Merge tag 'sched-core-2026-10-18' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull scheduler updates from Ingo Molnar:
 "Kconfig updates:

   - Simplify PREEMPT_DYNAMIC (Mark Rutland)

  Fair scheduler updates:

   - Remove redundant RUN_TO_PARITY check (Elif Topuz)

   - Declare augmented callbacks per field, fix
     rb_add_augmented_cached() descent (Yiwei Lin, Peter Zijlstra)

   - Restart fair hrtick after same-task repicks (Shubhang Kaushik)

   - Improve the scheduling latency of short slice tasks (Vincent
     Guittot)

  Deadline/RT scheduler updates:

   - Check start_dl_timer expiry with ktime_before() (Liang Hao)

   - irq_work: CPU-hotplug improvements on PREEMPT_RT (Sebastian Andrzej
     Siewior)

  Energy-aware scheduling updates:

   - Implement the steal_governor, introduce preferred CPUs and
     steal-driven vCPU backoff (Shrikanth Hegde)

  Multi-processor load-balancing updates:

   - Enable preferred SMT siblings on NVIDIA Olympus (Andrea Righi)

   - Randomize equally shallow idle CPU picks (Christian Loehle)

   - Remove redundant core_sched_seq (Hui Su)

  Cache-aware scheduling updates:

   - Avoid publishing partial LLC utilization (Jemmy Wong)

  NUMA-balancing updates:

   - Reset NUMA fault locality after scan period update (Eric Kim)

  Proxy execution updates:

   - Ongoing work to implement sleeping owner handling for proxy
     execution (John Stultz, Vasily Gorbik)

   - Preparatory steps to make sched_ext compatible with proxy execution
     (Andrea Righi)

  Scheduler infrastructure updates & fixes:

   - Remove the RCU wait from PSI trigger destruction (Shakeel Butt)

   - Avoid modifying wait-entry on failed priority registration
     (Hui Su)

   - Use the new static key API for sched_feat (Hongyan Xia)

   - Convert paravirt_steal to new static key APIs (Hongyan Xia)

   - cputime: Don't account idle time twice after dyntick-idle
     (Stian Halseth)

  Scheduler debugging updates:

   - Set TIF_NEED_RESCHED before calling __trace_set_need_resched()
     (Sechang Lim)

   - Fix context analysis errors in non-preferred CPU push (Shrikanth
     Hegde)

  Misc cleanups and fixes by Christian Loehle, Shubhang Kaushik,
  Thomas Huth and Vladimir Zapolskiy"

* tag 'sched-core-2026-10-18' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip: (64 commits)
  sched: Replace nr_iowait_cpu() with a bool accessor
  Revert "sched/fair: Remove dead code on enqueue_task_fair()"
  sched/cache: Avoid publishing partial LLC utilization
  sched/cputime: Don't account idle time twice after dyntick-idle
  sched/psi: Remove the RCU wait from trigger destruction
  sched/eevdf: Remove redundant RUN_TO_PARITY check
  sched/wait: Avoid modifying entry on failed priority registration
  rbtree_test: Use rb_add_augmented_cached() for the cached augmented test
  rbtree: Update augmented data on the way down in rb_add_augmented_cached()
  rbtree: Declare augmented callbacks per field with RB_AUG()
  rbtree_test: Use rb_add() and rb_add_cached() for the basic tests
  smpboot: Don't park the thread if work is pending
  irq_work: Flush lazy work CPU down on PREEMPT_RT
  irq_work: Update a comment regarding CPU hotplug invocation
  sched/topology: Add asymmetric SMT packing override
  sched/fair: Honor asymmetric SMT priority in idle selection
  sched: Set TIF_NEED_RESCHED before calling __trace_set_need_resched()
  sched/core: Fix context analysis errors in non-preferred CPU push
  virt/steal_governor: Enable the driver
  virt/steal_governor: Implement steal_governor policy loop
  ...

5 files changed, 328 insertions, 2 deletions

diff --git a/drivers/base/cpu.c b/drivers/base/cpu.c
index 69e52fed4241..747915ff974f 100644
--- a/drivers/base/cpu.c
+++ b/drivers/base/cpu.c
@@ -391,6 +391,15 @@ static int cpu_uevent(const struct device *dev, struct kobj_uevent_env *env)
 }
 #endif
 
