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
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@ -6661,15 +6661,15 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
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{
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unsigned long prev_delta = ULONG_MAX, best_delta = ULONG_MAX;
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struct root_domain *rd = cpu_rq(smp_processor_id())->rd;
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int cpu, best_energy_cpu = prev_cpu, target = -1;
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unsigned long cpu_cap, util, base_energy = 0;
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int cpu, best_energy_cpu = prev_cpu;
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struct sched_domain *sd;
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struct perf_domain *pd;
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rcu_read_lock();
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pd = rcu_dereference(rd->pd);
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if (!pd || READ_ONCE(rd->overutilized))
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goto fail;
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goto unlock;
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/*
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* Energy-aware wake-up happens on the lowest sched_domain starting
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@ -6679,7 +6679,9 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
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while (sd && !cpumask_test_cpu(prev_cpu, sched_domain_span(sd)))
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sd = sd->parent;
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if (!sd)
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goto fail;
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goto unlock;
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target = prev_cpu;
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sync_entity_load_avg(&p->se);
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if (!task_util_est(p))
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@ -6734,6 +6736,8 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
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/* Evaluate the energy impact of using prev_cpu. */
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if (compute_prev_delta) {
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prev_delta = compute_energy(p, prev_cpu, pd);
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if (prev_delta < base_energy_pd)
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goto unlock;
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prev_delta -= base_energy_pd;
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best_delta = min(best_delta, prev_delta);
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}
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@ -6741,6 +6745,8 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
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/* Evaluate the energy impact of using max_spare_cap_cpu. */
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if (max_spare_cap_cpu >= 0) {
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cur_delta = compute_energy(p, max_spare_cap_cpu, pd);
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if (cur_delta < base_energy_pd)
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goto unlock;
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cur_delta -= base_energy_pd;
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if (cur_delta < best_delta) {
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best_delta = cur_delta;
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@ -6748,25 +6754,22 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
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}
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}
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}
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unlock:
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rcu_read_unlock();
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/*
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* Pick the best CPU if prev_cpu cannot be used, or if it saves at
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* least 6% of the energy used by prev_cpu.
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*/
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if (prev_delta == ULONG_MAX)
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return best_energy_cpu;
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if ((prev_delta == ULONG_MAX) ||
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(prev_delta - best_delta) > ((prev_delta + base_energy) >> 4))
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target = best_energy_cpu;
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if ((prev_delta - best_delta) > ((prev_delta + base_energy) >> 4))
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return best_energy_cpu;
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return target;
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return prev_cpu;
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fail:
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unlock:
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rcu_read_unlock();
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return -1;
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return target;
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}
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/*
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