sched_ext: Move built-in idle CPU selection policy to a separate file

As ext.c is becoming quite large, move the idle CPU selection policy to
separate files (ext_idle.c / ext_idle.h) for better code readability.

Moreover, group together all the idle CPU selection kfunc's to the same
btf_kfunc_id_set block.

No functional changes, this is purely code reorganization.

Suggested-by: Yury Norov <yury.norov@gmail.com>
Signed-off-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

1 files changed, 752 insertions, 0 deletions

diff --git a/kernel/sched/ext_idle.c b/kernel/sched/ext_idle.c
new file mode 100644
index 000000000000..cb981956005b
--- /dev/null
+++ b/kernel/sched/ext_idle.c
@@ -0,0 +1,752 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * BPF extensible scheduler class: Documentation/scheduler/sched-ext.rst
+ *
+ * Built-in idle CPU tracking policy.
+ *
+ * Copyright (c) 2022 Meta Platforms, Inc. and affiliates.
+ * Copyright (c) 2022 Tejun Heo <tj@kernel.org>
+ * Copyright (c) 2022 David Vernet <dvernet@meta.com>
+ * Copyright (c) 2024 Andrea Righi <arighi@nvidia.com>
+ */
+#include "ext_idle.h"
+
+/* Enable/disable built-in idle CPU selection policy */
+DEFINE_STATIC_KEY_FALSE(scx_builtin_idle_enabled);
+
+#ifdef CONFIG_SMP
+#ifdef CONFIG_CPUMASK_OFFSTACK
+#define CL_ALIGNED_IF_ONSTACK
+#else
+#define CL_ALIGNED_IF_ONSTACK __cacheline_aligned_in_smp
+#endif
+
+/* Enable/disable LLC aware optimizations */
+DEFINE_STATIC_KEY_FALSE(scx_selcpu_topo_llc);
+
+/* Enable/disable NUMA aware optimizations */
+DEFINE_STATIC_KEY_FALSE(scx_selcpu_topo_numa);
+
+static struct {
+	cpumask_var_t cpu;
+	cpumask_var_t smt;
+} idle_masks CL_ALIGNED_IF_ONSTACK;
+
+bool scx_idle_test_and_clear_cpu(int cpu)
+{
+#ifdef CONFIG_SCHED_SMT
+	/*
+	 * SMT mask should be cleared whether we can claim @cpu or not. The SMT
+	 * cluster is not wholly idle either way. This also prevents
+	 * scx_pick_idle_cpu() from getting caught in an infinite loop.
+	 */
+	if (sched_smt_active()) {
+		const struct cpumask *smt = cpu_smt_mask(cpu);
+
+		/*
+		 * If offline, @cpu is not its own sibling and
+		 * scx_pick_idle_cpu() can get caught in an infinite loop as
+		 * @cpu is never cleared from idle_masks.smt. Ensure that @cpu
+		 * is eventually cleared.
+		 *
+		 * NOTE: Use cpumask_intersects() and cpumask_test_cpu() to
+		 * reduce memory writes, which may help alleviate cache
+		 * coherence pressure.
+		 */
+		if (cpumask_intersects(smt, idle_masks.smt))
+			cpumask_andnot(idle_masks.smt, idle_masks.smt, smt);
+		else if (cpumask_test_cpu(cpu, idle_masks.smt))
+			__cpumask_clear_cpu(cpu, idle_masks.smt);
+	}
+#endif
+	return cpumask_test_and_clear_cpu(cpu, idle_masks.cpu);
+}
+
+s32 scx_pick_idle_cpu(const struct cpumask *cpus_allowed, u64 flags)
+{
+	int cpu;
+
+retry:
+	if (sched_smt_active()) {
+		cpu = cpumask_any_and_distribute(idle_masks.smt, cpus_allowed);
+		if (cpu < nr_cpu_ids)
+			goto found;
+
+		if (flags & SCX_PICK_IDLE_CORE)
+			return -EBUSY;
+	}
+
+	cpu = cpumask_any_and_distribute(idle_masks.cpu, cpus_allowed);
+	if (cpu >= nr_cpu_ids)
+		return -EBUSY;
+
+found:
+	if (scx_idle_test_and_clear_cpu(cpu))
+		return cpu;
+	else
+		goto retry;
+}
+
+/*
+ * Return the amount of CPUs in the same LLC domain of @cpu (or zero if the LLC
+ * domain is not defined).
