sched/uclamp: Set max_spare_cap_cpu even if max_spare_cap is 0

[ 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@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
Signed-off-by: Sasha Levin <sashal@kernel.org>

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 65cd5c1..95ccb88 100644 (file)
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7287,11 +7287,10 @@ 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);
@@ -7354,7 +7353,7 @@ 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
@@ -7366,7 +7365,7 @@ 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);
@@ -7374,7 +7373,7 @@ 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 */