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Message-Id: <20260203184939.2138022-1-christian.loehle@arm.com>
Date: Tue, 3 Feb 2026 18:49:39 +0000
From: Christian Loehle <christian.loehle@....com>
To: linux-kernel@...r.kernel.org,
peterz@...radead.org,
mingo@...hat.com,
vincent.guittot@...aro.org
Cc: juri.lelli@...hat.com,
dietmar.eggemann@....com,
kprateek.nayak@....com,
pierre.gondois@....com,
Christian Loehle <christian.loehle@....com>
Subject: [PATCHv2] sched/fair: Skip SCHED_IDLE rq for SCHED_IDLE task
CPUs whose rq only have SCHED_IDLE tasks running are considered to be
equivalent to truly idle CPUs during wakeup path. For fork and exec
SCHED_IDLE is even preferred.
This is based on the assumption that the SCHED_IDLE CPU is not in an
idle state and might be in a higher P-state, allowing the task/wakee
to run immediately without sharing the rq.
However this assumption doesn't hold if the wakee has SCHED_IDLE policy
itself, as it will share the rq with existing SCHED_IDLE tasks. In this
case, we are better off continuing to look for a truly idle CPU.
On a Intel Xeon 2-socket with 64 logical cores in total this yields
for kernel compilation using SCHED_IDLE:
+---------+----------------------+----------------------+--------+
| workers | mainline (seconds) | patch (seconds) | delta% |
+=========+======================+======================+========+
| 1 | 4384.728 ± 21.085 | 3843.250 ± 16.235 | -12.35 |
| 2 | 2242.513 ± 2.099 | 1971.696 ± 2.842 | -12.08 |
| 4 | 1199.324 ± 1.823 | 1033.744 ± 1.803 | -13.81 |
| 8 | 649.083 ± 1.959 | 559.123 ± 4.301 | -13.86 |
| 16 | 370.425 ± 0.915 | 325.906 ± 4.623 | -12.02 |
| 32 | 234.651 ± 2.255 | 217.266 ± 0.253 | -7.41 |
| 64 | 202.286 ± 1.452 | 197.977 ± 2.275 | -2.13 |
| 128 | 217.092 ± 1.687 | 212.164 ± 1.138 | -2.27 |
+---------+----------------------+----------------------+--------+
Signed-off-by: Christian Loehle <christian.loehle@....com>
---
v2: Reword commit message, SCHED_IDLE aren't always preferred,
but rather equivalent
Factor out choose_sched_idle_rq() too (Both Vincent)
kernel/sched/fair.c | 32 +++++++++++++++++++-------------
1 file changed, 19 insertions(+), 13 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 3eaeceda71b0..6510ab6eb44b 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6832,9 +6832,15 @@ static int sched_idle_rq(struct rq *rq)
rq->nr_running);
}
-static int sched_idle_cpu(int cpu)
+static int choose_sched_idle_rq(struct rq *rq, struct task_struct *p)
{
- return sched_idle_rq(cpu_rq(cpu));
+ return sched_idle_rq(rq) && !task_has_idle_policy(p);
+}
+
+static int choose_idle_cpu(int cpu, struct task_struct *p)
+{
+ return available_idle_cpu(cpu) ||
+ choose_sched_idle_rq(cpu_rq(cpu), p);
}
static void
@@ -7400,7 +7406,7 @@ sched_balance_find_dst_group_cpu(struct sched_group *group, struct task_struct *
if (!sched_core_cookie_match(rq, p))
continue;
- if (sched_idle_cpu(i))
+ if (choose_sched_idle_rq(rq, p))
return i;
if (available_idle_cpu(i)) {
@@ -7491,8 +7497,7 @@ static inline int sched_balance_find_dst_cpu(struct sched_domain *sd, struct tas
static inline int __select_idle_cpu(int cpu, struct task_struct *p)
{
- if ((available_idle_cpu(cpu) || sched_idle_cpu(cpu)) &&
- sched_cpu_cookie_match(cpu_rq(cpu), p))
+ if (choose_idle_cpu(cpu, p) && sched_cpu_cookie_match(cpu_rq(cpu), p))
return cpu;
return -1;
@@ -7565,7 +7570,8 @@ static int select_idle_core(struct task_struct *p, int core, struct cpumask *cpu
if (!available_idle_cpu(cpu)) {
idle = false;
if (*idle_cpu == -1) {
- if (sched_idle_cpu(cpu) && cpumask_test_cpu(cpu, cpus)) {
+ if (choose_sched_idle_rq(cpu_rq(cpu), p) &&
+ cpumask_test_cpu(cpu, cpus)) {
*idle_cpu = cpu;
break;
}
@@ -7600,7 +7606,7 @@ static int select_idle_smt(struct task_struct *p, struct sched_domain *sd, int t
*/
if (!cpumask_test_cpu(cpu, sched_domain_span(sd)))
continue;
- if (available_idle_cpu(cpu) || sched_idle_cpu(cpu))
+ if (choose_idle_cpu(cpu, p))
return cpu;
}
@@ -7722,7 +7728,7 @@ select_idle_capacity(struct task_struct *p, struct sched_domain *sd, int target)
for_each_cpu_wrap(cpu, cpus, target) {
unsigned long cpu_cap = capacity_of(cpu);
- if (!available_idle_cpu(cpu) && !sched_idle_cpu(cpu))
+ if (!choose_idle_cpu(cpu, p))
continue;
fits = util_fits_cpu(task_util, util_min, util_max, cpu);
@@ -7793,7 +7799,7 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
*/
lockdep_assert_irqs_disabled();
- if ((available_idle_cpu(target) || sched_idle_cpu(target)) &&
+ if (choose_idle_cpu(target, p) &&
asym_fits_cpu(task_util, util_min, util_max, target))
return target;
@@ -7801,7 +7807,7 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
* If the previous CPU is cache affine and idle, don't be stupid:
*/
if (prev != target && cpus_share_cache(prev, target) &&
- (available_idle_cpu(prev) || sched_idle_cpu(prev)) &&
+ choose_idle_cpu(prev, p) &&
asym_fits_cpu(task_util, util_min, util_max, prev)) {
if (!static_branch_unlikely(&sched_cluster_active) ||
@@ -7833,7 +7839,7 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
if (recent_used_cpu != prev &&
recent_used_cpu != target &&
cpus_share_cache(recent_used_cpu, target) &&
- (available_idle_cpu(recent_used_cpu) || sched_idle_cpu(recent_used_cpu)) &&
+ choose_idle_cpu(recent_used_cpu, p) &&
cpumask_test_cpu(recent_used_cpu, p->cpus_ptr) &&
asym_fits_cpu(task_util, util_min, util_max, recent_used_cpu)) {
@@ -12261,7 +12267,7 @@ static void sched_balance_domains(struct rq *rq, enum cpu_idle_type idle)
{
int continue_balancing = 1;
int cpu = rq->cpu;
- int busy = idle != CPU_IDLE && !sched_idle_cpu(cpu);
+ int busy = idle != CPU_IDLE && !sched_idle_rq(rq);
unsigned long interval;
struct sched_domain *sd;
/* Earliest time when we have to do rebalance again */
@@ -12299,7 +12305,7 @@ static void sched_balance_domains(struct rq *rq, enum cpu_idle_type idle)
* state even if we migrated tasks. Update it.
*/
idle = idle_cpu(cpu);
- busy = !idle && !sched_idle_cpu(cpu);
+ busy = !idle && !sched_idle_rq(rq);
}
sd->last_balance = jiffies;
interval = get_sd_balance_interval(sd, busy);
--
2.34.1
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