[PATCH 1/2] sched_ext: Add scx_tickbug, a scheduler that holds a task on an idle CPU

[Radu Toader <radu.m.toader@gmail.com>]

The state behind the tick_irq_exit() gate - the idle task running on a
CPU whose run queue is not empty - is reachable with the existing
example schedulers but not holdable with them, which makes everything
that depends on how long it lasts impossible to measure.

enqueue_task_scx() raises rq->nr_running before it calls ops.enqueue(),
so a scheduler that stashes the task in a map of its own leaves it
runnable and accounted on that CPU with nothing for the CPU to pick.
scx_central already does exactly that, and then calls
scx_bpf_kick_cpu() immediately, so the window is microseconds wide.

This scheduler is the same thing with the kick removed: a tagged task
is parked on the target CPU's run queue and released only by a timer
pinned to another CPU, so the window is as wide as -H says. Untagged
tasks go through a shared DSQ and per-CPU kthreads keep their local DSQ
and preemption, so the machine stays usable and the timer's softirq
cannot be starved.

Three things it has to get right. The release timer must kick
@target_cpu only when something is actually held: an unconditional
SCX_KICK_PREEMPT lands on the task released a moment earlier and throws
it off the CPU mid-run, whereupon ops.enqueue() holds it again, and the
load then reports using half the CPU it asked for. @target_cpu must not
take work from the shared DSQ while it is holding something, or the idle
stretch under test ends early and the hold reported for the task covers
time the CPU spent running. And a tagged task re-enqueued while it is
the current scheduling context goes back to its local DSQ instead of
being held, so a hold only ever delays the start of a run and never
interrupts one - with %SCX_ENQ_LAST excluded from that test, because it
arrives from put_prev_task_scx(), where rq->donor is still the task, so
scx_bpf_task_running() is true for every enqueue on that path while the
CPU is in fact about to go idle with that task as the only runnable one.

The hold is tracked per task rather than globally: the timestamp travels
with the pid through the queue, and ops.dequeue() gives up the hold when
a parked task leaves BPF custody behind the scheduler's back, so a stale
queue entry cannot leave the release timer kicking a CPU that is holding
nothing.

It reports the mean and worst hold it actually achieved, which is the
independent variable of any experiment using it and is not otherwise
observable from userspace. Do not use a hold that divides the load's
period: with a 100 ms spinner, holds of 5/10/20 ms phase-lock and the
task is only ever held for microseconds.

Tooling: written by Claude Opus 5 (Claude Code) while measuring what
the tick_irq_exit() idle gate changes. All of this patch is
tool-generated.

Signed-off-by: Radu Toader <radu.m.toader@gmail.com>
---
 tools/sched_ext/Makefile          |   2 +-
 tools/sched_ext/README.md         |  13 +
 tools/sched_ext/scx_tickbug.bpf.c | 534 ++++++++++++++++++++++++++++++
 tools/sched_ext/scx_tickbug.c     | 177 ++++++++++
 4 files changed, 725 insertions(+), 1 deletion(-)
 create mode 100644 tools/sched_ext/scx_tickbug.bpf.c
 create mode 100644 tools/sched_ext/scx_tickbug.c

diff --git a/tools/sched_ext/Makefile b/tools/sched_ext/Makefile
index 70d2503b6b33..02e1500bf402 100644
--- a/tools/sched_ext/Makefile
+++ b/tools/sched_ext/Makefile
@@ -191,7 +191,7 @@ $(INCLUDE_DIR)/%.bpf.skel.h: $(SCXOBJ_DIR)/%.bpf.o $(INCLUDE_DIR)/vmlinux.h $(BP
 
 SCX_COMMON_DEPS := include/scx/common.h include/scx/user_exit_info.h | $(BINDIR)
 
-c-sched-targets = scx_simple scx_cpu0 scx_qmap scx_central scx_flatcg scx_userland scx_pair scx_sdt
+c-sched-targets = scx_simple scx_cpu0 scx_qmap scx_central scx_tickbug scx_flatcg scx_userland scx_pair scx_sdt
 
 $(addprefix $(BINDIR)/,$(c-sched-targets)): \
 	$(BINDIR)/%: \
diff --git a/tools/sched_ext/README.md b/tools/sched_ext/README.md
index 0ee5a3d997e5..4addc7c2ed23 100644
--- a/tools/sched_ext/README.md
+++ b/tools/sched_ext/README.md
@@ -185,6 +185,19 @@ where this could be particularly useful is running VMs, where running with
 infinite slices and no timer ticks allows the VM to avoid unnecessary expensive
 vmexits.
 
