,8 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
                /* Evaluate the energy impact of using prev_cpu. */
                if (compute_prev_delta) {
                        prev_delta = compute_energy(p, prev_cpu, pd);
+                       if (prev_delta < base_energy_pd)
+                               goto unlock;
                        prev_delta -= base_energy_pd;
                        best_delta = min(best_delta, prev_delta);
                }
@@ -6741,6 +6745,8 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
                /* Evaluate the energy impact of using max_spare_cap_cpu. */
                if (max_spare_cap_cpu >= 0) {
                        cur_delta = compute_energy(p, max_spare_cap_cpu, pd);
+                       if (cur_delta < base_energy_pd)
+                               goto unlock;
                        cur_delta -= base_energy_pd;
                        if (cur_delta < best_delta) {
                                best_delta = cur_delta;
@@ -6748,25 +6754,22 @@ 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.
         */
-       if (prev_delta == ULONG_MAX)
-               return best_energy_cpu;
+       if ((prev_delta == ULONG_MAX) ||
+           (prev_delta - best_delta) > ((prev_delta + base_energy) >> 4))
+               target = best_energy_cpu;
 
-       if ((prev_delta - best_delta) > ((prev_delta + base_energy) >> 4))
-               return best_energy_cpu;
-
-       return prev_cpu;
+       return target;
 
-fail:
+unlock:
        rcu_read_unlock();
 
-       return -1;
+       return target;
 }
 
 /*