6.6-stable review patch. If anyone has any objections, please let me know.
------------------
From: Qais Yousef <qyousef@xxxxxxxxxxx>
[ Upstream commit 6b00a40147653c8ea748e8f4396510f252763364 ]
When uclamp_max is being used, the util of the task could be higher than
the spare capacity of the CPU, but due to uclamp_max value we force-fit
it there.
The way the condition for checking for max_spare_cap in
find_energy_efficient_cpu() was constructed; it ignored any CPU that has
its spare_cap less than or _equal_ to max_spare_cap. Since we initialize
max_spare_cap to 0; this lead to never setting max_spare_cap_cpu and
hence ending up never performing compute_energy() for this cluster and
missing an opportunity for a better energy efficient placement to honour
uclamp_max setting.
max_spare_cap = 0;
cpu_cap = capacity_of(cpu) - cpu_util(p); // 0 if cpu_util(p) is high
...
util_fits_cpu(...); // will return true if uclamp_max forces it to fit
...
// this logic will fail to update max_spare_cap_cpu if cpu_cap is 0
if (cpu_cap > max_spare_cap) {
max_spare_cap = cpu_cap;
max_spare_cap_cpu = cpu;
}
prev_spare_cap suffers from a similar problem.
Fix the logic by converting the variables into long and treating -1
value as 'not populated' instead of 0 which is a viable and correct
spare capacity value. We need to be careful signed comparison is used
when comparing with cpu_cap in one of the conditions.
Fixes: 1d42509e475c ("sched/fair: Make EAS wakeup placement consider uclamp restrictions")
Signed-off-by: Qais Yousef (Google) <qyousef@xxxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Reviewed-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Reviewed-by: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20230916232955.2099394-2-qyousef@xxxxxxxxxxx
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
kernel/sched/fair.c | 11 +++++------
1 file changed, 5 insertions(+), 6 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 6af1d48d467ed..a60b5d0edd039 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7764,11 +7764,10 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
for (; pd; pd = pd->next) {
unsigned long util_min = p_util_min, util_max = p_util_max;
unsigned long cpu_cap, cpu_thermal_cap, util;
- unsigned long cur_delta, max_spare_cap = 0;
+ long prev_spare_cap = -1, max_spare_cap = -1;
unsigned long rq_util_min, rq_util_max;
- unsigned long prev_spare_cap = 0;
+ unsigned long cur_delta, base_energy;
int max_spare_cap_cpu = -1;
- unsigned long base_energy;
int fits, max_fits = -1;
cpumask_and(cpus, perf_domain_span(pd), cpu_online_mask);
@@ -7831,7 +7830,7 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
prev_spare_cap = cpu_cap;
prev_fits = fits;
} else if ((fits > max_fits) ||
- ((fits == max_fits) && (cpu_cap > max_spare_cap))) {
+ ((fits == max_fits) && ((long)cpu_cap > max_spare_cap))) {
/*
* Find the CPU with the maximum spare capacity
* among the remaining CPUs in the performance
@@ -7843,7 +7842,7 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
}
}
- if (max_spare_cap_cpu < 0 && prev_spare_cap == 0)
+ if (max_spare_cap_cpu < 0 && prev_spare_cap < 0)
continue;
eenv_pd_busy_time(&eenv, cpus, p);
@@ -7851,7 +7850,7 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
base_energy = compute_energy(&eenv, pd, cpus, p, -1);
/* Evaluate the energy impact of using prev_cpu. */
- if (prev_spare_cap > 0) {
+ if (prev_spare_cap > -1) {
prev_delta = compute_energy(&eenv, pd, cpus, p,
prev_cpu);
/* CPU utilization has changed */
--
2.42.0
