[tip: sched/core] sched/fair: Proportional newidle balance

tip-bot2 for Peter Zijlstra
2025-11-14T12:19:02Z

The following commit has been merged into the sched/core branch of tip:

Commit-ID: 7c983640e4db0c1fd8ce6c6cd921c19954a8d479
Gitweb: https://git.kernel.org/tip/7c983640e4db0c1fd8ce6c6cd921c19954a8d479
Author: Peter Zijlstra <peterz@infradead.org>
AuthorDate: Fri, 07 Nov 2025 17:01:31 +01:00
Committer: Peter Zijlstra <peterz@infradead.org>
CommitterDate: Fri, 14 Nov 2025 13:03:08 +01:00

sched/fair: Proportional newidle balance

Add a randomized algorithm that runs newidle balancing proportional to
its success rate.

This improves schbench significantly:

6.18-rc4: 2.22 Mrps/s
6.18-rc4+revert: 2.04 Mrps/s
6.18-rc4+revert+random: 2.18 Mrps/S

Conversely, per Adam Li this affects SpecJBB slightly, reducing it by 1%:

6.17: -6%
6.17+revert: 0%
6.17+revert+random: -1%

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Tested-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Tested-by: Chris Mason <clm@meta.com>
Link: https://lkml.kernel.org/r/6825c50d-7fa7-45d8-9b81-c6e7e25738e2@meta.com
Link: https://patch.msgid.link/20251107161739.770122091@infradead.org
---
include/linux/sched/topology.h | 3 ++-
kernel/sched/core.c | 3 ++-
kernel/sched/fair.c | 44 ++++++++++++++++++++++++++++++---
kernel/sched/features.h | 5 ++++-
kernel/sched/sched.h | 7 +++++-
kernel/sched/topology.c | 6 +++++-
6 files changed, 64 insertions(+), 4 deletions(-)

diff --git a/include/linux/sched/topology.h b/include/linux/sched/topology.h
index bbcfdf1..45c0022 100644
--- a/include/linux/sched/topology.h
+++ b/include/linux/sched/topology.h
@@ -92,6 +92,9 @@ struct sched_domain {
unsigned int nr_balance_failed; /* initialise to 0 */

/* idle_balance() stats */
+ unsigned int newidle_call;
+ unsigned int newidle_success;
+ unsigned int newidle_ratio;
u64 max_newidle_lb_cost;
unsigned long last_decay_max_lb_cost;

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 699db3f..9f10cfb 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -121,6 +121,7 @@ EXPORT_TRACEPOINT_SYMBOL_GPL(sched_update_nr_running_tp);
EXPORT_TRACEPOINT_SYMBOL_GPL(sched_compute_energy_tp);

DEFINE_PER_CPU_SHARED_ALIGNED(struct rq, runqueues);
+DEFINE_PER_CPU(struct rnd_state, sched_rnd_state);

#ifdef CONFIG_SCHED_PROXY_EXEC
DEFINE_STATIC_KEY_TRUE(__sched_proxy_exec);
@@ -8489,6 +8490,8 @@ void __init sched_init_smp(void)
{
sched_init_numa(NUMA_NO_NODE);

+ prandom_init_once(&sched_rnd_state);
+
/*
* There's no userspace yet to cause hotplug operations; hence all the
* CPU masks are stable and all blatant races in the below code cannot
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 50461c9..aaa47ec 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -12223,11 +12223,27 @@ void update_max_interval(void)
max_load_balance_interval = HZ*num_online_cpus()/10;
}

-static inline bool update_newidle_cost(struct sched_domain *sd, u64 cost)
+static inline void update_newidle_stats(struct sched_domain *sd, unsigned int success)
+{
+ sd->newidle_call++;
+ sd->newidle_success += success;
+
+ if (sd->newidle_call >= 1024) {
+ sd->newidle_ratio = sd->newidle_success;
+ sd->newidle_call /= 2;
+ sd->newidle_success /= 2;
+ }
+}
+
+static inline bool
+update_newidle_cost(struct sched_domain *sd, u64 cost, unsigned int success)
{
unsigned long next_decay = sd->last_decay_max_lb_cost + HZ;
unsigned long now = jiffies;

