linux-kernel - [PATCH v2 1/3] sched/uclamp: Set max_spare_cap_cpu even if max_spare

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Date:   Sun,  5 Feb 2023 22:43:16 +0000
From:   Qais Yousef <qyousef@...alina.io>
To:     Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>
Cc:     linux-kernel@...r.kernel.org, Lukasz Luba <lukasz.luba@....com>,
        Wei Wang <wvw@...gle.com>, Xuewen Yan <xuewen.yan94@...il.com>,
        Hank <han.lin@...iatek.com>,
        Jonathan JMChen <Jonathan.JMChen@...iatek.com>,
        Qais Yousef <qyousef@...alina.io>
Subject: [PATCH v2 1/3] 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) - task_util(p);  // 0 if task_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.

Fixes: 1d42509e475c ("sched/fair: Make EAS wakeup placement consider uclamp restrictions")
Signed-off-by: Qais Yousef (Google) <qyousef@...alina.io>
---
 kernel/sched/fair.c | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index c6c8e7f52935..7a21ee74139f 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7382,11 +7382,10 @@ 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);
@@ -7461,7 +7460,7 @@ 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);
@@ -7469,7 +7468,7 @@ 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 */
-- 
2.25.1