+ */
+static unsigned int llc_weight(s32 cpu)
+{
+	struct sched_domain *sd;
+
+	sd = rcu_dereference(per_cpu(sd_llc, cpu));
+	if (!sd)
+		return 0;
+
+	return sd->span_weight;
+}
+
+/*
+ * Return the cpumask representing the LLC domain of @cpu (or NULL if the LLC
+ * domain is not defined).
+ */
+static struct cpumask *llc_span(s32 cpu)
+{
+	struct sched_domain *sd;
+
+	sd = rcu_dereference(per_cpu(sd_llc, cpu));
+	if (!sd)
+		return 0;
+
+	return sched_domain_span(sd);
+}
+
+/*
+ * Return the amount of CPUs in the same NUMA domain of @cpu (or zero if the
+ * NUMA domain is not defined).
+ */
+static unsigned int numa_weight(s32 cpu)
+{
+	struct sched_domain *sd;
+	struct sched_group *sg;
+
+	sd = rcu_dereference(per_cpu(sd_numa, cpu));
+	if (!sd)
+		return 0;
+	sg = sd->groups;
+	if (!sg)
+		return 0;
+
+	return sg->group_weight;
+}
+
+/*
+ * Return the cpumask representing the NUMA domain of @cpu (or NULL if the NUMA
+ * domain is not defined).
+ */
+static struct cpumask *numa_span(s32 cpu)
+{
+	struct sched_domain *sd;
+	struct sched_group *sg;
+
+	sd = rcu_dereference(per_cpu(sd_numa, cpu));
+	if (!sd)
+		return NULL;
+	sg = sd->groups;
+	if (!sg)
+		return NULL;
+
+	return sched_group_span(sg);
+}
+
+/*
+ * Return true if the LLC domains do not perfectly overlap with the NUMA
+ * domains, false otherwise.
+ */
+static bool llc_numa_mismatch(void)
+{
+	int cpu;
+
+	/*
+	 * We need to scan all online CPUs to verify whether their scheduling
+	 * domains overlap.
+	 *
+	 * While it is rare to encounter architectures with asymmetric NUMA
+	 * topologies, CPU hotplugging or virtualized environments can result
+	 * in asymmetric configurations.
+	 *
+	 * For example:
+	 *
+	 *  NUMA 0:
+	 *    - LLC 0: cpu0..cpu7
+	 *    - LLC 1: cpu8..cpu15 [offline]
+	 *
+	 *  NUMA 1:
+	 *    - LLC 0: cpu16..cpu23
+	 *    - LLC 1: cpu24..cpu31
+	 *
+	 * In this case, if we only check the first online CPU (cpu0), we might
+	 * incorrectly assume that the LLC and NUMA domains are fully
+	 * overlapping, which is incorrect (as NUMA 1 has two distinct LLC
+	 * domains).
+	 */
+	for_each_online_cpu(cpu)
+		if (llc_weight(cpu) != numa_weight(cpu))
+			return true;
+
+	return false;
+}
+
+/*
+ * Initialize topology-aware scheduling.
+ *
+ * Detect if the system has multiple LLC or multiple NUMA domains and enable
+ * cache-aware / NUMA-aware scheduling optimizations in the default CPU idle
+ * selection policy.
+ *
+ * Assumption: the kernel's internal topology representation assumes that each
+ * CPU belongs to a single LLC domain, and that each LLC domain is entirely
+ * contained within a single NUMA node.
+ */
+void scx_idle_update_selcpu_topology(void)
+{
+	bool enable_llc = false, enable_numa = false;
+	unsigned int nr_cpus;
+	s32 cpu = cpumask_first(cpu_online_mask);
+
+	/*
+	 * Enable LLC domain optimization only when there are multiple LLC
+	 * domains among the online CPUs. If all online CPUs are part of a
+	 * single LLC domain, the idle CPU selection logic can choose any
+	 * online CPU without bias.
+	 *
+	 * Note that it is sufficient to check the LLC domain of the first
+	 * online CPU to determine whether a single LLC domain includes all
+	 * CPUs.
