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sched/uclamp: Set max_spare_cap_cpu even if max_spare_cap is 0 

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: 1d42509e47 ("sched/fair: Make EAS wakeup placement consider uclamp restrictions")
Signed-off-by: Qais Yousef (Google) <qyousef@layalina.io>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20230916232955.2099394-2-qyousef@layalina.io
This commit is contained in:
Qais Yousef 2023-09-17 00:29:53 +01:00 committed by Ingo Molnar
parent 5fe7765997
commit 6b00a40147
1 changed files with 5 additions and 6 deletions
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11
kernel/sched/fair.c
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@ -7703,11 +7703,10 @@ 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);
@ -7770,7 +7769,7 @@ 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
@ -7782,7 +7781,7 @@ 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);
@ -7790,7 +7789,7 @@ 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 */