+#ifdef CONFIG_PREFERRED_CPU
+static ssize_t preferred_show(struct device *dev,
+			      struct device_attribute *attr, char *buf)
+{
+	return sysfs_emit(buf, "%*pbl\n", cpumask_pr_args(cpu_preferred_mask));
+}
+static DEVICE_ATTR_RO(preferred);
+#endif
+
 const struct bus_type cpu_subsys = {
 	.name = "cpu",
 	.dev_name = "cpu",
@@ -532,6 +541,9 @@ static struct attribute *cpu_root_attrs[] = {
 #ifdef CONFIG_GENERIC_CPU_AUTOPROBE
 	&dev_attr_modalias.attr,
 #endif
+#ifdef CONFIG_PREFERRED_CPU
+	&dev_attr_preferred.attr,
+#endif
 	NULL
 };
 
diff --git a/drivers/virt/Kconfig b/drivers/virt/Kconfig
index 52eb7e4ba71f..eeb84e578ddf 100644
--- a/drivers/virt/Kconfig
+++ b/drivers/virt/Kconfig
@@ -41,6 +41,23 @@ config FSL_HV_MANAGER
           4) A kernel interface for receiving callbacks when a managed
 	     partition shuts down.
 
+config STEAL_GOVERNOR
+	tristate "Dynamic vCPU management based on steal time"
+	depends on PARAVIRT && SMP
+	select PREFERRED_CPU
+	default m
+	help
+	  This driver helps to reduce the steal time in paravirtualized
+	  environments, thereby reducing vCPU preemption costs.
+
+	  By default preferred CPUs will be same as active CPUs. Depending
+	  on the steal time when steal_governor driver is enabled,
+	  preferred CPUs could become subset of active CPUs.
+	  More details are at: Documentation/driver-api/steal-governor.rst
+
+	  It is recommended to build it as module and load the module
+	  to enable it.
+
 source "drivers/virt/vboxguest/Kconfig"
 
 source "drivers/virt/nitro_enclaves/Kconfig"
diff --git a/drivers/virt/Makefile b/drivers/virt/Makefile
index f29901bd7820..05fb075ef5b8 100644
--- a/drivers/virt/Makefile
+++ b/drivers/virt/Makefile
@@ -5,6 +5,7 @@
 
 obj-$(CONFIG_FSL_HV_MANAGER)	+= fsl_hypervisor.o
 obj-$(CONFIG_VMGENID)		+= vmgenid.o
+obj-$(CONFIG_STEAL_GOVERNOR)	+= steal_governor.o
 obj-y				+= vboxguest/
 
