[PATCH sched_ext/for-6.14] sched_ext: Move built-in idle CPU selection policy to a separate file

Andrea Righi
2025-01-23T22:02:11Z

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>
---
MAINTAINERS | 2 +
kernel/sched/build_policy.c | 1 +
kernel/sched/ext.c | 753 +-----------------------------------
kernel/sched/ext_idle.c | 722 ++++++++++++++++++++++++++++++++++
kernel/sched/ext_idle.h | 45 +++
5 files changed, 787 insertions(+), 736 deletions(-)
create mode 100644 kernel/sched/ext_idle.c
create mode 100644 kernel/sched/ext_idle.h

diff --git a/MAINTAINERS b/MAINTAINERS
index 023df277737d..cd3d5b139a11 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -20935,6 +20935,8 @@ T: git://git.kernel.org/pub/scm/linux/kernel/git/tj/sched_ext.git
F: include/linux/sched/ext.h
F: kernel/sched/ext.h
F: kernel/sched/ext.c
+F: kernel/sched/ext_idle.c
+F: kernel/sched/ext_idle.h
F: tools/sched_ext/
F: tools/testing/selftests/sched_ext

diff --git a/kernel/sched/build_policy.c b/kernel/sched/build_policy.c
index fae1f5c921eb..72d97aa8b726 100644
--- a/kernel/sched/build_policy.c
+++ b/kernel/sched/build_policy.c
@@ -61,6 +61,7 @@

#ifdef CONFIG_SCHED_CLASS_EXT
# include "ext.c"
+# include "ext_idle.c"
#endif

#include "syscalls.c"
diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 564f250e7689..a24d48cebfb7 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -6,6 +6,9 @@
* Copyright (c) 2022 Tejun Heo <tj@kernel.org>
* Copyright (c) 2022 David Vernet <dvernet@meta.com>
*/
+#include <linux/btf_ids.h>
+#include "ext_idle.h"
+
#define SCX_OP_IDX(op) (offsetof(struct sched_ext_ops, op) / sizeof(void (*)(void)))

enum scx_consts {
@@ -883,12 +886,6 @@ static bool scx_warned_zero_slice;
static DEFINE_STATIC_KEY_FALSE(scx_ops_enq_last);
static DEFINE_STATIC_KEY_FALSE(scx_ops_enq_exiting);
static DEFINE_STATIC_KEY_FALSE(scx_ops_cpu_preempt);
-static DEFINE_STATIC_KEY_FALSE(scx_builtin_idle_enabled);
-
-#ifdef CONFIG_SMP
-static DEFINE_STATIC_KEY_FALSE(scx_selcpu_topo_llc);
-static DEFINE_STATIC_KEY_FALSE(scx_selcpu_topo_numa);
-#endif

static struct static_key_false scx_has_op[SCX_OPI_END] =
{ [0 ... SCX_OPI_END-1] = STATIC_KEY_FALSE_INIT };
@@ -896,6 +893,17 @@ static struct static_key_false scx_has_op[SCX_OPI_END] =
static atomic_t scx_exit_kind = ATOMIC_INIT(SCX_EXIT_DONE);
static struct scx_exit_info *scx_exit_info;

+#define scx_ops_error_kind(err, fmt, args...) \
+ scx_ops_exit_kind((err), 0, fmt, ##args)
+
+#define scx_ops_exit(code, fmt, args...) \
+ scx_ops_exit_kind(SCX_EXIT_UNREG_KERN, (code), fmt, ##args)
+
+#define scx_ops_error(fmt, args...) \
+ scx_ops_error_kind(SCX_EXIT_ERROR, fmt, ##args)
+
+#define SCX_HAS_OP(op) static_branch_likely(&scx_has_op[SCX_OP_IDX(op)])
+
static atomic_long_t scx_nr_rejected = ATOMIC_LONG_INIT(0);
static atomic_long_t scx_hotplug_seq = ATOMIC_LONG_INIT(0);

@@ -923,21 +931,6 @@ static unsigned long scx_watchdog_timestamp = INITIAL_JIFFIES;

static struct delayed_work scx_watchdog_work;

-/* idle tracking */
-#ifdef CONFIG_SMP
-#ifdef CONFIG_CPUMASK_OFFSTACK
-#define CL_ALIGNED_IF_ONSTACK
-#else
-#define CL_ALIGNED_IF_ONSTACK __cacheline_aligned_in_smp
-#endif
-
-static struct {
- cpumask_var_t cpu;
- cpumask_var_t smt;
-} idle_masks CL_ALIGNED_IF_ONSTACK;
-
-#endif /* CONFIG_SMP */
-
/* for %SCX_KICK_WAIT */
static unsigned long __percpu *scx_kick_cpus_pnt_seqs;

@@ -1024,17 +1017,6 @@ static __printf(3, 4) void scx_ops_exit_kind(enum scx_exit_kind kind,
s64 exit_code,
const char *fmt, ...);

-#define scx_ops_error_kind(err, fmt, args...) \
- scx_ops_exit_kind((err), 0, fmt, ##args)
-
-#define scx_ops_exit(code, fmt, args...) \
- scx_ops_exit_kind(SCX_EXIT_UNREG_KERN, (code), fmt, ##args)
-
-#define scx_ops_error(fmt, args...) \
- scx_ops_error_kind(SCX_EXIT_ERROR, fmt, ##args)
-
-#define SCX_HAS_OP(op) static_branch_likely(&scx_has_op[SCX_OP_IDX(op)])
-
static long jiffies_delta_msecs(unsigned long at, unsigned long now)
{
if (time_after(at, now))
@@ -3169,416 +3151,6 @@ bool scx_prio_less(const struct task_struct *a, const struct task_struct *b,

#ifdef CONFIG_SMP

-static bool test_and_clear_cpu_idle(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);
-}
-
-static 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 (test_and_clear_cpu_idle(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.
- */
-static void 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().
- */
-static 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) &&
- test_and_clear_cpu_idle(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) &&
- test_and_clear_cpu_idle(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 (test_and_clear_cpu_idle(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;
-}
-
static int select_task_rq_scx(struct task_struct *p, int prev_cpu, int wake_flags)
{
/*
@@ -3645,90 +3217,6 @@ static void set_cpus_allowed_scx(struct task_struct *p,
(struct cpumask *)p->cpus_ptr);
}

-static void reset_idle_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);
-}
-
-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);
-}
-
static void handle_hotplug(struct rq *rq, bool online)
{
int cpu = cpu_of(rq);
@@ -3768,12 +3256,6 @@ static void rq_offline_scx(struct rq *rq)
rq->scx.flags &= ~SCX_RQ_ONLINE;
}

-#else /* CONFIG_SMP */
-
-static bool test_and_clear_cpu_idle(int cpu) { return false; }
-static s32 scx_pick_idle_cpu(const struct cpumask *cpus_allowed, u64 flags) { return -EBUSY; }
-static void reset_idle_masks(void) {}
-
#endif /* CONFIG_SMP */

static bool check_rq_for_timeouts(struct rq *rq)
@@ -5632,9 +5114,8 @@ static int scx_ops_enable(struct sched_ext_ops *ops, struct bpf_link *link)
static_branch_enable_cpuslocked(&scx_has_op[i]);

check_hotplug_seq(ops);
-#ifdef CONFIG_SMP
update_selcpu_topology();
-#endif
+
cpus_read_unlock();

ret = validate_ops(ops);
@@ -6325,10 +5806,8 @@ void __init init_sched_ext_class(void)
SCX_TG_ONLINE);

BUG_ON(rhashtable_init(&dsq_hash, &dsq_hash_params));
-#ifdef CONFIG_SMP
- BUG_ON(!alloc_cpumask_var(&idle_masks.cpu, GFP_KERNEL));
- BUG_ON(!alloc_cpumask_var(&idle_masks.smt, GFP_KERNEL));
-#endif
+ init_idle_masks();
+
scx_kick_cpus_pnt_seqs =
__alloc_percpu(sizeof(scx_kick_cpus_pnt_seqs[0]) * nr_cpu_ids,
__alignof__(scx_kick_cpus_pnt_seqs[0]));
@@ -6361,62 +5840,6 @@ void __init init_sched_ext_class(void)
/********************************************************************************
* Helpers that can be called from the BPF scheduler.
*/
-#include <linux/btf_ids.h>
-
-__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;
-}
-
-__bpf_kfunc_end_defs();
-
-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,
-};
-
static bool scx_dsq_insert_preamble(struct task_struct *p, u64 enq_flags)
{
if (!scx_kf_allowed(SCX_KF_ENQUEUE | SCX_KF_DISPATCH))
@@ -7475,142 +6898,6 @@ __bpf_kfunc void scx_bpf_put_cpumask(const struct cpumask *cpumask)
*/
}

