[PATCH 19/30] sched_ext: Implement scx_bpf_kick_cpu() and task preemption support

[Tejun Heo <tj@kernel.org>]

It's often useful to wake up and/or trigger reschedule on other CPUs. This
patch adds scx_bpf_kick_cpu() kfunc helper that BPF scheduler can call to
kick the target CPU into the scheduling path.

As a sched_ext task relinquishes its CPU only after its slice is depleted,
this patch also adds SCX_KICK_PREEMPT and SCX_ENQ_PREEMPT which clears the
slice of the target CPU's current task to guarantee that sched_ext's
scheduling path runs on the CPU.

This patch also adds a new example scheduler, scx_example_central, which
demonstrates central scheduling where one CPU is responsible for making all
scheduling decisions in the system. The central CPU makes scheduling
decisions for all CPUs in the system, queues tasks on the appropriate local
dsq's and preempts the worker CPUs. The worker CPUs in turn preempt the
central CPU when it needs tasks to run.

Currently, every CPU depends on its own tick to expire the current task. A
follow-up patch implementing tickless support for sched_ext will allow the
worker CPUs to go full tickless so that they can run completely undisturbed.

v2: * Julia Lawall reported that scx_example_central can overflow the
      dispatch buffer and malfunction. As scheduling for other CPUs can't be
      handled by the automatic retry mechanism, fix by implementing an
      explicit overflow and retry handling.

    * Updated to use generic BPF cpumask helpers.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: David Vernet <dvernet@meta.com>
Acked-by: Josh Don <joshdon@google.com>
Acked-by: Hao Luo <haoluo@google.com>
Acked-by: Barret Rhoden <brho@google.com>
Cc: Julia Lawall <julia.lawall@inria.fr>
---
 include/linux/sched/ext.h                 |   4 +
 kernel/sched/ext.c                        |  82 +++++++-
 kernel/sched/ext.h                        |  12 ++
 kernel/sched/sched.h                      |   3 +
 tools/sched_ext/.gitignore                |   1 +
 tools/sched_ext/Makefile                  |   8 +-
 tools/sched_ext/scx_common.bpf.h          |   1 +
 tools/sched_ext/scx_example_central.bpf.c | 231 ++++++++++++++++++++++
 tools/sched_ext/scx_example_central.c     |  93 +++++++++
 9 files changed, 430 insertions(+), 5 deletions(-)
 create mode 100644 tools/sched_ext/scx_example_central.bpf.c
 create mode 100644 tools/sched_ext/scx_example_central.c

diff --git a/include/linux/sched/ext.h b/include/linux/sched/ext.h
index b4c4b83a07f6..10cd3ede5ae5 100644
--- a/include/linux/sched/ext.h
+++ b/include/linux/sched/ext.h
@@ -396,6 +396,10 @@ struct sched_ext_entity {
 	 * scx_bpf_dispatch() but can also be modified directly by the BPF
 	 * scheduler. Automatically decreased by SCX as the task executes. On
 	 * depletion, a scheduling event is triggered.
+	 *
+	 * This value is cleared to zero if the task is preempted by
+	 * %SCX_KICK_PREEMPT and shouldn't be used to determine how long the
+	 * task ran. Use p->se.sum_exec_runtime instead.
 	 */
 	u64			slice;
 
diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 63f0a3cf2d53..098edfc56a9b 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -466,7 +466,7 @@ static void dispatch_enqueue(struct scx_dispatch_q *dsq, struct task_struct *p,
 		}
 	}
 
-	if (enq_flags & SCX_ENQ_HEAD)
+	if (enq_flags & (SCX_ENQ_HEAD | SCX_ENQ_PREEMPT))
 		list_add(&p->scx.dsq_node, &dsq->fifo);
 	else
 		list_add_tail(&p->scx.dsq_node, &dsq->fifo);
@@ -482,8 +482,16 @@ static void dispatch_enqueue(struct scx_dispatch_q *dsq, struct task_struct *p,
 
 	if (is_local) {
 		struct rq *rq = container_of(dsq, struct rq, scx.local_dsq);
+		bool preempt = false;
 
