scx_cpu0.bpf.c

 1/* SPDX-License-Identifier: GPL-2.0 */
 2/*
 3 * A CPU0 scheduler.
 4 *
 5 * This scheduler queues all tasks to a shared DSQ and only dispatches them on
 6 * CPU0 in FIFO order. This is useful for testing bypass behavior when many
 7 * tasks are concentrated on a single CPU. If the load balancer doesn't work,
 8 * bypass mode can trigger task hangs or RCU stalls as the queue is long and
 9 * there's only one CPU working on it.
10 *
11 * - Statistics tracking how many tasks are queued to local and CPU0 DSQs.
12 * - Termination notification for userspace.
13 *
14 * Copyright (c) 2025 Meta Platforms, Inc. and affiliates.
15 * Copyright (c) 2025 Tejun Heo <tj@kernel.org>
16 */
17#include <scx/common.bpf.h>
18
19char _license[] SEC("license") = "GPL";
20
21const volatile u32 nr_cpus = 32;	/* !0 for veristat, set during init */
22
23UEI_DEFINE(uei);
24
25/*
26 * We create a custom DSQ with ID 0 that we dispatch to and consume from on
27 * CPU0.
28 */
29#define DSQ_CPU0 0
30
31struct {
32	__uint(type, BPF_MAP_TYPE_PERCPU_ARRAY);
33	__uint(key_size, sizeof(u32));
34	__uint(value_size, sizeof(u64));
35	__uint(max_entries, 2);			/* [local, cpu0] */
36} stats SEC(".maps");
37
38static void stat_inc(u32 idx)
39{
40	u64 *cnt_p = bpf_map_lookup_elem(&stats, &idx);
41	if (cnt_p)
42		(*cnt_p)++;
43}
44
45s32 BPF_STRUCT_OPS(cpu0_select_cpu, struct task_struct *p, s32 prev_cpu, u64 wake_flags)
46{
47	return 0;
48}
49
50void BPF_STRUCT_OPS(cpu0_enqueue, struct task_struct *p, u64 enq_flags)
51{
52	/*
53	 * select_cpu() always picks CPU0. If @p is not on CPU0, it can't run on
54	 * CPU 0. Queue on whichever CPU it's currently only.
55	 */
56	if (scx_bpf_task_cpu(p) != 0) {
57		stat_inc(0);	/* count local queueing */
58		scx_bpf_dsq_insert(p, SCX_DSQ_LOCAL, SCX_SLICE_DFL, 0);
59		return;
60	}
61
62	stat_inc(1);	/* count cpu0 queueing */
63	scx_bpf_dsq_insert(p, DSQ_CPU0, SCX_SLICE_DFL, enq_flags);
64}
65
66void BPF_STRUCT_OPS(cpu0_dispatch, s32 cpu, struct task_struct *prev)
67{
68	if (cpu == 0)
69		scx_bpf_dsq_move_to_local(DSQ_CPU0);
70}
71
72s32 BPF_STRUCT_OPS_SLEEPABLE(cpu0_init)
73{
74	return scx_bpf_create_dsq(DSQ_CPU0, -1);
75}
76
77void BPF_STRUCT_OPS(cpu0_exit, struct scx_exit_info *ei)
78{
79	UEI_RECORD(uei, ei);
80}
81
82SCX_OPS_DEFINE(cpu0_ops,
83	       .select_cpu		= (void *)cpu0_select_cpu,
84	       .enqueue			= (void *)cpu0_enqueue,
85	       .dispatch		= (void *)cpu0_dispatch,
86	       .init			= (void *)cpu0_init,
87	       .exit			= (void *)cpu0_exit,
88	       .name			= "cpu0");