+## scx_tickbug
+
+A scheduler whose only job is to keep one task runnable on a CPU that is in the
+idle loop, for a length of time given on the command line. That state - the
+idle task running while the run queue is not empty - is reachable by several of
+the schedulers here, but none of them holds it long enough to observe what
+depends on it. Tasks named "spin" are parked on the target CPU's run queue and
+released only by a periodic timer; everything else runs through a shared DSQ so
+the machine stays usable.
+
+This is a test scheduler, not a policy. It exists so that the idle-time
+bookkeeping skipped by an interrupt taken in that state can be measured.
+
 ## scx_flatcg
 
 A flattened cgroup hierarchy scheduler. This scheduler implements hierarchical
diff --git a/tools/sched_ext/scx_tickbug.bpf.c b/tools/sched_ext/scx_tickbug.bpf.c
new file mode 100644
index 000000000000..3d7ce7423628
--- /dev/null
+++ b/tools/sched_ext/scx_tickbug.bpf.c
@@ -0,0 +1,534 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * scx_tickbug: keep a task runnable on a CPU that is in the idle loop.
+ *
+ * This scheduler exists to hold one state for a controllable length of time:
+ * the idle task running on a CPU whose run queue is not empty.
+ *
+ * enqueue_task_scx() raises rq->nr_running before it calls ops.enqueue(), so a
+ * BPF scheduler that stashes the task somewhere of its own - here a BPF queue
+ * map - leaves it runnable and accounted on that CPU with nothing for the CPU
+ * to pick. The CPU enters the idle loop and stays there. In that state
+ * idle_cpu() is false, because idle_rq() tests rq->nr_running, while
+ * is_idle_task(current) is true.
+ *
+ * tick_irq_exit() used to ask idle_cpu(), so every interrupt taken in that
+ * idle loop skipped tick_nohz_irq_exit(). Each skip leaves TS_FLAG_IDLE_ACTIVE
+ * clear, and the next exit from the idle loop then also skips
+ * sched_clock_idle_wakeup_event(), so with an unstable sched_clock the CPU's
+ * clock is never resynchronized: the task picked next starts with a stale
+ * exec_start and the next tick charges it the whole idle stretch.
+ *
+ * The in-tree example schedulers can reach this state but not hold it.
+ * scx_central stashes a task in a BPF queue exactly like this, then
+ * immediately calls scx_bpf_kick_cpu(), so the window is microseconds wide and
+ * nothing measurable accumulates. Here the only thing that releases a held
+ * task is a periodic timer, so the window is hold_ns wide and can be swept.
+ *
+ * Only tasks named by @tag_comm are held; everything else runs through a
+ * shared DSQ so the machine stays usable while the test runs. @target_cpu does
+ * not pull from that shared DSQ while it is holding something: shared work
+ * running there would take the CPU out of the state under test and make the
+ * hold this scheduler reports an overestimate.
+ *
+ * Copyright (c) 2026 Radu Toader
+ */
+#include <scx/common.bpf.h>
+
+char _license[] SEC("license") = "GPL";
+
+enum {
+	SHARED_DSQ	= 0,
+	MS_TO_NS	= 1000LLU * 1000,
+};
+
+/* CPU whose run queue holds the tagged task while the CPU idles. */
+const volatile s32 target_cpu;
+/* CPU the release timer is pinned to. Must not be @target_cpu. */
+const volatile s32 timer_cpu = 1;
+/* How long a tagged task is held before it is allowed to run. */
+const volatile u64 hold_ns = 10 * MS_TO_NS;
+const volatile u64 slice_ns;
+
+bool timer_pinned = true;
+bool timer_started;
+
+/*
+ * Set by the release timer, consumed by ops.dispatch() on @target_cpu. Without
+ * it ops.dispatch() would hand the held task straight back out, because the
+ * core calls ops.dispatch() whenever the CPU is about to go idle.
