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Subject[tip: sched/core] sched/fair: Fix negative energy delta in find_energy_efficient_cpu()
The following commit has been merged into the sched/core branch of tip:

Commit-ID: 619e090c8e409e09bd3e8edcd5a73d83f689890c
Gitweb: https://git.kernel.org/tip/619e090c8e409e09bd3e8edcd5a73d83f689890c
Author: Pierre Gondois <Pierre.Gondois@arm.com>
AuthorDate: Tue, 04 May 2021 10:07:43 +01:00
Committer: Peter Zijlstra <peterz@infradead.org>
CommitterDate: Wed, 12 May 2021 11:43:23 +02:00

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
---
kernel/sched/fair.c | 27 +++++++++++++++------------
1 file changed, 15 insertions(+), 12 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index b229d0c..c209f68 100644
--- 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;
}

/*
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