[PATCH 3/4] sched_ext: idle: Introduce scx_bpf_select_cpu_pref()

Andrea Righi
2025-03-06T18:26:25Z

Provide a new kfunc that can be used to apply the built-in idle CPU
selection policy to a subset of preferred CPU:

s32 scx_bpf_select_cpu_pref(struct task_struct *p,
const struct cpumask *preferred_cpus,
s32 prev_cpu, u64 wake_flags, u64 flags);

This new helper is basically an extension of scx_bpf_select_cpu_dfl().
However, when an idle CPU can't be found, it returns a negative value
instead of @prev_cpu, aligning its behavior more closely with
scx_bpf_pick_idle_cpu().

It also accepts %SCX_PICK_IDLE_* flags, which can be used to enforce
strict selection to the preferred CPUs (%SCX_PICK_IDLE_IN_PREF) or to
@prev_cpu's node (%SCX_PICK_IDLE_IN_NODE), or to request only a
full-idle SMT core (%SCX_PICK_IDLE_CORE), while applying the built-in
selection logic.

With this helper, BPF schedulers can apply the built-in idle CPU
selection policy to a generic CPU domain, with strict or soft selection
requirements.

In the future we can also consider to deprecate scx_bpf_select_cpu_dfl()
and replace it with scx_bpf_select_cpu_pref(), as the latter provides
the same functionality, with the addition of the preferred domain logic.

Example usage
=============

Possible usage in ops.select_cpu():

s32 BPF_STRUCT_OPS(foo_select_cpu, struct task_struct *p,
s32 prev_cpu, u64 wake_flags)
{
const struct cpumask *dom = task_domain(p) ?: p->cpus_ptr;
s32 cpu;

/*
* Pick an idle CPU in the task's domain. If no CPU is found,
* extend the search outside the domain.
*/
cpu = scx_bpf_select_cpu_pref(p, dom, prev_cpu, wake_flags, 0);
if (cpu >= 0) {
scx_bpf_dsq_insert(p, SCX_DSQ_LOCAL, SCX_SLICE_DFL, 0);
return cpu;
}

return prev_cpu;
}

Results
=======

Load distribution on a 4 sockets / 4 cores per socket system, simulated
using virtme-ng, running a modified version of scx_bpfland that uses the
new helper scx_bpf_select_cpu_pref() and 0xff00 as preferred domain:

$ vng --cpu 16,sockets=4,cores=4,threads=1

Starting 12 CPU hogs to fill the preferred domain:

$ stress-ng -c 12
...
0[|||||||||||||||||||||||100.0%] 8[||||||||||||||||||||||||100.0%]
1[| 1.3%] 9[||||||||||||||||||||||||100.0%]
2[|||||||||||||||||||||||100.0%] 10[||||||||||||||||||||||||100.0%]
3[|||||||||||||||||||||||100.0%] 11[||||||||||||||||||||||||100.0%]
4[|||||||||||||||||||||||100.0%] 12[||||||||||||||||||||||||100.0%]
5[|| 2.6%] 13[||||||||||||||||||||||||100.0%]
6[| 0.6%] 14[||||||||||||||||||||||||100.0%]
7| 0.0%] 15[||||||||||||||||||||||||100.0%]

Passing %SCX_PICK_IDLE_IN_PREF to scx_bpf_select_cpu_pref() to enforce
strict selection on the preferred CPUs (with the same workload):

0[ 0.0%] 8[||||||||||||||||||||||||100.0%]
1[ 0.0%] 9[||||||||||||||||||||||||100.0%]
2[ 0.0%] 10[||||||||||||||||||||||||100.0%]
3[ 0.0%] 11[||||||||||||||||||||||||100.0%]
4[ 0.0%] 12[||||||||||||||||||||||||100.0%]
5[ 0.0%] 13[||||||||||||||||||||||||100.0%]
6[ 0.0%] 14[||||||||||||||||||||||||100.0%]
7[ 0.0%] 15[||||||||||||||||||||||||100.0%]

Signed-off-by: Andrea Righi <arighi@nvidia.com>
---
kernel/sched/ext.c | 1 +
kernel/sched/ext_idle.c | 60 ++++++++++++++++++++++++
tools/sched_ext/include/scx/common.bpf.h | 2 +
3 files changed, 63 insertions(+)

