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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_idle_cpu(const struct task_struct *p, const struct cpumask *allowed, s32 cpu)
{
if (scx_bpf_test_and_clear_cpu_idle(cpu))
scx_bpf_error("CPU %d should be marked as busy", cpu);
if (bpf_cpumask_subset(allowed, p->cpus_ptr) &&
!bpf_cpumask_test_cpu(cpu, allowed))
scx_bpf_error("CPU %d not in the allowed domain 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;
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_idle_cpu(p, allowed, 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);
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_idle_cpu(p, allowed, 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 {
pid_t pid;
};
SEC("syscall")
int select_cpu_from_user(struct task_cpu_arg *input)
{
struct task_struct *p;
int cpu;
p = bpf_task_from_pid(input->pid);
if (!p)
return -EINVAL;
bpf_rcu_read_lock();
cpu = scx_bpf_select_cpu_and(p, bpf_get_smp_processor_id(), 0, p->cpus_ptr, 0);
bpf_rcu_read_unlock();
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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