-		if (sched_class_above(&ext_sched_class, rq->curr->sched_class))
+		if ((enq_flags & SCX_ENQ_PREEMPT) && p != rq->curr &&
+		    rq->curr->sched_class == &ext_sched_class) {
+			rq->curr->scx.slice = 0;
+			preempt = true;
+		}
+
+		if (preempt || sched_class_above(&ext_sched_class,
+						 rq->curr->sched_class))
 			resched_curr(rq);
 	} else {
 		raw_spin_unlock(&dsq->lock);
@@ -1839,7 +1847,9 @@ int scx_check_setscheduler(struct task_struct *p, int policy)
  * Omitted operations:
  *
  * - check_preempt_curr: NOOP as it isn't useful in the wakeup path because the
- *   task isn't tied to the CPU at that point.
+ *   task isn't tied to the CPU at that point. Preemption is implemented by
+ *   resetting the victim task's slice to 0 and triggering reschedule on the
+ *   target CPU.
  *
  * - migrate_task_rq: Unncessary as task to cpu mapping is transient.
  *
@@ -2672,6 +2682,32 @@ static const struct sysrq_key_op sysrq_sched_ext_reset_op = {
 	.enable_mask	= SYSRQ_ENABLE_RTNICE,
 };
 
+static void kick_cpus_irq_workfn(struct irq_work *irq_work)
+{
+	struct rq *this_rq = this_rq();
+	int this_cpu = cpu_of(this_rq);
+	int cpu;
+
+	for_each_cpu(cpu, this_rq->scx.cpus_to_kick) {
+		struct rq *rq = cpu_rq(cpu);
+		unsigned long flags;
+
+		raw_spin_rq_lock_irqsave(rq, flags);
+
+		if (cpu_online(cpu) || cpu == this_cpu) {
+			if (cpumask_test_cpu(cpu, this_rq->scx.cpus_to_preempt) &&
+			    rq->curr->sched_class == &ext_sched_class)
+				rq->curr->scx.slice = 0;
+			resched_curr(rq);
+		}
+
+		raw_spin_rq_unlock_irqrestore(rq, flags);
+	}
+
+	cpumask_clear(this_rq->scx.cpus_to_kick);
+	cpumask_clear(this_rq->scx.cpus_to_preempt);
+}
+
 void __init init_sched_ext_class(void)
 {
 	int cpu;
@@ -2695,6 +2731,10 @@ void __init init_sched_ext_class(void)
 
 		init_dsq(&rq->scx.local_dsq, SCX_DSQ_LOCAL);
 		INIT_LIST_HEAD(&rq->scx.watchdog_list);
+
+		BUG_ON(!zalloc_cpumask_var(&rq->scx.cpus_to_kick, GFP_KERNEL));
+		BUG_ON(!zalloc_cpumask_var(&rq->scx.cpus_to_preempt, GFP_KERNEL));
+		init_irq_work(&rq->scx.kick_cpus_irq_work, kick_cpus_irq_workfn);
 	}
 
 	register_sysrq_key('S', &sysrq_sched_ext_reset_op);
@@ -2917,6 +2957,41 @@ static const struct btf_kfunc_id_set scx_kfunc_set_dispatch = {
 	.set			= &scx_kfunc_ids_dispatch,
 };
 
+/**
+ * scx_bpf_kick_cpu - Trigger reschedule on a CPU
+ * @cpu: cpu to kick
+ * @flags: SCX_KICK_* flags
+ *
+ * Kick @cpu into rescheduling. This can be used to wake up an idle CPU or
+ * trigger rescheduling on a busy CPU. This can be called from any online
+ * scx_ops operation and the actual kicking is performed asynchronously through
+ * an irq work.
+ */
+void scx_bpf_kick_cpu(s32 cpu, u64 flags)
+{
+	struct rq *rq;
+
+	if (!ops_cpu_valid(cpu)) {
+		scx_ops_error("invalid cpu %d", cpu);
+		return;
+	}
+
+	preempt_disable();
+	rq = this_rq();
+
+	/*
+	 * Actual kicking is bounced to kick_cpus_irq_workfn() to avoid nesting
+	 * rq locks. We can probably be smarter and avoid bouncing if called
+	 * from ops which don't hold a rq lock.
+	 */
+	cpumask_set_cpu(cpu, rq->scx.cpus_to_kick);
+	if (flags & SCX_KICK_PREEMPT)
+		cpumask_set_cpu(cpu, rq->scx.cpus_to_preempt);
+
+	irq_work_queue(&rq->scx.kick_cpus_irq_work);
+	preempt_enable();
+}
+
 /**
  * scx_bpf_dsq_nr_queued - Return the number of queued tasks
  * @dsq_id: id of the DSQ
@@ -3138,6 +3213,7 @@ s32 scx_bpf_task_cpu(const struct task_struct *p)
 }
 