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index a28ddd7655ba8..8ee4818de908b 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -465,6 +465,7 @@ struct sched_ext_ops {
* idle CPU tracking and the following helpers become unavailable:
*
* - scx_bpf_select_cpu_dfl()
+ * - scx_bpf_select_cpu_pref()
* - scx_bpf_test_and_clear_cpu_idle()
* - scx_bpf_pick_idle_cpu()
*
diff --git a/kernel/sched/ext_idle.c b/kernel/sched/ext_idle.c
index 9b002e109404b..24cba7ddceec4 100644
--- a/kernel/sched/ext_idle.c
+++ b/kernel/sched/ext_idle.c
@@ -907,6 +907,65 @@ __bpf_kfunc s32 scx_bpf_select_cpu_dfl(struct task_struct *p, s32 prev_cpu,
return prev_cpu;
}

+/**
+ * scx_bpf_select_cpu_pref - Pick an idle CPU usable by task @p,
+ * prioritizing those in @preferred_cpus
+ * @p: task_struct to select a CPU for
+ * @preferred_cpus: cpumask of preferred CPUs
+ * @prev_cpu: CPU @p was on previously
+ * @wake_flags: %SCX_WAKE_* flags
+ * @flags: %SCX_PICK_IDLE* flags
+ *
+ * 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 selected idle CPU, which will be automatically awakened upon
+ * returning from ops.select_cpu() and can be used for direct dispatch, or
+ * a negative value if no idle CPU is available.
+ */
+__bpf_kfunc s32 scx_bpf_select_cpu_pref(struct task_struct *p,
+ const struct cpumask *preferred_cpus,
+ s32 prev_cpu, u64 wake_flags, u64 flags)
+{
+#ifdef CONFIG_SMP
+ struct cpumask *preferred = NULL;
+ bool is_idle = false;
+#endif
+
+ if (!ops_cpu_valid(prev_cpu, NULL))
+ return -EINVAL;
+
+ if (!check_builtin_idle_enabled())
+ return -EBUSY;
+
+ if (!scx_kf_allowed(SCX_KF_SELECT_CPU))
+ return -EPERM;
+
+#ifdef CONFIG_SMP
+ preempt_disable();
+
+ /*
+ * As an optimization, do not update the local idle mask when
+ * p->cpus_ptr is passed directly in @preferred_cpus.
+ */
+ if (preferred_cpus != p->cpus_ptr) {
+ preferred = this_cpu_cpumask_var_ptr(local_idle_cpumask);
+ if (!cpumask_and(preferred, p->cpus_ptr, preferred_cpus))
+ preferred = NULL;
+ }
+ prev_cpu = scx_select_cpu_dfl(p, preferred, prev_cpu, wake_flags, flags, &is_idle);
+ if (!is_idle)
+ prev_cpu = -EBUSY;
+
+ preempt_enable();
+#else
+ prev_cpu = -EBUSY;
+#endif
+
+ return prev_cpu;
+}
+
/**
* scx_bpf_get_idle_cpumask_node - Get a referenced kptr to the
* idle-tracking per-CPU cpumask of a target NUMA node.
@@ -1215,6 +1274,7 @@ static const struct btf_kfunc_id_set scx_kfunc_set_idle = {

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_select_cpu_pref, KF_RCU)
BTF_KFUNCS_END(scx_kfunc_ids_select_cpu)

static const struct btf_kfunc_id_set scx_kfunc_set_select_cpu = {
diff --git a/tools/sched_ext/include/scx/common.bpf.h b/tools/sched_ext/include/scx/common.bpf.h
index dc4333d23189f..a33e709ec12ab 100644
--- a/tools/sched_ext/include/scx/common.bpf.h
+++ b/tools/sched_ext/include/scx/common.bpf.h
@@ -47,6 +47,8 @@ static inline void ___vmlinux_h_sanity_check___(void)
}

s32 scx_bpf_create_dsq(u64 dsq_id, s32 node) __ksym;
+s32 scx_bpf_select_cpu_pref(struct task_struct *p, const struct cpumask *preferred_cpus,
+ s32 prev_cpu, u64 wake_flags, u64 flags) __ksym __weak;
s32 scx_bpf_select_cpu_dfl(struct task_struct *p, s32 prev_cpu, u64 wake_flags, bool *is_idle) __ksym;
void scx_bpf_dsq_insert(struct task_struct *p, u64 dsq_id, u64 slice, u64 enq_flags) __ksym __weak;
void scx_bpf_dsq_insert_vtime(struct task_struct *p, u64 dsq_id, u64 slice, u64 vtime, u64 enq_flags) __ksym __weak;
--
2.48.1