+	 */
+	rcu_read_lock();
+	nr_cpus = llc_weight(cpu);
+	if (nr_cpus > 0) {
+		if (nr_cpus < num_online_cpus())
+			enable_llc = true;
+		pr_debug("sched_ext: LLC=%*pb weight=%u\n",
+			 cpumask_pr_args(llc_span(cpu)), llc_weight(cpu));
+	}
+
+	/*
+	 * Enable NUMA optimization only when there are multiple NUMA domains
+	 * among the online CPUs and the NUMA domains don't perfectly overlaps
+	 * with the LLC domains.
+	 *
+	 * If all CPUs belong to the same NUMA node and the same LLC domain,
+	 * enabling both NUMA and LLC optimizations is unnecessary, as checking
+	 * for an idle CPU in the same domain twice is redundant.
+	 */
+	nr_cpus = numa_weight(cpu);
+	if (nr_cpus > 0) {
+		if (nr_cpus < num_online_cpus() && llc_numa_mismatch())
+			enable_numa = true;
+		pr_debug("sched_ext: NUMA=%*pb weight=%u\n",
+			 cpumask_pr_args(numa_span(cpu)), numa_weight(cpu));
+	}
+	rcu_read_unlock();
+
+	pr_debug("sched_ext: LLC idle selection %s\n",
+		 str_enabled_disabled(enable_llc));
+	pr_debug("sched_ext: NUMA idle selection %s\n",
+		 str_enabled_disabled(enable_numa));
+
+	if (enable_llc)
+		static_branch_enable_cpuslocked(&scx_selcpu_topo_llc);
+	else
+		static_branch_disable_cpuslocked(&scx_selcpu_topo_llc);
+	if (enable_numa)
+		static_branch_enable_cpuslocked(&scx_selcpu_topo_numa);
+	else
+		static_branch_disable_cpuslocked(&scx_selcpu_topo_numa);
+}
+
+/*
+ * Built-in CPU idle selection policy:
+ *
+ * 1. Prioritize full-idle cores:
+ *   - always prioritize CPUs from fully idle cores (both logical CPUs are
+ *     idle) to avoid interference caused by SMT.
+ *
+ * 2. Reuse the same CPU:
+ *   - prefer the last used CPU to take advantage of cached data (L1, L2) and
+ *     branch prediction optimizations.
+ *
+ * 3. Pick a CPU within the same LLC (Last-Level Cache):
+ *   - if the above conditions aren't met, pick a CPU that shares the same LLC
+ *     to maintain cache locality.
+ *
+ * 4. Pick a CPU within the same NUMA node, if enabled:
+ *   - choose a CPU from the same NUMA node to reduce memory access latency.
+ *
+ * 5. Pick any idle CPU usable by the task.
+ *
+ * Step 3 and 4 are performed only if the system has, respectively, multiple
+ * LLC domains / multiple NUMA nodes (see scx_selcpu_topo_llc and
+ * scx_selcpu_topo_numa).
+ *
+ * NOTE: tasks that can only run on 1 CPU are excluded by this logic, because
+ * we never call ops.select_cpu() for them, see select_task_rq().
+ */
+s32 scx_select_cpu_dfl(struct task_struct *p, s32 prev_cpu, u64 wake_flags, bool *found)
+{
+	const struct cpumask *llc_cpus = NULL;
+	const struct cpumask *numa_cpus = NULL;
+	s32 cpu;
+
+	*found = false;
+
+	/*
+	 * This is necessary to protect llc_cpus.
+	 */
+	rcu_read_lock();
+
+	/*
+	 * Determine the scheduling domain only if the task is allowed to run
+	 * on all CPUs.
+	 *
+	 * This is done primarily for efficiency, as it avoids the overhead of
+	 * updating a cpumask every time we need to select an idle CPU (which
+	 * can be costly in large SMP systems), but it also aligns logically:
+	 * if a task's scheduling domain is restricted by user-space (through
+	 * CPU affinity), the task will simply use the flat scheduling domain
+	 * defined by user-space.
+	 */
+	if (p->nr_cpus_allowed >= num_possible_cpus()) {
+		if (static_branch_maybe(CONFIG_NUMA, &scx_selcpu_topo_numa))
+			numa_cpus = numa_span(prev_cpu);
+
+		if (static_branch_maybe(CONFIG_SCHED_MC, &scx_selcpu_topo_llc))
+			llc_cpus = llc_span(prev_cpu);
+	}
+
+	/*
+	 * If WAKE_SYNC, try to migrate the wakee to the waker's CPU.
+	 */
+	if (wake_flags & SCX_WAKE_SYNC) {
+		cpu = smp_processor_id();
+
+		/*
+		 * If the waker's CPU is cache affine and prev_cpu is idle,
+		 * then avoid a migration.
+		 */
+		if (cpus_share_cache(cpu, prev_cpu) &&
+		    scx_idle_test_and_clear_cpu(prev_cpu)) {
+			cpu = prev_cpu;
+			goto cpu_found;
+		}
+
+		/*
+		 * If the waker's local DSQ is empty, and the system is under
+		 * utilized, try to wake up @p to the local DSQ of the waker.
+		 *
+		 * Checking only for an empty local DSQ is insufficient as it
+		 * could give the wakee an unfair advantage when the system is
+		 * oversaturated.
+		 *
+		 * Checking only for the presence of idle CPUs is also
+		 * insufficient as the local DSQ of the waker could have tasks
+		 * piled up on it even if there is an idle core elsewhere on
+		 * the system.
+		 */
+		if (!cpumask_empty(idle_masks.cpu) &&
+		    !(current->flags & PF_EXITING) &&
+		    cpu_rq(cpu)->scx.local_dsq.nr == 0) {
+			if (cpumask_test_cpu(cpu, p->cpus_ptr))
+				goto cpu_found;
+		}
+	}
+
+	/*
+	 * If CPU has SMT, any wholly idle CPU is likely a better pick than
+	 * partially idle @prev_cpu.
+	 */
+	if (sched_smt_active()) {
+		/*
+		 * Keep using @prev_cpu if it's part of a fully idle core.
+		 */
+		if (cpumask_test_cpu(prev_cpu, idle_masks.smt) &&
+		    scx_idle_test_and_clear_cpu(prev_cpu)) {
+			cpu = prev_cpu;
+			goto cpu_found;
+		}
+
+		/*
+		 * Search for any fully idle core in the same LLC domain.
+		 */
+		if (llc_cpus) {
+			cpu = scx_pick_idle_cpu(llc_cpus, SCX_PICK_IDLE_CORE);
+			if (cpu >= 0)
+				goto cpu_found;
+		}
+
+		/*
+		 * Search for any fully idle core in the same NUMA node.
+		 */
+		if (numa_cpus) {
+			cpu = scx_pick_idle_cpu(numa_cpus, SCX_PICK_IDLE_CORE);
+			if (cpu >= 0)
+				goto cpu_found;
+		}
+
+		/*
+		 * Search for any full idle core usable by the task.
+		 */
+		cpu = scx_pick_idle_cpu(p->cpus_ptr, SCX_PICK_IDLE_CORE);
+		if (cpu >= 0)
+			goto cpu_found;
+	}
+
+	/*
+	 * Use @prev_cpu if it's idle.
+	 */
+	if (scx_idle_test_and_clear_cpu(prev_cpu)) {
+		cpu = prev_cpu;
+		goto cpu_found;
+	}
+
+	/*
+	 * Search for any idle CPU in the same LLC domain.
+	 */
+	if (llc_cpus) {
+		cpu = scx_pick_idle_cpu(llc_cpus, 0);
+		if (cpu >= 0)
+			goto cpu_found;
+	}
+
+	/*
+	 * Search for any idle CPU in the same NUMA node.
+	 */
+	if (numa_cpus) {
+		cpu = scx_pick_idle_cpu(numa_cpus, 0);
+		if (cpu >= 0)
+			goto cpu_found;
+	}
+
+	/*
+	 * Search for any idle CPU usable by the task.
+	 */
+	cpu = scx_pick_idle_cpu(p->cpus_ptr, 0);
+	if (cpu >= 0)
+		goto cpu_found;
+
+	rcu_read_unlock();
+	return prev_cpu;
+
+cpu_found:
+	rcu_read_unlock();
+
+	*found = true;
+	return cpu;
+}
+
+void scx_idle_reset_masks(void)
+{
+	/*
+	 * Consider all online cpus idle. Should converge to the actual state
+	 * quickly.