 BTF_SET8_START(scx_kfunc_ids_any)
+BTF_ID_FLAGS(func, scx_bpf_kick_cpu)
 BTF_ID_FLAGS(func, scx_bpf_dsq_nr_queued)
 BTF_ID_FLAGS(func, scx_bpf_test_and_clear_cpu_idle)
 BTF_ID_FLAGS(func, scx_bpf_pick_idle_cpu, KF_TRUSTED_ARGS)
diff --git a/kernel/sched/ext.h b/kernel/sched/ext.h
index a4fe649e649d..0b04626e8ca2 100644
--- a/kernel/sched/ext.h
+++ b/kernel/sched/ext.h
@@ -19,6 +19,14 @@ enum scx_enq_flags {
 
 	/* high 32bits are SCX specific */
 
+	/*
+	 * Set the following to trigger preemption when calling
+	 * scx_bpf_dispatch() with a local dsq as the target. The slice of the
+	 * current task is cleared to zero and the CPU is kicked into the
+	 * scheduling path. Implies %SCX_ENQ_HEAD.
+	 */
+	SCX_ENQ_PREEMPT		= 1LLU << 32,
+
 	/*
 	 * The task being enqueued is the only task available for the cpu. By
 	 * default, ext core keeps executing such tasks but when
@@ -51,6 +59,10 @@ enum scx_deq_flags {
 	SCX_DEQ_SLEEP		= DEQUEUE_SLEEP,
 };
 
+enum scx_kick_flags {
+	SCX_KICK_PREEMPT	= 1LLU << 0,	/* force scheduling on the CPU */
+};
+
 #ifdef CONFIG_SCHED_CLASS_EXT
 
 extern const struct sched_class ext_sched_class;
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 112c2f127c95..3c16caecd3a5 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -690,6 +690,9 @@ struct scx_rq {
 	struct list_head	watchdog_list;
 	u64			ops_qseq;
 	u32			nr_running;
+	cpumask_var_t		cpus_to_kick;
+	cpumask_var_t		cpus_to_preempt;
+	struct irq_work		kick_cpus_irq_work;
 };
 #endif /* CONFIG_SCHED_CLASS_EXT */
 
diff --git a/tools/sched_ext/.gitignore b/tools/sched_ext/.gitignore
index 6734f7fd9324..389f0e5b0970 100644
--- a/tools/sched_ext/.gitignore
+++ b/tools/sched_ext/.gitignore
@@ -1,5 +1,6 @@
 scx_example_dummy
 scx_example_qmap
+scx_example_central
 *.skel.h
 *.subskel.h
 /tools/
diff --git a/tools/sched_ext/Makefile b/tools/sched_ext/Makefile
index 926b0a36c221..c6c3669c47b9 100644
--- a/tools/sched_ext/Makefile
+++ b/tools/sched_ext/Makefile
@@ -115,7 +115,7 @@ BPF_CFLAGS = -g -D__TARGET_ARCH_$(SRCARCH)					\
 	     -Wno-compare-distinct-pointer-types				\
 	     -O2 -mcpu=v3
 
-all: scx_example_dummy scx_example_qmap
+all: scx_example_dummy scx_example_qmap scx_example_central
 
 # sort removes libbpf duplicates when not cross-building
 MAKE_DIRS := $(sort $(BUILD_DIR)/libbpf $(HOST_BUILD_DIR)/libbpf		\
@@ -174,10 +174,14 @@ scx_example_qmap: scx_example_qmap.c scx_example_qmap.skel.h user_exit_info.h
 	$(CC) $(CFLAGS) -c $< -o $@.o
 	$(CC) -o $@ $@.o $(HOST_BPFOBJ) $(LDFLAGS)
 
+scx_example_central: scx_example_central.c scx_example_central.skel.h user_exit_info.h
+	$(CC) $(CFLAGS) -c $< -o $@.o
+	$(CC) -o $@ $@.o $(HOST_BPFOBJ) $(LDFLAGS)
+
 clean:
 	rm -rf $(SCRATCH_DIR) $(HOST_SCRATCH_DIR)
 	rm -f *.o *.bpf.o *.skel.h *.subskel.h
-	rm -f scx_example_dummy scx_example_qmap
+	rm -f scx_example_dummy scx_example_qmap scx_example_central
 
 .PHONY: all clean
 
diff --git a/tools/sched_ext/scx_common.bpf.h b/tools/sched_ext/scx_common.bpf.h
index fec19e8d0681..ff32e4dd30a6 100644
--- a/tools/sched_ext/scx_common.bpf.h
+++ b/tools/sched_ext/scx_common.bpf.h
@@ -71,6 +71,7 @@ s32 scx_bpf_create_dsq(u64 dsq_id, s32 node) __ksym;
 bool scx_bpf_consume(u64 dsq_id) __ksym;
 u32 scx_bpf_dispatch_nr_slots(void) __ksym;
 void scx_bpf_dispatch(struct task_struct *p, u64 dsq_id, u64 slice, u64 enq_flags) __ksym;
+void scx_bpf_kick_cpu(s32 cpu, u64 flags) __ksym;
 s32 scx_bpf_dsq_nr_queued(u64 dsq_id) __ksym;
 bool scx_bpf_test_and_clear_cpu_idle(s32 cpu) __ksym;
 s32 scx_bpf_pick_idle_cpu(const cpumask_t *cpus_allowed) __ksym;
diff --git a/tools/sched_ext/scx_example_central.bpf.c b/tools/sched_ext/scx_example_central.bpf.c
new file mode 100644
index 000000000000..6b246bf308d0
--- /dev/null
+++ b/tools/sched_ext/scx_example_central.bpf.c
@@ -0,0 +1,231 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * A central FIFO sched_ext scheduler which demonstrates the followings:
+ *
+ * a. Making all scheduling decisions from one CPU:
+ *
+ *    The central CPU is the only one making scheduling decisions. All other
+ *    CPUs kick the central CPU when they run out of tasks to run.
+ *
+ *    There is one global BPF queue and the central CPU schedules all CPUs by
+ *    dispatching from the global queue to each CPU's local dsq from dispatch().
+ *    This isn't the most straightforward. e.g. It'd be easier to bounce
+ *    through per-CPU BPF queues. The current design is chosen to maximally
+ *    utilize and verify various scx mechanisms such as LOCAL_ON dispatching.
+ *
+ * b. Preemption
+ *
+ *    SCX_KICK_PREEMPT is used to trigger scheduling and CPUs to move to the
+ *    next tasks.
+ *
+ * Copyright (c) 2022 Meta Platforms, Inc. and affiliates.
+ * Copyright (c) 2022 Tejun Heo <tj@kernel.org>
+ * Copyright (c) 2022 David Vernet <dvernet@meta.com>
+ */
+#include "scx_common.bpf.h"
+
+char _license[] SEC("license") = "GPL";
+
+enum {
+	FALLBACK_DSQ_ID		= 0,
+	MAX_CPUS		= 4096,
+	MS_TO_NS		= 1000LLU * 1000,
+	TIMER_INTERVAL_NS	= 1 * MS_TO_NS,
+};
+
+const volatile bool switch_all;
+const volatile s32 central_cpu;
+const volatile u32 nr_cpu_ids;
+
+u64 nr_total, nr_locals, nr_queued, nr_lost_pids;
+u64 nr_dispatches, nr_mismatches, nr_retries;
+u64 nr_overflows;
+
+struct user_exit_info uei;
+
+struct {
+	__uint(type, BPF_MAP_TYPE_QUEUE);
+	__uint(max_entries, 4096);
+	__type(value, s32);
+} central_q SEC(".maps");
+
+/* can't use percpu map due to bad lookups */
+static bool cpu_gimme_task[MAX_CPUS];
+
+struct central_timer {
+	struct bpf_timer timer;
+};
+
+struct {
+	__uint(type, BPF_MAP_TYPE_ARRAY);
+	__uint(max_entries, 1);
+	__type(key, u32);
+	__type(value, struct central_timer);
+} central_timer SEC(".maps");
+
+static bool vtime_before(u64 a, u64 b)
+{
+	return (s64)(a - b) < 0;
+}
+
+s32 BPF_STRUCT_OPS(central_select_cpu, struct task_struct *p,
+		   s32 prev_cpu, u64 wake_flags)
+{
+	/*
+	 * Steer wakeups to the central CPU as much as possible to avoid
+	 * disturbing other CPUs. It's safe to blindly return the central cpu as
+	 * select_cpu() is a hint and if @p can't be on it, the kernel will