 obj-$(CONFIG_NITRO_ENCLAVES)	+= nitro_enclaves/
diff --git a/drivers/virt/steal_governor.c b/drivers/virt/steal_governor.c
new file mode 100644
index 000000000000..6e31f9923dea
--- /dev/null
+++ b/drivers/virt/steal_governor.c
@@ -0,0 +1,296 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Steal time governor driver periodically computes steal time.
+ * Based on the thresholds it either reduce/increase the preferred
+ * CPUs which can be used by the workload to avoid vCPU preemption
+ * to an extent possible in paravirtualized environment.
+ *
+ * Available with CONFIG_STEAL_GOVERNOR
+ *
+ * Copyright (C) 2026 IBM
+ * Author: Shrikanth Hegde <sshegde@linux.ibm.com>
+ */
+
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+
+#include <linux/cleanup.h>
+#include <linux/cpuhplock.h>
+#include <linux/cpumask.h>
+#include <linux/init.h>
+#include <linux/kernel.h>
+#include <linux/kernel_stat.h>
+#include <linux/kconfig.h>
+#include <linux/ktime.h>
+#include <linux/math64.h>
+#include <linux/module.h>
+#include <linux/sched/isolation.h>
+#include <linux/topology.h>
+#include <linux/types.h>
+#include <linux/workqueue.h>
+#ifdef CONFIG_XEN
+#include <xen/xen.h>
+#endif
+
+#if !IS_ENABLED(CONFIG_PREFERRED_CPU)
+#error "Steal Governor requires CONFIG_PREFERRED_CPU"
+#endif
+
+struct steal_governor {
+	ktime_t			time;
+	u64			steal;
+	unsigned long		delay;
+	unsigned int		interval_ms;
+	unsigned int		high_threshold;
+	unsigned int		low_threshold;
+	struct delayed_work	work;
+};
+
+static struct steal_governor sg_ctx = {
+	.interval_ms	=	1000,	/* 1 second */
+	.high_threshold =	500,	/* 5% */
+	.low_threshold	=	200,	/* 2% */
+};
+
+static void restore_preferred_to_active(void)
+{
+	int cpu;
+
+	guard(cpus_read_lock)();
+	for_each_cpu(cpu, cpu_active_mask)
+		set_cpu_preferred(cpu, true);
+}
+
+static int param_set_interval_ms(const char *val, const struct kernel_param *kp)
+{
+	unsigned int interval;
+	int ret;
+
+	ret = kstrtouint(val, 0, &interval);
+	if (ret)
+		return ret;
+
+	if (interval < 100 || interval > 100000) {
+		pr_err("interval_ms must be between 100 and 100000\n");
+		return -EINVAL;
+	}
+
+	return param_set_uint(val, kp);
+}
+
+static const struct kernel_param_ops interval_ms_ops = {
+	.set = param_set_interval_ms,
+	.get = param_get_uint,
+};
+
+module_param_cb(interval_ms, &interval_ms_ops, &sg_ctx.interval_ms, 0444);
+MODULE_PARM_DESC(interval_ms,
+		 "Sampling frequency in milliseconds. default: 1000");
+
+static int param_set_high_threshold(const char *val, const struct kernel_param *kp)
+{
+	unsigned int threshold;
+	int ret;
+
+	ret = kstrtouint(val, 0, &threshold);
+	if (ret)
+		return ret;
+
+	if (threshold >= 100 * 100) {
+		pr_err("high_threshold (%u) can't be more than 99.99%%\n", threshold);
+		return -EINVAL;
+	}
+
+	return param_set_uint(val, kp);
+}
+
+static const struct kernel_param_ops high_threshold_ops = {
+	.set = param_set_high_threshold,
+	.get = param_get_uint,
+};
+
+module_param_cb(high_threshold, &high_threshold_ops, &sg_ctx.high_threshold, 0444);
+MODULE_PARM_DESC(high_threshold,
+		 "High steal threshold. default: 500 i.e 5%. Must be > low_threshold");
+
+module_param_named(low_threshold, sg_ctx.low_threshold, uint, 0444);
+MODULE_PARM_DESC(low_threshold,
+		 "Low steal threshold. default: 200 i.e 2%. Must be < high_threshold");
+
+/* Return collective steal time across system. */
+static u64 get_system_steal_time(void)
+{
+	return kcpustat_field_total(CPUTIME_STEAL, cpu_possible_mask);
+}
+
+/* Return number of CPUs to consider for steal ratio. */
+static unsigned int get_system_cpus(void)
+{
+	return num_active_cpus();
+}
+
+/*
+ * Called when the steal governor detects high physical CPU contention.
+ * It finds the last active core in the preferred mask and mark those
+ * CPUs as non-preferred.
+ *
+ * Must ensure:
+ * - at least one core is always kept as preferred
+ * - preferred is always subset of active.
+ */
+static void decrease_preferred_cpus(void)
+{
+	const struct cpumask *first_hk_core;
+	int target_cpu = nr_cpu_ids;
+	int cpu;
+
+	guard(cpus_read_lock)();
+	cpu = cpumask_first_and(housekeeping_cpumask(HK_TYPE_KERNEL_NOISE),
+				cpu_preferred_mask);
+	if (cpu >= nr_cpu_ids)
+		return;
+
+	/* Always leave first housekeeping core as preferred. */
+	first_hk_core = topology_sibling_cpumask(cpu);
+	cpu = cpumask_last(cpu_preferred_mask);
+	if (cpu >= nr_cpu_ids)
+		return;
+