+ */
+bool release_now;
+
+u64 nr_enq, nr_held, nr_released, nr_timers;
+u64 nr_passthru, nr_locals, nr_lost, nr_other_cpu, nr_overflow, nr_requeue;
+u64 nr_last, nr_dup, nr_stale, nr_undone;
+/*
+ * How many tagged tasks are held right now. The release timer only kicks when
+ * this is non-zero: a kick with nothing held would preempt whatever is running
+ * on @target_cpu, and since that is normally the task just released, the task
+ * would lose most of its CPU to its own release mechanism. The first version of
+ * this scheduler did exactly that and reported half the usage it should.
+ */
+u64 nr_held_now;
+/* Proof that the hold actually happened, and how wide it was. */
+u64 total_held_ns, max_held_ns;
+
+UEI_DEFINE(uei);
+
+/*
+ * One queued hold. The timestamp travels with the pid: a single global would
+ * be overwritten by the second task to be held and would then mismeasure both.
+ */
+struct hold_entry {
+	s32	pid;
+	u64	at;
+};
+
+struct {
+	__uint(type, BPF_MAP_TYPE_QUEUE);
+	__uint(max_entries, 1024);
+	__type(value, struct hold_entry);
+} hold_q SEC(".maps");
+
+/*
+ * Whether this task's pid is parked in @hold_q. @hold_q cannot be searched or
+ * amended, so this flag is the authority on whether a queued entry is still
+ * live: ops.dequeue() clears it when the task leaves BPF custody behind the
+ * scheduler's back, and release_one() then drops the entry it finds.
+ */
+struct task_ctx {
+	bool	held;
+};
+
+struct {
+	__uint(type, BPF_MAP_TYPE_TASK_STORAGE);
+	__uint(map_flags, BPF_F_NO_PREALLOC);
+	__type(key, int);
+	__type(value, struct task_ctx);
+} task_ctx_stor SEC(".maps");
+
+struct tickbug_timer {
+	struct bpf_timer timer;
+};
+
+struct {
+	__uint(type, BPF_MAP_TYPE_ARRAY);
+	__uint(max_entries, 1);
+	__type(key, u32);
+	__type(value, struct tickbug_timer);
+} release_timer SEC(".maps");
+
+static struct task_ctx *lookup_task_ctx(struct task_struct *p)
+{
+	struct task_ctx *tctx;
+
+	tctx = bpf_task_storage_get(&task_ctx_stor, p, 0, 0);
+	if (!tctx)
+		scx_bpf_error("no task_ctx for %d (%s)", p->pid, p->comm);
+	return tctx;
+}
+
+/*
+ * The load generator is called "spin". Compare 5 bytes so the terminating NUL
+ * has to match too and a longer name starting with "spin" is not held.
+ */
+static bool is_tagged(struct task_struct *p)
+{
+	return !bpf_strncmp(p->comm, 5, "spin");
+}
+
+s32 BPF_STRUCT_OPS(tickbug_select_cpu, struct task_struct *p,
+		   s32 prev_cpu, u64 wake_flags)
+{
+	if (is_tagged(p))
+		return target_cpu;
+	return prev_cpu;
+}
+
+/*
+ * Deal with a tagged task: park it on @target_cpu's run queue, or decide that
+ * it cannot be held. Returns true if @p has been taken care of and must not go
+ * on to the shared DSQ.
+ */
+static bool handle_tagged(struct task_struct *p, struct task_ctx *tctx,
+			  u64 enq_flags)
+{
+	struct hold_entry ent = { .pid = p->pid };
+
+	if (scx_bpf_task_cpu(p) != target_cpu) {
+		/*
+		 * ops.select_cpu() is only a hint and is not called on every
+		 * path. Counted rather than forced: a run with a non-zero
+		 * count here did not test what it meant to.
+		 */
+		__sync_fetch_and_add(&nr_other_cpu, 1);
+		return false;
+	}
+
+	if (tctx->held) {
+		/*
+		 * Already parked. Pushing the pid again would hold one task
+		 * twice and leave an entry behind for the release timer to
+		 * trip over.
+		 */
+		__sync_fetch_and_add(&nr_dup, 1);
+		return true;
+	}
+
+	if (!(enq_flags & SCX_ENQ_LAST) && scx_bpf_task_running(p)) {
+		/*
+		 * Re-enqueued while it is the current scheduling context, e.g.
+		 * after a kthread preempted it. Holding it here would take CPU
+		 * away from work already in progress, which is not the state