+	 * automatically pick a fallback CPU.
+	 */
+	return central_cpu;
+}
+
+void BPF_STRUCT_OPS(central_enqueue, struct task_struct *p, u64 enq_flags)
+{
+	s32 pid = p->pid;
+
+	__sync_fetch_and_add(&nr_total, 1);
+
+	if (bpf_map_push_elem(&central_q, &pid, 0)) {
+		__sync_fetch_and_add(&nr_overflows, 1);
+		scx_bpf_dispatch(p, FALLBACK_DSQ_ID, SCX_SLICE_DFL, enq_flags);
+		return;
+	}
+
+	__sync_fetch_and_add(&nr_queued, 1);
+
+	if (!scx_bpf_task_running(p))
+		scx_bpf_kick_cpu(central_cpu, SCX_KICK_PREEMPT);
+}
+
+static int dispatch_a_task_loopfn(u32 idx, void *data)
+{
+	s32 cpu = *(s32 *)data;
+	s32 pid;
+	struct task_struct *p;
+	bool *gimme;
+
+	if (bpf_map_pop_elem(&central_q, &pid))
+		return 1;
+
+	__sync_fetch_and_sub(&nr_queued, 1);
+
+	p = bpf_task_from_pid(pid);
+	if (!p) {
+		__sync_fetch_and_add(&nr_lost_pids, 1);
+		return 0;
+	}
+
+	/*
+	 * If we can't run the task at the top, do the dumb thing and bounce it
+	 * to the fallback dsq.
+	 */
+	if (!bpf_cpumask_test_cpu(cpu, p->cpus_ptr)) {
+		__sync_fetch_and_add(&nr_mismatches, 1);
+		scx_bpf_dispatch(p, FALLBACK_DSQ_ID, SCX_SLICE_DFL, 0);
+		bpf_task_release(p);
+		return 0;
+	}
+
+	/* dispatch to the local and mark that @cpu doesn't need more tasks */
+	scx_bpf_dispatch(p, SCX_DSQ_LOCAL_ON | cpu, SCX_SLICE_DFL, 0);
+
+	if (cpu != central_cpu)
+		scx_bpf_kick_cpu(cpu, 0);
+
+	gimme = MEMBER_VPTR(cpu_gimme_task, [cpu]);
+	if (gimme)
+		*gimme = false;
+
+	bpf_task_release(p);
+	return 1;
+}
+
+static int dispatch_to_one_cpu_loopfn(u32 idx, void *data)
+{
+	s32 cpu = idx;
+
+	if (!scx_bpf_dispatch_nr_slots())
+		return 1;
+
+	if (cpu >= 0 && cpu < MAX_CPUS) {
+		bool *gimme = MEMBER_VPTR(cpu_gimme_task, [cpu]);
+		if (gimme && !*gimme)
+			return 0;
+	}
+
+	bpf_loop(1 << 23, dispatch_a_task_loopfn, &cpu, 0);
+	return 0;
+}
+
+void BPF_STRUCT_OPS(central_dispatch, s32 cpu, struct task_struct *prev)
+{
+	if (cpu == central_cpu) {
+		/* dispatch for all other CPUs first */
+		__sync_fetch_and_add(&nr_dispatches, 1);
+		bpf_loop(nr_cpu_ids, dispatch_to_one_cpu_loopfn, NULL, 0);
+
+		/*
+		 * Retry if we ran out of dispatch buffer slots as we might have
+		 * skipped some CPUs and also need to dispatch for self. The ext
+		 * core automatically retries if the local dsq is empty but we
+		 * can't rely on that as we're dispatching for other CPUs too.
+		 * Kick self explicitly to retry.
+		 */
+		if (!scx_bpf_dispatch_nr_slots()) {
+			__sync_fetch_and_add(&nr_retries, 1);
+			scx_bpf_kick_cpu(central_cpu, SCX_KICK_PREEMPT);
+			return;
+		}
+
+		/* look for a task to run on the central CPU */
+		if (scx_bpf_consume(FALLBACK_DSQ_ID))
+			return;
+		bpf_loop(1 << 23, dispatch_a_task_loopfn, &cpu, 0);
+	} else {
+		bool *gimme;
+
+		if (scx_bpf_consume(FALLBACK_DSQ_ID))
+			return;
+
+		gimme = MEMBER_VPTR(cpu_gimme_task, [cpu]);
+		if (gimme)
+			*gimme = true;
+
+		/*
+		 * Force dispatch on the scheduling CPU so that it finds a task
+		 * to run for us.
+		 */
+		scx_bpf_kick_cpu(central_cpu, SCX_KICK_PREEMPT);
+	}
+}
+
+int BPF_STRUCT_OPS_SLEEPABLE(central_init)
+{
+	if (switch_all)
+		scx_bpf_switch_all();
+
+	return scx_bpf_create_dsq(FALLBACK_DSQ_ID, -1);
+}
+