+	 */
+	cpumask_copy(idle_masks.cpu, cpu_online_mask);
+	cpumask_copy(idle_masks.smt, cpu_online_mask);
+}
+
+void scx_idle_init_masks(void)
+{
+	BUG_ON(!alloc_cpumask_var(&idle_masks.cpu, GFP_KERNEL));
+	BUG_ON(!alloc_cpumask_var(&idle_masks.smt, GFP_KERNEL));
+}
+
+static void update_builtin_idle(int cpu, bool idle)
+{
+	assign_cpu(cpu, idle_masks.cpu, idle);
+
+#ifdef CONFIG_SCHED_SMT
+	if (sched_smt_active()) {
+		const struct cpumask *smt = cpu_smt_mask(cpu);
+
+		if (idle) {
+			/*
+			 * idle_masks.smt handling is racy but that's fine as
+			 * it's only for optimization and self-correcting.
+			 */
+			if (!cpumask_subset(smt, idle_masks.cpu))
+				return;
+			cpumask_or(idle_masks.smt, idle_masks.smt, smt);
+		} else {
+			cpumask_andnot(idle_masks.smt, idle_masks.smt, smt);
+		}
+	}
+#endif
+}
+
+/*
+ * Update the idle state of a CPU to @idle.
+ *
+ * If @do_notify is true, ops.update_idle() is invoked to notify the scx
+ * scheduler of an actual idle state transition (idle to busy or vice
+ * versa). If @do_notify is false, only the idle state in the idle masks is
+ * refreshed without invoking ops.update_idle().
+ *
+ * This distinction is necessary, because an idle CPU can be "reserved" and
+ * awakened via scx_bpf_pick_idle_cpu() + scx_bpf_kick_cpu(), marking it as
+ * busy even if no tasks are dispatched. In this case, the CPU may return
+ * to idle without a true state transition. Refreshing the idle masks
+ * without invoking ops.update_idle() ensures accurate idle state tracking
+ * while avoiding unnecessary updates and maintaining balanced state
+ * transitions.
+ */
+void __scx_update_idle(struct rq *rq, bool idle, bool do_notify)
+{
+	int cpu = cpu_of(rq);
+
+	lockdep_assert_rq_held(rq);
+
+	/*
+	 * Trigger ops.update_idle() only when transitioning from a task to
+	 * the idle thread and vice versa.
+	 *
+	 * Idle transitions are indicated by do_notify being set to true,
+	 * managed by put_prev_task_idle()/set_next_task_idle().
+	 */
+	if (SCX_HAS_OP(update_idle) && do_notify && !scx_rq_bypassing(rq))
+		SCX_CALL_OP(SCX_KF_REST, update_idle, cpu_of(rq), idle);
+
+	/*
+	 * Update the idle masks:
+	 * - for real idle transitions (do_notify == true)
+	 * - for idle-to-idle transitions (indicated by the previous task
+	 *   being the idle thread, managed by pick_task_idle())
+	 *
+	 * Skip updating idle masks if the previous task is not the idle
+	 * thread, since set_next_task_idle() has already handled it when
+	 * transitioning from a task to the idle thread (calling this
+	 * function with do_notify == true).
+	 *
+	 * In this way we can avoid updating the idle masks twice,
+	 * unnecessarily.
+	 */
+	if (static_branch_likely(&scx_builtin_idle_enabled))
+		if (do_notify || is_idle_task(rq->curr))
+			update_builtin_idle(cpu, idle);
+}
+#endif	/* CONFIG_SMP */
+
+/********************************************************************************
+ * Helpers that can be called from the BPF scheduler.
+ */
+__bpf_kfunc_start_defs();
+
+static bool check_builtin_idle_enabled(void)
+{
+	if (static_branch_likely(&scx_builtin_idle_enabled))
+		return true;
+
+	scx_ops_error("built-in idle tracking is disabled");
+	return false;
+}
+
+/**
+ * scx_bpf_select_cpu_dfl - The default implementation of ops.select_cpu()
+ * @p: task_struct to select a CPU for
+ * @prev_cpu: CPU @p was on previously
+ * @wake_flags: %SCX_WAKE_* flags
+ * @is_idle: out parameter indicating whether the returned CPU is idle
+ *
+ * Can only be called from ops.select_cpu() if the built-in CPU selection is
+ * enabled - ops.update_idle() is missing or %SCX_OPS_KEEP_BUILTIN_IDLE is set.
+ * @p, @prev_cpu and @wake_flags match ops.select_cpu().
+ *
+ * Returns the picked CPU with *@is_idle indicating whether the picked CPU is
+ * currently idle and thus a good candidate for direct dispatching.
+ */
+__bpf_kfunc s32 scx_bpf_select_cpu_dfl(struct task_struct *p, s32 prev_cpu,
+				       u64 wake_flags, bool *is_idle)
+{
+	if (!check_builtin_idle_enabled())
+		goto prev_cpu;
+
+	if (!scx_kf_allowed(SCX_KF_SELECT_CPU))
+		goto prev_cpu;
+
+#ifdef CONFIG_SMP
+	return scx_select_cpu_dfl(p, prev_cpu, wake_flags, is_idle);
+#endif
+
+prev_cpu:
+	*is_idle = false;
+	return prev_cpu;
+}
+
+/**
+ * scx_bpf_get_idle_cpumask - Get a referenced kptr to the idle-tracking
+ * per-CPU cpumask.
+ *
+ * Returns NULL if idle tracking is not enabled, or running on a UP kernel.
+ */
+__bpf_kfunc const struct cpumask *scx_bpf_get_idle_cpumask(void)
+{
+	if (!check_builtin_idle_enabled())
+		return cpu_none_mask;
+
+#ifdef CONFIG_SMP
+	return idle_masks.cpu;
+#else
+	return cpu_none_mask;
+#endif
+}
+
+/**
+ * scx_bpf_get_idle_smtmask - Get a referenced kptr to the idle-tracking,
+ * per-physical-core cpumask. Can be used to determine if an entire physical
+ * core is free.
+ *
+ * Returns NULL if idle tracking is not enabled, or running on a UP kernel.
+ */
+__bpf_kfunc const struct cpumask *scx_bpf_get_idle_smtmask(void)
+{
+	if (!check_builtin_idle_enabled())
+		return cpu_none_mask;
+
+#ifdef CONFIG_SMP
+	if (sched_smt_active())
+		return idle_masks.smt;
+	else
+		return idle_masks.cpu;
+#else
+	return cpu_none_mask;
+#endif
+}
+
+/**
+ * scx_bpf_put_idle_cpumask - Release a previously acquired referenced kptr to
+ * either the percpu, or SMT idle-tracking cpumask.
+ * @idle_mask: &cpumask to use
+ */
+__bpf_kfunc void scx_bpf_put_idle_cpumask(const struct cpumask *idle_mask)
+{
+	/*
+	 * Empty function body because we aren't actually acquiring or releasing
+	 * a reference to a global idle cpumask, which is read-only in the
+	 * caller and is never released. The acquire / release semantics here
+	 * are just used to make the cpumask a trusted pointer in the caller.
+	 */
+}
+
+/**
+ * scx_bpf_test_and_clear_cpu_idle - Test and clear @cpu's idle state
+ * @cpu: cpu to test and clear idle for
+ *
+ * Returns %true if @cpu was idle and its idle state was successfully cleared.
+ * %false otherwise.
+ *
+ * Unavailable if ops.update_idle() is implemented and
+ * %SCX_OPS_KEEP_BUILTIN_IDLE is not set.
+ */
+__bpf_kfunc bool scx_bpf_test_and_clear_cpu_idle(s32 cpu)
+{
+	if (!check_builtin_idle_enabled())
+		return false;
+
+	if (ops_cpu_valid(cpu, NULL))
+		return scx_idle_test_and_clear_cpu(cpu);
+	else
+		return false;
+}
+
+/**
+ * scx_bpf_pick_idle_cpu - Pick and claim an idle cpu
+ * @cpus_allowed: Allowed cpumask
+ * @flags: %SCX_PICK_IDLE_CPU_* flags
+ *
+ * Pick and claim an idle cpu in @cpus_allowed. Returns the picked idle cpu
+ * number on success. -%EBUSY if no matching cpu was found.
