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// SPDX-License-Identifier: GPL-2.0
/*
* A scheduler that validates the behavior of scx_bpf_select_cpu_and() by
* selecting idle CPUs strictly within a subset of allowed CPUs.
*
* Copyright (c) 2025 Andrea Righi <arighi@nvidia.com>
*/
#include <scx/common.bpf.h>
char _license[] SEC("license") = "GPL";
UEI_DEFINE(uei);
private(PREF_CPUS) struct bpf_cpumask __kptr * allowed_cpumask;
static void
validate_local_idle_state(void)
{
const struct cpumask *idle;
struct task_struct *curr;
s32 cpu = bpf_get_smp_processor_id();
bool cpu_is_idle, curr_is_idle;
bpf_rcu_read_lock();
curr = scx_bpf_cpu_curr(cpu);
curr_is_idle = curr && (curr->flags & PF_IDLE);
bpf_rcu_read_unlock();
idle = scx_bpf_get_idle_cpumask();
cpu_is_idle = bpf_cpumask_test_cpu(cpu, idle);
scx_bpf_put_idle_cpumask(idle);
/*
* Unlike a remote selected CPU, the local CPU cannot go through an
* idle re-pick while this callback is running. If it is running a
* non-idle scheduling context, it must not be advertised as idle.
*/
if (!curr_is_idle && cpu_is_idle)
scx_bpf_error("running CPU %d should be marked as busy", cpu);
}
static void
validate_selected_cpu(const struct task_struct *p, s32 cpu)
{
const struct cpumask *allowed = cast_mask(allowed_cpumask);
if (!allowed) {
scx_bpf_error("allowed domain not initialized");
return;
}
if (!bpf_cpumask_test_cpu(cpu, allowed))
scx_bpf_error("CPU %d not in the allowed domain for %d (%s)",
cpu, p->pid, p->comm);
if (!bpf_cpumask_test_cpu(cpu, p->cpus_ptr))
scx_bpf_error("CPU %d not in the affinity mask for %d (%s)",
cpu, p->pid, p->comm);
}
s32 BPF_STRUCT_OPS(allowed_cpus_select_cpu,
struct task_struct *p, s32 prev_cpu, u64 wake_flags)
{
const struct cpumask *allowed;
s32 cpu;
validate_local_idle_state();
allowed = cast_mask(allowed_cpumask);
if (!allowed) {
scx_bpf_error("allowed domain not initialized");
return -EINVAL;
}
/*
* Select an idle CPU strictly within the allowed domain.
*/
cpu = scx_bpf_select_cpu_and(p, prev_cpu, wake_flags, allowed, 0);
if (cpu >= 0) {
validate_selected_cpu(p, cpu);
scx_bpf_dsq_insert(p, SCX_DSQ_LOCAL, SCX_SLICE_DFL, 0);
return cpu;
}
return prev_cpu;
}
void BPF_STRUCT_OPS(allowed_cpus_enqueue, struct task_struct *p, u64 enq_flags)
{
const struct cpumask *allowed;
s32 prev_cpu = scx_bpf_task_cpu(p), cpu;
scx_bpf_dsq_insert(p, SCX_DSQ_GLOBAL, SCX_SLICE_DFL, 0);
validate_local_idle_state();
allowed = cast_mask(allowed_cpumask);
if (!allowed) {
scx_bpf_error("allowed domain not initialized");
return;
}
/*
* Use scx_bpf_select_cpu_and() to proactively kick an idle CPU
* within @allowed_cpumask, usable by @p.
*/
cpu = scx_bpf_select_cpu_and(p, prev_cpu, 0, allowed, 0);
if (cpu >= 0) {
validate_selected_cpu(p, cpu);
scx_bpf_kick_cpu(cpu, SCX_KICK_IDLE);
}
}
s32 BPF_STRUCT_OPS_SLEEPABLE(allowed_cpus_init)
{
struct bpf_cpumask *mask;
mask = bpf_cpumask_create();
if (!mask)
return -ENOMEM;
mask = bpf_kptr_xchg(&allowed_cpumask, mask);
if (mask)
bpf_cpumask_release(mask);
bpf_rcu_read_lock();
/*
* Assign the first online CPU to the allowed domain.
*/
mask = allowed_cpumask;
if (mask) {
const struct cpumask *online = scx_bpf_get_online_cpumask();
bpf_cpumask_set_cpu(bpf_cpumask_first(online), mask);
scx_bpf_put_cpumask(online);
}
bpf_rcu_read_unlock();
return 0;
}
void BPF_STRUCT_OPS(allowed_cpus_exit, struct scx_exit_info *ei)
{
UEI_RECORD(uei, ei);
}
struct task_cpu_arg {
u64 pid;
s64 custom_cpu;
};
SEC("syscall")
int select_cpu_from_user(struct task_cpu_arg *input)
{
struct task_struct *p;
struct bpf_cpumask *mask;
s32 cpu;
p = bpf_task_from_pid(input->pid);
if (!p)
return -EINVAL;
mask = bpf_cpumask_create();
if (!mask) {
bpf_task_release(p);
return -ENOMEM;
}
/* A negative custom_cpu leaves the custom mask empty. */
if (input->custom_cpu >= 0)
bpf_cpumask_set_cpu(input->custom_cpu, mask);
bpf_rcu_read_lock();
cpu = scx_bpf_select_cpu_and(p, bpf_get_smp_processor_id(), 0,
cast_mask(mask), 0);
if (cpu >= 0 &&
(!bpf_cpumask_test_cpu(cpu, cast_mask(mask)) ||
!bpf_cpumask_test_cpu(cpu, &p->cpus_mask)))
cpu = -ERANGE;
bpf_rcu_read_unlock();
bpf_cpumask_release(mask);
bpf_task_release(p);
return cpu;
}
SEC(".struct_ops.link")
struct sched_ext_ops allowed_cpus_ops = {
.select_cpu = (void *)allowed_cpus_select_cpu,
.enqueue = (void *)allowed_cpus_enqueue,
.init = (void *)allowed_cpus_init,
.exit = (void *)allowed_cpus_exit,
.name = "allowed_cpus",
};
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