+		 * under test: the hold belongs at the start of a run, not in
+		 * the middle of one. Put it straight back so it carries on.
+		 *
+		 * %SCX_ENQ_LAST is excluded because it is not that case. It
+		 * arrives from put_prev_task_scx(), where rq->donor is still
+		 * @p - put_prev_set_next_task() asserts as much - so
+		 * scx_bpf_task_running() is true for every enqueue on that
+		 * path, while the CPU is in fact about to go idle with @p as
+		 * the only runnable task. That is what has to be held.
+		 */
+		__sync_fetch_and_add(&nr_requeue, 1);
+		scx_bpf_dsq_insert(p, SCX_DSQ_LOCAL, slice_ns, enq_flags);
+		return true;
+	}
+
+	ent.at = scx_bpf_now() ?: 1;
+	if (bpf_map_push_elem(&hold_q, &ent, 0)) {
+		__sync_fetch_and_add(&nr_overflow, 1);
+		return false;
+	}
+
+	/*
+	 * This is the whole scheduler. The task is already in target_cpu's
+	 * rq->nr_running; parking its pid here and deliberately not kicking
+	 * the CPU leaves it in the idle loop with a runnable task queued.
+	 */
+	tctx->held = true;
+	__sync_fetch_and_add(&nr_held, 1);
+	__sync_fetch_and_add(&nr_held_now, 1);
+	return true;
+}
+
+void BPF_STRUCT_OPS(tickbug_enqueue, struct task_struct *p, u64 enq_flags)
+{
+	bool last = enq_flags & SCX_ENQ_LAST;
+	struct task_ctx *tctx;
+
+	__sync_fetch_and_add(&nr_enq, 1);
+	if (last)
+		__sync_fetch_and_add(&nr_last, 1);
+
+	/*
+	 * ops.init() kicks @timer_cpu so that ops.dispatch() runs there and
+	 * arms the timer, but a kick does not guarantee an ops.dispatch()
+	 * call: dispatch_one() keeps a @prev that still has slice left, and
+	 * takes the local DSQ when it is not empty, without asking the
+	 * scheduler. Retry from here, because a hold taken before the timer is
+	 * up would never be released.
+	 */
+	if (unlikely(!timer_started))
+		scx_bpf_kick_cpu(timer_cpu, SCX_KICK_PREEMPT);
+
+	/*
+	 * Per-CPU kthreads go to the head of their own CPU's local DSQ with
+	 * preemption. The release timer runs from a softirq, so if a kthread
+	 * such as ksoftirqd were starved behind a user task the held task
+	 * would never be released.
+	 */
+	if ((p->flags & PF_KTHREAD) && p->nr_cpus_allowed == 1) {
+		__sync_fetch_and_add(&nr_locals, 1);
+		scx_bpf_dsq_insert(p, SCX_DSQ_LOCAL, slice_ns,
+				   enq_flags | SCX_ENQ_PREEMPT);
+		return;
+	}
+
+	tctx = lookup_task_ctx(p);
+	if (!tctx)
+		return;
+
+	if (is_tagged(p) && handle_tagged(p, tctx, enq_flags))
+		return;
+
+	__sync_fetch_and_add(&nr_passthru, 1);
+
+	/*
+	 * A %SCX_ENQ_LAST task stays on its own CPU. put_prev_task_scx() sets
+	 * need_resched on the incoming idle task only if the enqueue landed on
+	 * the local DSQ, so a task left in a shared DSQ here would sit there
+	 * until some other CPU happened to drain it.
+	 */
+	if (last)
+		scx_bpf_dsq_insert(p, SCX_DSQ_LOCAL, slice_ns, enq_flags);
+	else
+		scx_bpf_dsq_insert(p, SHARED_DSQ, slice_ns, enq_flags);
+}
+
+void BPF_STRUCT_OPS(tickbug_dequeue, struct task_struct *p, u64 deq_flags)
+{
+	struct task_ctx *tctx;
+
+	tctx = bpf_task_storage_get(&task_ctx_stor, p, 0, 0);
+	if (!tctx || !tctx->held)
+		return;
+
+	/*
+	 * A parked task is leaving BPF custody without going through
+	 * release_one(): an affinity or property change, a class change, or
+	 * exit. Give up the hold now. Its entry stays in @hold_q, where
+	 * release_one() recognises it by this flag and drops it.
+	 */
+	tctx->held = false;
+	__sync_fetch_and_add(&nr_undone, 1);
+	__sync_fetch_and_sub(&nr_held_now, 1);
+}
+
+/* Let one held task run on the CPU we are dispatching for. */
+static bool release_one(void)
+{
+	struct hold_entry ent;
+	struct task_ctx *tctx;
+	struct task_struct *p;
