[PATCH 11/17] sched_ext: Add cid-form kfunc wrappers alongside cpu-form

Tejun Heo
2026-04-24T17:27:34Z

cpumask is awkward from BPF and unusable from arena; cid/cmask work in
both. Sub-sched enqueue will need cmask. Without full cid coverage a
scheduler has to mix cid and cpu forms, which is a subtle-bug factory.
Close the gap with a cid-native interface.

Pair every cpu-form kfunc that takes a cpu id with a cid-form
equivalent (kick, task placement, cpuperf query/set, per-cpu current
task, nr-cpu-ids). Add two cid-natives with no cpu-form sibling:
scx_bpf_this_cid() (cid of the running cpu, scx equivalent of
bpf_get_smp_processor_id) and scx_bpf_nr_online_cids().

scx_bpf_cpu_rq is deprecated; no cid-form counterpart. NUMA node info
is reachable via scx_bpf_cid_topo() on the BPF side.

Each cid-form wrapper is a thin cid -> cpu translation that delegates
to the cpu path, registered in the same context sets so usage
constraints match.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Cheng-Yang Chou <yphbchou0911@gmail.com>
---
kernel/sched/ext.c | 184 +++++++++++++++++++++++
tools/sched_ext/include/scx/common.bpf.h | 9 ++
2 files changed, 193 insertions(+)

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 271399b9faa4..914e72da1c33 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -8751,6 +8751,28 @@ __bpf_kfunc void scx_bpf_kick_cpu(s32 cpu, u64 flags, const struct bpf_prog_aux
scx_kick_cpu(sch, cpu, flags);
}

+/**
+ * scx_bpf_kick_cid - Trigger reschedule on the CPU mapped to @cid
+ * @cid: cid to kick
+ * @flags: %SCX_KICK_* flags
+ * @aux: implicit BPF argument to access bpf_prog_aux hidden from BPF progs
+ *
+ * cid-addressed equivalent of scx_bpf_kick_cpu().
+ */
+__bpf_kfunc void scx_bpf_kick_cid(s32 cid, u64 flags, const struct bpf_prog_aux *aux)
+{
+ struct scx_sched *sch;
+ s32 cpu;
+
+ guard(rcu)();
+ sch = scx_prog_sched(aux);
+ if (unlikely(!sch))
+ return;
+ cpu = scx_cid_to_cpu(sch, cid);
+ if (cpu >= 0)
+ scx_kick_cpu(sch, cpu, flags);
+}
+
/**
* scx_bpf_dsq_nr_queued - Return the number of queued tasks
* @dsq_id: id of the DSQ
@@ -9173,6 +9195,29 @@ __bpf_kfunc u32 scx_bpf_cpuperf_cap(s32 cpu, const struct bpf_prog_aux *aux)
return SCX_CPUPERF_ONE;
}

+/**
+ * scx_bpf_cidperf_cap - Query the maximum relative capacity of the CPU at @cid
+ * @cid: cid of the CPU to query
+ * @aux: implicit BPF argument to access bpf_prog_aux hidden from BPF progs
+ *
+ * cid-addressed equivalent of scx_bpf_cpuperf_cap().
+ */
+__bpf_kfunc u32 scx_bpf_cidperf_cap(s32 cid, const struct bpf_prog_aux *aux)
+{
+ struct scx_sched *sch;
+ s32 cpu;
+
+ guard(rcu)();
+
+ sch = scx_prog_sched(aux);
+ if (unlikely(!sch))
+ return SCX_CPUPERF_ONE;
+ cpu = scx_cid_to_cpu(sch, cid);
+ if (cpu < 0)
+ return SCX_CPUPERF_ONE;
+ return arch_scale_cpu_capacity(cpu);
+}
+
/**
* scx_bpf_cpuperf_cur - Query the current relative performance of a CPU
* @cpu: CPU of interest
@@ -9201,6 +9246,29 @@ __bpf_kfunc u32 scx_bpf_cpuperf_cur(s32 cpu, const struct bpf_prog_aux *aux)
return SCX_CPUPERF_ONE;
}

+/**
+ * scx_bpf_cidperf_cur - Query the current performance of the CPU at @cid
+ * @cid: cid of the CPU to query
+ * @aux: implicit BPF argument to access bpf_prog_aux hidden from BPF progs
+ *
+ * cid-addressed equivalent of scx_bpf_cpuperf_cur().
+ */
+__bpf_kfunc u32 scx_bpf_cidperf_cur(s32 cid, const struct bpf_prog_aux *aux)
+{
+ struct scx_sched *sch;
+ s32 cpu;
+
+ guard(rcu)();
+
+ sch = scx_prog_sched(aux);
+ if (unlikely(!sch))
+ return SCX_CPUPERF_ONE;
+ cpu = scx_cid_to_cpu(sch, cid);
+ if (cpu < 0)
+ return SCX_CPUPERF_ONE;
+ return arch_scale_freq_capacity(cpu);
+}
+
/**
* scx_bpf_cpuperf_set - Set the relative performance target of a CPU
* @cpu: CPU of interest
@@ -9261,6 +9329,31 @@ __bpf_kfunc void scx_bpf_cpuperf_set(s32 cpu, u32 perf, const struct bpf_prog_au
}
}

+/**
+ * scx_bpf_cidperf_set - Set the performance target of the CPU at @cid
+ * @cid: cid of the CPU to target
+ * @perf: target performance level [0, %SCX_CPUPERF_ONE]
+ * @aux: implicit BPF argument to access bpf_prog_aux hidden from BPF progs
+ *
+ * cid-addressed equivalent of scx_bpf_cpuperf_set().
+ */
+__bpf_kfunc void scx_bpf_cidperf_set(s32 cid, u32 perf,
+ const struct bpf_prog_aux *aux)
+{
+ struct scx_sched *sch;
+ s32 cpu;
+
+ guard(rcu)();
+
+ sch = scx_prog_sched(aux);
+ if (unlikely(!sch))
+ return;
+ cpu = scx_cid_to_cpu(sch, cid);
+ if (cpu < 0)
+ return;
+ scx_bpf_cpuperf_set(cpu, perf, aux);
+}
+
/**
* scx_bpf_nr_node_ids - Return the number of possible node IDs
*
@@ -9281,6 +9374,47 @@ __bpf_kfunc u32 scx_bpf_nr_cpu_ids(void)
return nr_cpu_ids;
}

+/**
+ * scx_bpf_nr_cids - Return the size of the cid space
+ *
+ * Equals num_possible_cpus(). All valid cids are in [0, return value).
+ */
+__bpf_kfunc u32 scx_bpf_nr_cids(void)
+{
+ return num_possible_cpus();
+}
+
+/**
+ * scx_bpf_nr_online_cids - Return current count of online CPUs in cid space
+ *
+ * Return num_online_cpus(). The standard model restarts the scheduler on
+ * hotplug, which lets schedulers treat [0, nr_online_cids) as the online
+ * range. Schedulers that prefer to handle hotplug without a restart should
+ * install a custom mapping via scx_bpf_cid_override() and track onlining
+ * through the ops.cid_online / ops.cid_offline callbacks.
+ */
+__bpf_kfunc u32 scx_bpf_nr_online_cids(void)
+{
+ return num_online_cpus();
+}
+
+/**
+ * scx_bpf_this_cid - Return the cid of the CPU this program is running on
+ *
+ * cid-addressed equivalent of bpf_get_smp_processor_id() for scx programs.
+ * The current cpu is trivially valid, so this is just a table lookup. Return
+ * -EINVAL if called from a non-SCX program before any scheduler has ever
+ * been enabled (the cid table is still unallocated at that point).
+ */
+__bpf_kfunc s32 scx_bpf_this_cid(void)
+{
+ s16 *tbl = READ_ONCE(scx_cpu_to_cid_tbl);
+
+ if (!tbl)
+ return -EINVAL;
+ return tbl[raw_smp_processor_id()];
+}
+
/**
* scx_bpf_get_possible_cpumask - Get a referenced kptr to cpu_possible_mask
*/
@@ -9329,6 +9463,23 @@ __bpf_kfunc s32 scx_bpf_task_cpu(const struct task_struct *p)
return task_cpu(p);
}

+/**
+ * scx_bpf_task_cid - cid a task is currently associated with
+ * @p: task of interest
+ *
+ * cid-addressed equivalent of scx_bpf_task_cpu(). task_cpu(p) is always a
+ * valid cpu, so this is just a table lookup. Return -EINVAL if called from
+ * a non-SCX program before any scheduler has ever been enabled.
+ */
+__bpf_kfunc s32 scx_bpf_task_cid(const struct task_struct *p)
+{
+ s16 *tbl = READ_ONCE(scx_cpu_to_cid_tbl);
+
+ if (!tbl)
+ return -EINVAL;
+ return tbl[task_cpu(p)];
+}
+
/**
* scx_bpf_cpu_rq - Fetch the rq of a CPU
* @cpu: CPU of the rq
@@ -9407,6 +9558,30 @@ __bpf_kfunc struct task_struct *scx_bpf_cpu_curr(s32 cpu, const struct bpf_prog_
return rcu_dereference(cpu_rq(cpu)->curr);
}

