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Message-ID: <20260122154227.130767-1-realwujing@gmail.com>
Date: Thu, 22 Jan 2026 10:42:27 -0500
From: Qiliang Yuan <realwujing@...il.com>
To: Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Juri Lelli <juri.lelli@...hat.com>,
Vincent Guittot <vincent.guittot@...aro.org>
Cc: Dietmar Eggemann <dietmar.eggemann@....com>,
Steven Rostedt <rostedt@...dmis.org>,
Ben Segall <bsegall@...gle.com>,
Mel Gorman <mgorman@...e.de>,
Valentin Schneider <vschneid@...hat.com>,
linux-kernel@...r.kernel.org,
Qiliang Yuan <realwujing@...il.com>,
Qiliang Yuan <yuanql9@...natelecom.cn>
Subject: [PATCH] sched/fair: Optimize EAS energy calculation complexity from O(N) to O(1) inside inner loop
By pre-calculating the base max utilization of each performance domain
at the start of find_energy_efficient_cpu(), we can avoid the repetitive
O(M) scan in eenv_pd_max_util() for every candidate CPU.
This reduces the complexity of energy calculation from O(P*N) to O(N + P^2),
where P is the number of performance domains. For systems with many
performance domains or high core counts, this results in significant
reduction in wakeup latency.
Signed-off-by: Qiliang Yuan <realwujing@...il.com>
Signed-off-by: Qiliang Yuan <yuanql9@...natelecom.cn>
---
kernel/sched/fair.c | 46 ++++++++++++++++++++++++-----------------
kernel/sched/sched.h | 4 ++++
kernel/sched/topology.c | 1 +
3 files changed, 32 insertions(+), 19 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 458324d240e9..de5bfdfe553f 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8236,41 +8236,32 @@ static inline void eenv_pd_busy_time(struct energy_env *eenv,
* exceed @eenv->cpu_cap.
*/
static inline unsigned long
-eenv_pd_max_util(struct energy_env *eenv, struct cpumask *pd_cpus,
+eenv_pd_max_util(struct energy_env *eenv, struct perf_domain *pd,
struct task_struct *p, int dst_cpu)
{
- unsigned long max_util = 0;
- int cpu;
+ unsigned long max_util = pd->max_util;
- for_each_cpu(cpu, pd_cpus) {
- struct task_struct *tsk = (cpu == dst_cpu) ? p : NULL;
- unsigned long util = cpu_util(cpu, p, dst_cpu, 1);
+ if (dst_cpu >= 0 && cpumask_test_cpu(dst_cpu, perf_domain_span(pd))) {
+ unsigned long util = cpu_util(dst_cpu, p, dst_cpu, 1);
unsigned long eff_util, min, max;
- /*
- * Performance domain frequency: utilization clamping
- * must be considered since it affects the selection
- * of the performance domain frequency.
- * NOTE: in case RT tasks are running, by default the min
- * utilization can be max OPP.
- */
- eff_util = effective_cpu_util(cpu, util, &min, &max);
+ eff_util = effective_cpu_util(dst_cpu, util, &min, &max);
/* Task's uclamp can modify min and max value */
- if (tsk && uclamp_is_used()) {
+ if (uclamp_is_used()) {
min = max(min, uclamp_eff_value(p, UCLAMP_MIN));
/*
* If there is no active max uclamp constraint,
* directly use task's one, otherwise keep max.
*/
- if (uclamp_rq_is_idle(cpu_rq(cpu)))
+ if (uclamp_rq_is_idle(cpu_rq(dst_cpu)))
max = uclamp_eff_value(p, UCLAMP_MAX);
else
max = max(max, uclamp_eff_value(p, UCLAMP_MAX));
}
- eff_util = sugov_effective_cpu_perf(cpu, eff_util, min, max);
+ eff_util = sugov_effective_cpu_perf(dst_cpu, eff_util, min, max);
max_util = max(max_util, eff_util);
}
@@ -8286,7 +8277,7 @@ static inline unsigned long
compute_energy(struct energy_env *eenv, struct perf_domain *pd,
struct cpumask *pd_cpus, struct task_struct *p, int dst_cpu)
{
- unsigned long max_util = eenv_pd_max_util(eenv, pd_cpus, p, dst_cpu);
+ unsigned long max_util = eenv_pd_max_util(eenv, pd, p, dst_cpu);
unsigned long busy_time = eenv->pd_busy_time;
unsigned long energy;
@@ -8351,7 +8342,7 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
unsigned long best_actual_cap = 0;
unsigned long prev_actual_cap = 0;
struct sched_domain *sd;
- struct perf_domain *pd;
+ struct perf_domain *pd, *tmp_pd;
struct energy_env eenv;
rcu_read_lock();
@@ -8377,6 +8368,23 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
eenv_task_busy_time(&eenv, p, prev_cpu);
+ /* Algorithmic Optimization: Pre-calculate max_util for O(1) energy estimation */
+ for (tmp_pd = pd; tmp_pd; tmp_pd = tmp_pd->next) {
+ unsigned long max_u = 0;
+ int i;
+
+ for_each_cpu(i, perf_domain_span(tmp_pd)) {
+ unsigned long util = cpu_util(i, p, -1, 1);
+ unsigned long eff_util, min, max;
+
+ eff_util = effective_cpu_util(i, util, &min, &max);
+ eff_util = sugov_effective_cpu_perf(i, eff_util, min, max);
+ if (eff_util > max_u)
+ max_u = eff_util;
+ }
+ tmp_pd->max_util = max_u;
+ }
+
for (; pd; pd = pd->next) {
unsigned long util_min = p_util_min, util_max = p_util_max;
unsigned long cpu_cap, cpu_actual_cap, util;
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index d30cca6870f5..f308d335ca77 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -972,6 +972,10 @@ struct perf_domain {
struct em_perf_domain *em_pd;
struct perf_domain *next;
struct rcu_head rcu;
+
+ /* O(1) optimization hints */
+ unsigned long max_util;
+ int max_spare_cap_cpu;
};
/*
diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 5a3f29a26bdb..b9de022ddb53 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -354,6 +354,7 @@ static struct perf_domain *pd_init(int cpu)
if (!pd)
return NULL;
pd->em_pd = obj;
+ pd->max_spare_cap_cpu = -1;
return pd;
}
--
2.51.0
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