+ *
+ * Idle CPU tracking may race against CPU scheduling state transitions. For
+ * example, this function may return -%EBUSY as CPUs are transitioning into the
+ * idle state. If the caller then assumes that there will be dispatch events on
+ * the CPUs as they were all busy, the scheduler may end up stalling with CPUs
+ * idling while there are pending tasks. Use scx_bpf_pick_any_cpu() and
+ * scx_bpf_kick_cpu() to guarantee that there will be at least one dispatch
+ * event in the near future.
+ *
+ * Unavailable if ops.update_idle() is implemented and
+ * %SCX_OPS_KEEP_BUILTIN_IDLE is not set.
+ */
+__bpf_kfunc s32 scx_bpf_pick_idle_cpu(const struct cpumask *cpus_allowed,
+				      u64 flags)
+{
+	if (!check_builtin_idle_enabled())
+		return -EBUSY;
+
+	return scx_pick_idle_cpu(cpus_allowed, flags);
+}
+
+/**
+ * scx_bpf_pick_any_cpu - Pick and claim an idle cpu if available or pick any CPU
+ * @cpus_allowed: Allowed cpumask
+ * @flags: %SCX_PICK_IDLE_CPU_* flags
+ *
+ * Pick and claim an idle cpu in @cpus_allowed. If none is available, pick any
+ * CPU in @cpus_allowed. Guaranteed to succeed and returns the picked idle cpu
+ * number if @cpus_allowed is not empty. -%EBUSY is returned if @cpus_allowed is
+ * empty.
+ *
+ * If ops.update_idle() is implemented and %SCX_OPS_KEEP_BUILTIN_IDLE is not
+ * set, this function can't tell which CPUs are idle and will always pick any
+ * CPU.
+ */
+__bpf_kfunc s32 scx_bpf_pick_any_cpu(const struct cpumask *cpus_allowed,
+				     u64 flags)
+{
+	s32 cpu;
+
+	if (static_branch_likely(&scx_builtin_idle_enabled)) {
+		cpu = scx_pick_idle_cpu(cpus_allowed, flags);
+		if (cpu >= 0)
+			return cpu;
+	}
+
+	cpu = cpumask_any_distribute(cpus_allowed);
+	if (cpu < nr_cpu_ids)
+		return cpu;
+	else
+		return -EBUSY;
+}
+
+__bpf_kfunc_end_defs();
+
+BTF_KFUNCS_START(scx_kfunc_ids_idle)
+BTF_ID_FLAGS(func, scx_bpf_get_idle_cpumask, KF_ACQUIRE)
+BTF_ID_FLAGS(func, scx_bpf_get_idle_smtmask, KF_ACQUIRE)
+BTF_ID_FLAGS(func, scx_bpf_put_idle_cpumask, KF_RELEASE)
+BTF_ID_FLAGS(func, scx_bpf_test_and_clear_cpu_idle)
+BTF_ID_FLAGS(func, scx_bpf_pick_idle_cpu, KF_RCU)
+BTF_ID_FLAGS(func, scx_bpf_pick_any_cpu, KF_RCU)
+BTF_KFUNCS_END(scx_kfunc_ids_idle)
+
+static const struct btf_kfunc_id_set scx_kfunc_set_idle = {
+	.owner			= THIS_MODULE,
+	.set			= &scx_kfunc_ids_idle,
+};
+
+BTF_KFUNCS_START(scx_kfunc_ids_select_cpu)
+BTF_ID_FLAGS(func, scx_bpf_select_cpu_dfl, KF_RCU)
+BTF_KFUNCS_END(scx_kfunc_ids_select_cpu)
+
+static const struct btf_kfunc_id_set scx_kfunc_set_select_cpu = {
+	.owner			= THIS_MODULE,
+	.set			= &scx_kfunc_ids_select_cpu,
+};
+
+int scx_idle_init(void)
+{
+	int ret;
+
+	ret = register_btf_kfunc_id_set(BPF_PROG_TYPE_STRUCT_OPS, &scx_kfunc_set_select_cpu) ||
+	      register_btf_kfunc_id_set(BPF_PROG_TYPE_STRUCT_OPS, &scx_kfunc_set_idle) ||
+	      register_btf_kfunc_id_set(BPF_PROG_TYPE_TRACING, &scx_kfunc_set_idle) ||
+	      register_btf_kfunc_id_set(BPF_PROG_TYPE_SYSCALL, &scx_kfunc_set_idle);
+
+	return ret;
+}