+/**
+ * scx_bpf_cid_curr - Return the curr task on the CPU at @cid
+ * @cid: cid of interest
+ * @aux: implicit BPF argument to access bpf_prog_aux hidden from BPF progs
+ *
+ * cid-addressed equivalent of scx_bpf_cpu_curr(). Callers must hold RCU
+ * read lock (KF_RCU).
+ */
+__bpf_kfunc struct task_struct *scx_bpf_cid_curr(s32 cid, const struct bpf_prog_aux *aux)
+{
+ struct scx_sched *sch;
+ s32 cpu;
+
+ guard(rcu)();
+
+ sch = scx_prog_sched(aux);
+ if (unlikely(!sch))
+ return NULL;
+ cpu = scx_cid_to_cpu(sch, cid);
+ if (cpu < 0)
+ return NULL;
+ return rcu_dereference(cpu_rq(cpu)->curr);
+}
+
/**
* scx_bpf_tid_to_task - Look up a task by its scx tid
* @tid: task ID previously read from p->scx.tid
@@ -9594,6 +9769,7 @@ BTF_KFUNCS_START(scx_kfunc_ids_any)
BTF_ID_FLAGS(func, scx_bpf_task_set_slice, KF_IMPLICIT_ARGS | KF_RCU);
BTF_ID_FLAGS(func, scx_bpf_task_set_dsq_vtime, KF_IMPLICIT_ARGS | KF_RCU);
BTF_ID_FLAGS(func, scx_bpf_kick_cpu, KF_IMPLICIT_ARGS)
+BTF_ID_FLAGS(func, scx_bpf_kick_cid, KF_IMPLICIT_ARGS)
BTF_ID_FLAGS(func, scx_bpf_dsq_nr_queued)
BTF_ID_FLAGS(func, scx_bpf_destroy_dsq)
BTF_ID_FLAGS(func, scx_bpf_dsq_peek, KF_IMPLICIT_ARGS | KF_RCU_PROTECTED | KF_RET_NULL)
@@ -9608,16 +9784,24 @@ BTF_ID_FLAGS(func, scx_bpf_dump_bstr, KF_IMPLICIT_ARGS)
BTF_ID_FLAGS(func, scx_bpf_cpuperf_cap, KF_IMPLICIT_ARGS)
BTF_ID_FLAGS(func, scx_bpf_cpuperf_cur, KF_IMPLICIT_ARGS)
BTF_ID_FLAGS(func, scx_bpf_cpuperf_set, KF_IMPLICIT_ARGS)
+BTF_ID_FLAGS(func, scx_bpf_cidperf_cap, KF_IMPLICIT_ARGS)
+BTF_ID_FLAGS(func, scx_bpf_cidperf_cur, KF_IMPLICIT_ARGS)
+BTF_ID_FLAGS(func, scx_bpf_cidperf_set, KF_IMPLICIT_ARGS)
BTF_ID_FLAGS(func, scx_bpf_nr_node_ids)
BTF_ID_FLAGS(func, scx_bpf_nr_cpu_ids)
+BTF_ID_FLAGS(func, scx_bpf_nr_cids)
+BTF_ID_FLAGS(func, scx_bpf_nr_online_cids)
+BTF_ID_FLAGS(func, scx_bpf_this_cid)
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_task_running, KF_RCU)
BTF_ID_FLAGS(func, scx_bpf_task_cpu, KF_RCU)
+BTF_ID_FLAGS(func, scx_bpf_task_cid, KF_RCU)
BTF_ID_FLAGS(func, scx_bpf_cpu_rq, KF_IMPLICIT_ARGS)
BTF_ID_FLAGS(func, scx_bpf_locked_rq, KF_IMPLICIT_ARGS | KF_RET_NULL)
BTF_ID_FLAGS(func, scx_bpf_cpu_curr, KF_IMPLICIT_ARGS | KF_RET_NULL | KF_RCU_PROTECTED)
+BTF_ID_FLAGS(func, scx_bpf_cid_curr, KF_IMPLICIT_ARGS | KF_RET_NULL | KF_RCU_PROTECTED)
BTF_ID_FLAGS(func, scx_bpf_tid_to_task, KF_RET_NULL | KF_RCU_PROTECTED)
BTF_ID_FLAGS(func, scx_bpf_now)
BTF_ID_FLAGS(func, scx_bpf_events)
diff --git a/tools/sched_ext/include/scx/common.bpf.h b/tools/sched_ext/include/scx/common.bpf.h
index 3e353dfafb46..4acfc9e8a645 100644
--- a/tools/sched_ext/include/scx/common.bpf.h
+++ b/tools/sched_ext/include/scx/common.bpf.h
@@ -105,6 +105,15 @@ void scx_bpf_events(struct scx_event_stats *events, size_t events__sz) __ksym __
s32 scx_bpf_cpu_to_cid(s32 cpu) __ksym __weak;
s32 scx_bpf_cid_to_cpu(s32 cid) __ksym __weak;
void scx_bpf_cid_topo(s32 cid, struct scx_cid_topo *out) __ksym __weak;
+void scx_bpf_kick_cid(s32 cid, u64 flags) __ksym __weak;
+s32 scx_bpf_task_cid(const struct task_struct *p) __ksym __weak;
+s32 scx_bpf_this_cid(void) __ksym __weak;
+struct task_struct *scx_bpf_cid_curr(s32 cid) __ksym __weak;
+u32 scx_bpf_nr_cids(void) __ksym __weak;
+u32 scx_bpf_nr_online_cids(void) __ksym __weak;
+u32 scx_bpf_cidperf_cap(s32 cid) __ksym __weak;
+u32 scx_bpf_cidperf_cur(s32 cid) __ksym __weak;
+void scx_bpf_cidperf_set(s32 cid, u32 perf) __ksym __weak;

/*
* Use the following as @it__iter when calling scx_bpf_dsq_move[_vtime]() from
--
2.53.0