+
+	bpf_repeat(BPF_MAX_LOOPS) {
+		u64 now;
+
+		if (bpf_map_pop_elem(&hold_q, &ent))
+			return false;
+
+		/*
+		 * Gone, or the pid belongs to someone else by now.
+		 * ops.dequeue() has already accounted the hold.
+		 */
+		p = bpf_task_from_pid(ent.pid);
+		if (!p) {
+			__sync_fetch_and_add(&nr_lost, 1);
+			continue;
+		}
+
+		tctx = bpf_task_storage_get(&task_ctx_stor, p, 0, 0);
+		if (!tctx || !tctx->held) {
+			__sync_fetch_and_add(&nr_stale, 1);
+			bpf_task_release(p);
+			continue;
+		}
+
+		tctx->held = false;
+		__sync_fetch_and_sub(&nr_held_now, 1);
+
+		/*
+		 * Affinity may have changed while the task sat in the queue.
+		 * Such a task goes to the shared DSQ instead of being pinned
+		 * to a CPU it is no longer allowed on.
+		 */
+		if (!bpf_cpumask_test_cpu(target_cpu, p->cpus_ptr) ||
+		    is_migration_disabled(p)) {
+			scx_bpf_dsq_insert(p, SHARED_DSQ, slice_ns, 0);
+			bpf_task_release(p);
+			if (!scx_bpf_dispatch_nr_slots())
+				return false;
+			continue;
+		}
+
+		scx_bpf_dsq_insert(p, SCX_DSQ_LOCAL, slice_ns, 0);
+		bpf_task_release(p);
+
+		now = scx_bpf_now();
+		if (ent.at && now > ent.at) {
+			u64 d = now - ent.at;
+
+			__sync_fetch_and_add(&total_held_ns, d);
+			if (d > max_held_ns)
+				max_held_ns = d;
+		}
+
+		__sync_fetch_and_add(&nr_released, 1);
+		return true;
+	}
+
+	return false;
+}
+
+static void start_release_timer(void)
+{
+	struct bpf_timer *timer;
+	u32 key = 0;
+	int ret;
+
+	if (likely(timer_started))
+		return;
+
+	/*
+	 * BPF_F_TIMER_CPU_PIN pins to the CPU running bpf_timer_start(), which
+	 * is not necessarily the CPU ops.dispatch() was called for: under core
+	 * scheduling a pick runs for every SMT sibling from one CPU. Arming it
+	 * from the wrong CPU could pin the timer to @target_cpu, which is the
+	 * one CPU the release path cannot run on.
+	 */
+	if (bpf_get_smp_processor_id() != timer_cpu)
+		return;
+
+	timer = bpf_map_lookup_elem(&release_timer, &key);
+	if (!timer) {
+		scx_bpf_error("failed to look up the release timer");
+		return;
+	}
+
+	ret = bpf_timer_start(timer, hold_ns, BPF_F_TIMER_CPU_PIN);
+	if (ret == -EINVAL) {
+		/* BPF_F_TIMER_CPU_PIN needs >= 6.7 */
+		timer_pinned = false;
+		ret = bpf_timer_start(timer, hold_ns, 0);
+	}
+	if (ret) {
+		scx_bpf_error("bpf_timer_start failed (%d)", ret);
+		return;
+	}
+
+	timer_started = true;
+}
+
+void BPF_STRUCT_OPS(tickbug_dispatch, s32 cpu, struct task_struct *prev)
+{
+	/*
+	 * Arm the timer from the CPU it must be pinned to. ops.init() runs in
+	 * whatever context loaded the scheduler, which is the wrong CPU.
+	 */
+	if (cpu == timer_cpu)
+		start_release_timer();
+
+	if (cpu == target_cpu) {
+		if (release_now) {
+			release_now = false;
+			if (release_one())
+				return;
+		}
+
+		/*
+		 * Leave @target_cpu in the idle loop for as long as it is
+		 * holding something. Taking shared work here would end the
+		 * idle stretch under test, and the hold reported for the task
+		 * would cover time the CPU spent running.
+		 */
+		if (nr_held_now)
+			return;
+	}
+
+	scx_bpf_dsq_move_to_local(SHARED_DSQ, 0);
+}
+
+static int release_timerfn(void *map, int *key, struct bpf_timer *timer)
+{
+	s32 curr_cpu = bpf_get_smp_processor_id();
+	int ret;
+
+	if (timer_pinned && curr_cpu != timer_cpu) {
+		scx_bpf_error("release timer ran on CPU %d, not timer CPU %d",
+			      curr_cpu, timer_cpu);
+		return 0;
+	}
+
+	/*
+	 * Nothing held means @target_cpu is either idle with an empty run
+	 * queue or busy with the task released last time. Kicking in either
+	 * case is at best pointless and at worst preempts that task.
+	 */
+	if (!nr_held_now)
+		goto rearm;
+
+	/*
+	 * