[PATCH 4/4] sched/fair: Proportional newidle balance

Peter Zijlstra
2025-11-07T16:18:19Z

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>
---
include/linux/sched/topology.h | 3 ++
kernel/sched/core.c | 3 ++
kernel/sched/fair.c | 43 +++++++++++++++++++++++++++++++++++++----
kernel/sched/features.h | 5 ++++
kernel/sched/sched.h | 7 ++++++
kernel/sched/topology.c | 6 +++++
6 files changed, 63 insertions(+), 4 deletions(-)

--- 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;

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -121,6 +121,7 @@ EXPORT_TRACEPOINT_SYMBOL_GPL(sched_updat
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);
@@ -8589,6 +8590,8 @@ 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
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -12146,11 +12146,26 @@ 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;

+ 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
@@ -12198,7 +12213,7 @@ static void sched_balance_domains(struct
* 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;

/*
@@ -12843,6 +12858,22 @@ static int sched_balance_newidle(struct
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,
@@ -12850,10 +12881,14 @@ static int sched_balance_newidle(struct

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);
}

/*
--- 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)
--- 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
}

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)
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -1662,6 +1662,12 @@ sd_init(struct sched_domain_topology_lev

.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,