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// SPDX-License-Identifier: GPL-2.0
/*
* A scheduler that keeps the target task waiting on a DSQ when it wakes up,
* until it's switched out of SCHED_EXT or, if @park_ns is set, for @park_ns.
* This lets userspace switch the target out while it's waiting.
*
* Copyright (c) 2026 Google LLC.
*/
#include <scx/common.bpf.h>
char _license[] SEC("license") = "GPL";
#define PARK_DSQ 0
UEI_DEFINE(uei);
s32 target_pid; /* set by userspace */
bool park; /* set by userspace, cleared when parking */
u64 park_ns; /* set by userspace, how long to park, 0 for no limit */
u64 nr_parked; /* how many times the target has been parked */
u64 nr_released; /* how many times the target has been released */
static bool released; /* set by the timer, cleared when parking */
static s32 target_cpu; /* CPU to kick when the timer fires */
struct release_timer {
struct bpf_timer timer;
};
struct {
__uint(type, BPF_MAP_TYPE_ARRAY);
__uint(max_entries, 1);
__type(key, u32);
__type(value, struct release_timer);
} release_timer SEC(".maps");
static int release_timerfn(void *map, int *key, struct bpf_timer *timer)
{
released = true;
scx_bpf_kick_cpu(target_cpu, 0);
return 0;
}
s32 BPF_STRUCT_OPS(class_switch_stall_select_cpu, struct task_struct *p,
s32 prev_cpu, u64 wake_flags)
{
/* Don't dispatch directly so that wakeups go to ops.enqueue(). */
return prev_cpu;
}
void BPF_STRUCT_OPS(class_switch_stall_enqueue, struct task_struct *p,
u64 enq_flags)
{
struct release_timer *rt;
u32 key = 0;
if (p->pid != target_pid || !park || !(enq_flags & SCX_ENQ_WAKEUP)) {
scx_bpf_dsq_insert(p, SCX_DSQ_GLOBAL, SCX_SLICE_DFL, enq_flags);
return;
}
park = false;
released = false;
target_cpu = scx_bpf_task_cpu(p);
scx_bpf_dsq_insert(p, PARK_DSQ, SCX_SLICE_DFL, enq_flags);
__sync_fetch_and_add(&nr_parked, 1);
if (!park_ns)
return;
rt = bpf_map_lookup_elem(&release_timer, &key);
if (!rt) {
scx_bpf_error("failed to look up the release timer");
return;
}
bpf_timer_start(&rt->timer, park_ns, 0);
}
void BPF_STRUCT_OPS(class_switch_stall_dispatch, s32 cpu,
struct task_struct *prev)
{
if (released && scx_bpf_dsq_move_to_local(PARK_DSQ, 0))
__sync_fetch_and_add(&nr_released, 1);
}
s32 BPF_STRUCT_OPS_SLEEPABLE(class_switch_stall_init)
{
struct release_timer *rt;
u32 key = 0;
s32 ret;
ret = scx_bpf_create_dsq(PARK_DSQ, -1);
if (ret)
return ret;
rt = bpf_map_lookup_elem(&release_timer, &key);
if (!rt)
return -ESRCH;
bpf_timer_init(&rt->timer, &release_timer, CLOCK_MONOTONIC);
bpf_timer_set_callback(&rt->timer, release_timerfn);
return 0;
}
void BPF_STRUCT_OPS(class_switch_stall_exit, struct scx_exit_info *ei)
{
UEI_RECORD(uei, ei);
}
SEC(".struct_ops.link")
struct sched_ext_ops class_switch_stall_ops = {
.select_cpu = (void *)class_switch_stall_select_cpu,
.enqueue = (void *)class_switch_stall_enqueue,
.dispatch = (void *)class_switch_stall_dispatch,
.init = (void *)class_switch_stall_init,
.exit = (void *)class_switch_stall_exit,
.flags = SCX_OPS_SWITCH_PARTIAL,
.name = "class_switch_stall",
};
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