+ if (cost)
+ update_newidle_stats(sd, success);
+
if (cost > sd->max_newidle_lb_cost) {
/*
* Track max cost of a domain to make sure to not delay the
@@ -12275,7 +12291,7 @@ static void sched_balance_domains(struct rq *rq, enum cpu_idle_type idle)
* Decay the newidle max times here because this is a regular
* visit to all the domains.
*/
- need_decay = update_newidle_cost(sd, 0);
+ need_decay = update_newidle_cost(sd, 0, 0);
max_cost += sd->max_newidle_lb_cost;

/*
@@ -12911,6 +12927,22 @@ static int sched_balance_newidle(struct rq *this_rq, struct rq_flags *rf)
break;

if (sd->flags & SD_BALANCE_NEWIDLE) {
+ unsigned int weight = 1;
+
+ if (sched_feat(NI_RANDOM)) {
+ /*
+ * Throw a 1k sided dice; and only run
+ * newidle_balance according to the success
+ * rate.
+ */
+ u32 d1k = sched_rng() % 1024;
+ weight = 1 + sd->newidle_ratio;
+ if (d1k > weight) {
+ update_newidle_stats(sd, 0);
+ continue;
+ }
+ weight = (1024 + weight/2) / weight;
+ }

pulled_task = sched_balance_rq(this_cpu, this_rq,
sd, CPU_NEWLY_IDLE,
@@ -12918,10 +12950,14 @@ static int sched_balance_newidle(struct rq *this_rq, struct rq_flags *rf)

t1 = sched_clock_cpu(this_cpu);
domain_cost = t1 - t0;
- update_newidle_cost(sd, domain_cost);
-
curr_cost += domain_cost;
t0 = t1;
+
+ /*
+ * Track max cost of a domain to make sure to not delay the
+ * next wakeup on the CPU.
+ */
+ update_newidle_cost(sd, domain_cost, weight * !!pulled_task);
}

/*
diff --git a/kernel/sched/features.h b/kernel/sched/features.h
index 0607def..980d92b 100644
--- a/kernel/sched/features.h
+++ b/kernel/sched/features.h
@@ -121,3 +121,8 @@ SCHED_FEAT(WA_BIAS, true)
SCHED_FEAT(UTIL_EST, true)

SCHED_FEAT(LATENCY_WARN, false)
+
+/*
+ * Do newidle balancing proportional to its success rate using randomization.
+ */
+SCHED_FEAT(NI_RANDOM, true)
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index def9ab7..b419a4d 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -5,6 +5,7 @@
#ifndef _KERNEL_SCHED_SCHED_H
#define _KERNEL_SCHED_SCHED_H

+#include <linux/prandom.h>
#include <linux/sched/affinity.h>
#include <linux/sched/autogroup.h>
#include <linux/sched/cpufreq.h>
@@ -1348,6 +1349,12 @@ static inline bool is_migration_disabled(struct task_struct *p)
}

DECLARE_PER_CPU_SHARED_ALIGNED(struct rq, runqueues);
+DECLARE_PER_CPU(struct rnd_state, sched_rnd_state);
+
+static inline u32 sched_rng(void)
+{
+ return prandom_u32_state(this_cpu_ptr(&sched_rnd_state));
+}

#define cpu_rq(cpu) (&per_cpu(runqueues, (cpu)))
#define this_rq() this_cpu_ptr(&runqueues)
diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 711076a..cf643a5 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -1669,6 +1669,12 @@ sd_init(struct sched_domain_topology_level *tl,

.last_balance = jiffies,
.balance_interval = sd_weight,
+
+ /* 50% success rate */
+ .newidle_call = 512,
+ .newidle_success = 256,
+ .newidle_ratio = 512,
+
.max_newidle_lb_cost = 0,
.last_decay_max_lb_cost = jiffies,
.child = child,