-/**
- * 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 test_and_clear_cpu_idle(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;
-}
-
/**
* scx_bpf_task_running - Is task currently running?
* @p: task of interest
@@ -7750,12 +7037,6 @@ BTF_ID_FLAGS(func, scx_bpf_nr_cpu_ids)
BTF_ID_FLAGS(func, scx_bpf_get_possible_cpumask, KF_ACQUIRE)
BTF_ID_FLAGS(func, scx_bpf_get_online_cpumask, KF_ACQUIRE)
BTF_ID_FLAGS(func, scx_bpf_put_cpumask, KF_RELEASE)
-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_ID_FLAGS(func, scx_bpf_task_running, KF_RCU)
BTF_ID_FLAGS(func, scx_bpf_task_cpu, KF_RCU)
BTF_ID_FLAGS(func, scx_bpf_cpu_rq)
diff --git a/kernel/sched/ext_idle.c b/kernel/sched/ext_idle.c
new file mode 100644
index 000000000000..ca99fc58af91
--- /dev/null
+++ b/kernel/sched/ext_idle.c
@@ -0,0 +1,722 @@
+// 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>
+ */
+#ifdef CONFIG_SMP
+#ifdef CONFIG_CPUMASK_OFFSTACK
+#define CL_ALIGNED_IF_ONSTACK
+#else
+#define CL_ALIGNED_IF_ONSTACK __cacheline_aligned_in_smp
+#endif
+
+static struct {
+ cpumask_var_t cpu;
+ cpumask_var_t smt;
+} idle_masks CL_ALIGNED_IF_ONSTACK;
+
+static bool test_and_clear_cpu_idle(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);
+}
+
+static 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 (test_and_clear_cpu_idle(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.
+ */
+static void 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().
+ */
+static 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) &&
+ test_and_clear_cpu_idle(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) &&
+ test_and_clear_cpu_idle(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 (test_and_clear_cpu_idle(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;
+}
+
+static void reset_idle_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);
+}
+
+static void init_idle_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 test_and_clear_cpu_idle(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_select_cpu)
+BTF_ID_FLAGS(func, scx_bpf_select_cpu_dfl, KF_RCU)
+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_select_cpu)
+
+const struct btf_kfunc_id_set scx_kfunc_set_select_cpu = {
+ .owner = THIS_MODULE,
+ .set = &scx_kfunc_ids_select_cpu,
+};
diff --git a/kernel/sched/ext_idle.h b/kernel/sched/ext_idle.h
new file mode 100644
index 000000000000..c1385af1ceeb
--- /dev/null
+++ b/kernel/sched/ext_idle.h
@@ -0,0 +1,45 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * BPF extensible scheduler class: Documentation/scheduler/sched-ext.rst
+ *
+ * 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>
+ */
+#ifndef _KERNEL_SCHED_EXT_IDLE_H
+
+static DEFINE_STATIC_KEY_FALSE(scx_builtin_idle_enabled);
+
+#ifdef CONFIG_SMP
+static DEFINE_STATIC_KEY_FALSE(scx_selcpu_topo_llc);
+static DEFINE_STATIC_KEY_FALSE(scx_selcpu_topo_numa);
+
+static void update_selcpu_topology(void);
+static void reset_idle_masks(void);
+static void init_idle_masks(void);
+static bool test_and_clear_cpu_idle(int cpu);
+
+static s32 scx_pick_idle_cpu(const struct cpumask *cpus_allowed, u64 flags);
+static s32 scx_select_cpu_dfl(struct task_struct *p,
+ s32 prev_cpu, u64 wake_flags, bool *found);
+#else /* !CONFIG_SMP */
+static inline void update_selcpu_topology(void) {}
+static inline void reset_idle_masks(void) {}
+static inline void init_idle_masks(void) {}
+static inline bool test_and_clear_cpu_idle(int cpu) { return false; }
+
+static inline s32 scx_pick_idle_cpu(const struct cpumask *cpus_allowed, u64 flags)
+{
+ return -EBUSY;
+}
+static inline s32 scx_select_cpu_dfl(struct task_struct *p,
+ s32 prev_cpu, u64 wake_flags, bool *found)
+{
+ return -EBUSY;
+}
+#endif /* CONFIG_SMP */
+
+extern const struct btf_kfunc_id_set scx_kfunc_set_select_cpu;
+
+#endif /* _KERNEL_SCHED_EXT_IDLE_H */
--
2.48.1