+void BPF_STRUCT_OPS(central_exit, struct scx_exit_info *ei)
+{
+	uei_record(&uei, ei);
+}
+
+SEC(".struct_ops")
+struct sched_ext_ops central_ops = {
+	/*
+	 * We are offloading all scheduling decisions to the central CPU and
+	 * thus being the last task on a given CPU doesn't mean anything
+	 * special. Enqueue the last tasks like any other tasks.
+	 */
+	.flags			= SCX_OPS_ENQ_LAST,
+
+	.select_cpu		= (void *)central_select_cpu,
+	.enqueue		= (void *)central_enqueue,
+	.dispatch		= (void *)central_dispatch,
+	.init			= (void *)central_init,
+	.exit			= (void *)central_exit,
+	.name			= "central",
+};
diff --git a/tools/sched_ext/scx_example_central.c b/tools/sched_ext/scx_example_central.c
new file mode 100644
index 000000000000..14f6598c03df
--- /dev/null
+++ b/tools/sched_ext/scx_example_central.c
@@ -0,0 +1,93 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Copyright (c) 2022 Meta Platforms, Inc. and affiliates.
+ * Copyright (c) 2022 Tejun Heo <tj@kernel.org>
+ * Copyright (c) 2022 David Vernet <dvernet@meta.com>
+ */
+#define _GNU_SOURCE
+#include <stdio.h>
+#include <unistd.h>
+#include <signal.h>
+#include <assert.h>
+#include <libgen.h>
+#include <bpf/bpf.h>
+#include "user_exit_info.h"
+#include "scx_example_central.skel.h"
+
+const char help_fmt[] =
+"A central FIFO sched_ext scheduler.\n"
+"\n"
+"See the top-level comment in .bpf.c for more details.\n"
+"\n"
+"Usage: %s [-a] [-c CPU]\n"
+"\n"
+"  -a            Switch all tasks\n"
+"  -c CPU        Override the central CPU (default: 0)\n"
+"  -h            Display this help and exit\n";
+
+static volatile int exit_req;
+
+static void sigint_handler(int dummy)
+{
+	exit_req = 1;
+}
+
+int main(int argc, char **argv)
+{
+	struct scx_example_central *skel;
+	struct bpf_link *link;
+	u64 seq = 0;
+	s32 opt;
+
+	signal(SIGINT, sigint_handler);
+	signal(SIGTERM, sigint_handler);
+
+	libbpf_set_strict_mode(LIBBPF_STRICT_ALL);
+
+	skel = scx_example_central__open();
+	assert(skel);
+
+	skel->rodata->central_cpu = 0;
+	skel->rodata->nr_cpu_ids = libbpf_num_possible_cpus();
+
+	while ((opt = getopt(argc, argv, "ahc:")) != -1) {
+		switch (opt) {
+		case 'a':
+			skel->rodata->switch_all = true;
+			break;
+		case 'c':
+			skel->rodata->central_cpu = strtoul(optarg, NULL, 0);
+			break;
+		default:
+			fprintf(stderr, help_fmt, basename(argv[0]));
+			return opt != 'h';
+		}
+	}
+
+	assert(!scx_example_central__load(skel));
+
+	link = bpf_map__attach_struct_ops(skel->maps.central_ops);
+	assert(link);
+
+	while (!exit_req && !uei_exited(&skel->bss->uei)) {
+		printf("[SEQ %lu]\n", seq++);
+		printf("total   :%10lu    local:%10lu   queued:%10lu  lost:%10lu\n",
+		       skel->bss->nr_total,
+		       skel->bss->nr_locals,
+		       skel->bss->nr_queued,
+		       skel->bss->nr_lost_pids);
+		printf("                    dispatch:%10lu mismatch:%10lu retry:%10lu\n",
+		       skel->bss->nr_dispatches,
+		       skel->bss->nr_mismatches,
+		       skel->bss->nr_retries);
+		printf("overflow:%10lu\n",
+		       skel->bss->nr_overflows);
+		fflush(stdout);
+		sleep(1);
+	}
+
+	bpf_link__destroy(link);
+	uei_print(&skel->bss->uei);
+	scx_example_central__destroy(skel);
+	return 0;
+}
-- 
2.39.1