+	/* Find the last CPU which doesn't belong to that first hk_core. */
+	if (!cpumask_test_cpu(cpu, first_hk_core)) {
+		target_cpu = cpu;
+	} else {
+		for_each_cpu_andnot(cpu, cpu_preferred_mask, first_hk_core)
+			target_cpu = cpu;
+	}
+
+	/* Only the first housekeeping core remains */
+	if (target_cpu >= nr_cpu_ids)
+		return;
+
+	for_each_cpu_and(cpu, topology_sibling_cpumask(target_cpu),
+			 cpu_preferred_mask)
+		set_cpu_preferred(cpu, false);
+}
+
+/*
+ * Called when the steal governor detects no/low physical CPU contention.
+ * It finds the first active core outside of preferred mask and mark
+ * those CPUs as preferred.
+ *
+ * Must ensure preferred is subset of active.
+ */
+static void increase_preferred_cpus(void)
+{
+	int first_cpu, cpu;
+
+	guard(cpus_read_lock)();
+	first_cpu = cpumask_first_andnot(cpu_active_mask, cpu_preferred_mask);
+
+	/* All CPUs are preferred. Nothing to increase further */
+	if (first_cpu >= nr_cpu_ids)
+		return;
+
+	for_each_cpu_and(cpu, topology_sibling_cpumask(first_cpu),
+			 cpu_active_mask)
+		set_cpu_preferred(cpu, true);
+}
+
+static bool preferred_cpus_valid(void)
+{
+	if (cpumask_empty(cpu_preferred_mask)) {
+		pr_err("empty preferred mask. stopping\n");
+		return false;
+	}
+
+	if (!cpumask_subset(cpu_preferred_mask, cpu_active_mask)) {
+		pr_err("preferred: %*pbl is not subset of active: %*pbl, stopping\n",
+		       cpumask_pr_args(cpu_preferred_mask),
+		       cpumask_pr_args(cpu_active_mask));
+		return false;
+	}
+
+	return true;
+}
+
+static void steal_governor_loop(struct work_struct *work)
+{
+	u64 curr_steal, delta_steal, delta_ns, steal_ratio;
+	ktime_t now;
+
+	now = ktime_get();
+	delta_ns = ktime_to_ns(ktime_sub(now, sg_ctx.time));
+
+	if (unlikely(delta_ns < NSEC_PER_MSEC)) {
+		pr_err_ratelimited("work scheduled too soon delta_ns: %llu\n", delta_ns);
+		goto requeue_work;
+	}
+
+	curr_steal = get_system_steal_time();
+	delta_steal = curr_steal > sg_ctx.steal ? curr_steal - sg_ctx.steal : 0;
+	sg_ctx.steal = curr_steal;
+	sg_ctx.time = now;
+
+	/*
+	 * steal_ratio = (delta_steal * 100*100)/(delta_ns * num_cpus())
+	 * To avoid possible overflow, divide the denominator early.
+	 * Note minimum interval is 100ms.
+	 */
+	delta_ns = max_t(u64, div_u64(delta_ns * get_system_cpus(), 10000), 1);
+	steal_ratio = div64_u64(delta_steal, delta_ns);
+
+	if (steal_ratio > sg_ctx.high_threshold)
+		decrease_preferred_cpus();
+	else if (steal_ratio <= sg_ctx.low_threshold)
+		increase_preferred_cpus();
+	/*
+	 * else: steal ratio is within bounds. Still do design checks so that
+	 * module restores to active if CPU hotplug breaks those assumptions.
+	 */
+	if (!preferred_cpus_valid()) {
+		restore_preferred_to_active();
+		return;
+	}
+
+requeue_work:
+	schedule_delayed_work(&sg_ctx.work, sg_ctx.delay);
+}
+
+static int __init steal_governor_init(void)
+{
+#ifdef CONFIG_XEN
+	if (xen_initial_domain()) {
+		pr_err("Cannot load in Xen Dom0 (Host OS). Driver is for guests only.\n");
+		return -ENODEV;
+	}
+#endif
+
+	if (sg_ctx.low_threshold >= sg_ctx.high_threshold) {
+		pr_err("low_threshold (%u) must be less than high_threshold (%u)\n",
+		       sg_ctx.low_threshold, sg_ctx.high_threshold);
+		return -EINVAL;
+	}
+
+	sg_ctx.delay = msecs_to_jiffies(sg_ctx.interval_ms);
+	INIT_DELAYED_WORK(&sg_ctx.work, steal_governor_loop);
+	sg_ctx.steal = get_system_steal_time();
+	sg_ctx.time = ktime_get();
+	schedule_delayed_work(&sg_ctx.work, sg_ctx.delay);
+	pr_info("enabled. interval: %ums, high_threshold: %u, low_threshold: %u\n",
+		sg_ctx.interval_ms, sg_ctx.high_threshold, sg_ctx.low_threshold);
+
+	return 0;
+}
+
+static void __exit steal_governor_exit(void)
+{
+	disable_delayed_work_sync(&sg_ctx.work);
+	restore_preferred_to_active();
+	pr_info("disabled\n");
+}
+
+module_init(steal_governor_init);
+module_exit(steal_governor_exit);
+
+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("IBM Corporation");
+MODULE_DESCRIPTION("Virtualization Steal Time Governor");
diff --git a/drivers/xen/time.c b/drivers/xen/time.c
index a2be0a4d45b0..a02d48a2aa68 100644
--- a/drivers/xen/time.c
+++ b/drivers/xen/time.c
@@ -169,7 +169,7 @@ void __init xen_time_setup_guest(void)
 
 	static_call_update(pv_steal_clock, xen_steal_clock);
 
-	static_key_slow_inc(&paravirt_steal_enabled);
+	static_branch_inc(&paravirt_steal_enabled);
 	if (xen_runstate_remote)
-		static_key_slow_inc(&paravirt_steal_rq_enabled);
+		static_branch_inc(&paravirt_steal_rq_enabled);
 }