[PATCH 1/4] sched_ext: idle: Honor idle flags in the built-in idle selection policy

Andrea Righi
2025-03-06T18:26:10Z

Enable passing idle flags (%SCX_PICK_IDLE_*) to scx_select_cpu_dfl(),
to enforce strict selection criteria, such as selecting an idle CPU
strictly within @prev_cpu's node or choosing only a fully idle SMT core.

This functionality will be exposed through a dedicated kfunc in a
separate patch.

Signed-off-by: Andrea Righi <arighi@nvidia.com>
---
kernel/sched/ext.c | 2 +-
kernel/sched/ext_idle.c | 41 ++++++++++++++++++++++++++++++-----------
kernel/sched/ext_idle.h | 2 +-
3 files changed, 32 insertions(+), 13 deletions(-)

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index debcd1cf2de9b..5cd878bbd0e39 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -3396,7 +3396,7 @@ static int select_task_rq_scx(struct task_struct *p, int prev_cpu, int wake_flag
bool found;
s32 cpu;

- cpu = scx_select_cpu_dfl(p, prev_cpu, wake_flags, &found);
+ cpu = scx_select_cpu_dfl(p, prev_cpu, wake_flags, 0, &found);
p->scx.selected_cpu = cpu;
if (found) {
p->scx.slice = SCX_SLICE_DFL;
diff --git a/kernel/sched/ext_idle.c b/kernel/sched/ext_idle.c
index 15e9d1c8b2815..16981456ec1ed 100644
--- a/kernel/sched/ext_idle.c
+++ b/kernel/sched/ext_idle.c
@@ -418,7 +418,7 @@ void scx_idle_update_selcpu_topology(struct sched_ext_ops *ops)
* 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)
+s32 scx_select_cpu_dfl(struct task_struct *p, s32 prev_cpu, u64 wake_flags, u64 flags, bool *found)
{
const struct cpumask *llc_cpus = NULL;
const struct cpumask *numa_cpus = NULL;
@@ -455,12 +455,13 @@ s32 scx_select_cpu_dfl(struct task_struct *p, s32 prev_cpu, u64 wake_flags, bool
* If WAKE_SYNC, try to migrate the wakee to the waker's CPU.
*/
if (wake_flags & SCX_WAKE_SYNC) {
- cpu = smp_processor_id();
+ int waker_node;

/*
* If the waker's CPU is cache affine and prev_cpu is idle,
* then avoid a migration.
*/
+ cpu = smp_processor_id();
if (cpus_share_cache(cpu, prev_cpu) &&
scx_idle_test_and_clear_cpu(prev_cpu)) {
cpu = prev_cpu;
@@ -480,9 +481,11 @@ s32 scx_select_cpu_dfl(struct task_struct *p, s32 prev_cpu, u64 wake_flags, bool
* piled up on it even if there is an idle core elsewhere on
* the system.
*/
+ waker_node = cpu_to_node(cpu);
if (!(current->flags & PF_EXITING) &&
cpu_rq(cpu)->scx.local_dsq.nr == 0 &&
- !cpumask_empty(idle_cpumask(cpu_to_node(cpu))->cpu)) {
+ (!(flags & SCX_PICK_IDLE_IN_NODE) || (waker_node == node)) &&
+ !cpumask_empty(idle_cpumask(waker_node)->cpu)) {
if (cpumask_test_cpu(cpu, p->cpus_ptr))
goto cpu_found;
}
@@ -521,15 +524,25 @@ s32 scx_select_cpu_dfl(struct task_struct *p, s32 prev_cpu, u64 wake_flags, bool
}

/*
- * Search for any full idle core usable by the task.
+ * Search for any full-idle core usable by the task.
*
- * If NUMA aware idle selection is enabled, the search will
+ * If the node-aware idle CPU selection policy is enabled
+ * (%SCX_OPS_BUILTIN_IDLE_PER_NODE), the search will always
* begin in prev_cpu's node and proceed to other nodes in
* order of increasing distance.
*/
- cpu = scx_pick_idle_cpu(p->cpus_ptr, node, SCX_PICK_IDLE_CORE);
+ cpu = scx_pick_idle_cpu(p->cpus_ptr, node, flags | SCX_PICK_IDLE_CORE);
if (cpu >= 0)
goto cpu_found;
+
+ /*
+ * Give up if we're strictly looking for a full-idle SMT
+ * core.
+ */
+ if (flags & SCX_PICK_IDLE_CORE) {
+ cpu = prev_cpu;
+ goto out_unlock;
+ }
}

/*
@@ -560,18 +573,24 @@ s32 scx_select_cpu_dfl(struct task_struct *p, s32 prev_cpu, u64 wake_flags, bool

/*
* Search for any idle CPU usable by the task.
+ *
+ * If the node-aware idle CPU selection policy is enabled
+ * (%SCX_OPS_BUILTIN_IDLE_PER_NODE), the search will always begin
+ * in prev_cpu's node and proceed to other nodes in order of
+ * increasing distance.
*/
- cpu = scx_pick_idle_cpu(p->cpus_ptr, node, 0);
+ cpu = scx_pick_idle_cpu(p->cpus_ptr, node, flags);
if (cpu >= 0)
goto cpu_found;

- rcu_read_unlock();
- return prev_cpu;
+ cpu = prev_cpu;
+ goto out_unlock;

cpu_found:
+ *found = true;
+out_unlock:
rcu_read_unlock();

- *found = true;
return cpu;
}

@@ -810,7 +829,7 @@ __bpf_kfunc s32 scx_bpf_select_cpu_dfl(struct task_struct *p, s32 prev_cpu,
goto prev_cpu;

#ifdef CONFIG_SMP
- return scx_select_cpu_dfl(p, prev_cpu, wake_flags, is_idle);
+ return scx_select_cpu_dfl(p, prev_cpu, wake_flags, 0, is_idle);
#endif

prev_cpu:
diff --git a/kernel/sched/ext_idle.h b/kernel/sched/ext_idle.h
index 68c4307ce4f6f..5c1db6b315f7a 100644
--- a/kernel/sched/ext_idle.h
+++ b/kernel/sched/ext_idle.h
@@ -27,7 +27,7 @@ static inline s32 scx_pick_idle_cpu(const struct cpumask *cpus_allowed, int node
}
#endif /* CONFIG_SMP */

-s32 scx_select_cpu_dfl(struct task_struct *p, s32 prev_cpu, u64 wake_flags, bool *found);
+s32 scx_select_cpu_dfl(struct task_struct *p, s32 prev_cpu, u64 wake_flags, u64 flags, bool *found);
void scx_idle_enable(struct sched_ext_ops *ops);
void scx_idle_disable(void);
int scx_idle_init(void);
--
2.48.1