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selftests/sched_ext: Make allowed_cpus idle validation race-free

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A remotely selected CPU can be re-advertised as idle by an idle-to-idle
re-pick before the BPF program validates the selection. Checking that
the selected CPU remains absent from the idle mask is therefore
inherently racy.

Validate a stable local invariant instead: a CPU executing
ops.select_cpu() or ops.enqueue() in a non-idle scheduling context must
not be advertised as idle. Read the idle mask without modifying it and
also validate selected CPUs against the requested domain and task
affinity.

Suggested-by: Kuba Piecuch <jpiecuch@google.com>
Signed-off-by: Andrea Righi <arighi@nvidia.com>
Reviewed-by: Kuba Piecuch <jpiecuch@google.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
This commit is contained in:
Andrea Righi
2026-08-03 06:21:11 -10:00
committed by Tejun Heo
parent ba190ed3f4
commit 12da4723b6
1 changed files with 42 additions and 7 deletions
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tools/testing/selftests/sched_ext/allowed_cpus.bpf.c
+42 -7
View File
@@ -15,15 +15,48 @@ 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)
validate_local_idle_state(void)
{
if (scx_bpf_test_and_clear_cpu_idle(cpu))
scx_bpf_error("CPU %d should be marked as busy", cpu);
const struct cpumask *idle;
struct task_struct *curr;
s32 cpu = bpf_get_smp_processor_id();
bool cpu_is_idle, curr_is_idle;
if (bpf_cpumask_subset(allowed, p->cpus_ptr) &&
!bpf_cpumask_test_cpu(cpu, allowed))
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,
@@ -32,6 +65,7 @@ s32 BPF_STRUCT_OPS(allowed_cpus_select_cpu,
const struct cpumask *allowed;
s32 cpu;
validate_local_idle_state();
allowed = cast_mask(allowed_cpumask);
if (!allowed) {
scx_bpf_error("allowed domain not initialized");
@@ -43,7 +77,7 @@ s32 BPF_STRUCT_OPS(allowed_cpus_select_cpu,
*/
cpu = scx_bpf_select_cpu_and(p, prev_cpu, wake_flags, allowed, 0);
if (cpu >= 0) {
validate_idle_cpu(p, allowed, cpu);
validate_selected_cpu(p, cpu);
scx_bpf_dsq_insert(p, SCX_DSQ_LOCAL, SCX_SLICE_DFL, 0);
return cpu;
@@ -59,6 +93,7 @@ void BPF_STRUCT_OPS(allowed_cpus_enqueue, struct task_struct *p, u64 enq_flags)
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");
@@ -71,7 +106,7 @@ void BPF_STRUCT_OPS(allowed_cpus_enqueue, struct task_struct *p, u64 enq_flags)
*/
cpu = scx_bpf_select_cpu_and(p, prev_cpu, 0, allowed, 0);
if (cpu >= 0) {
validate_idle_cpu(p, allowed, cpu);
validate_selected_cpu(p, cpu);
scx_bpf_kick_cpu(cpu, SCX_KICK_IDLE);
}
}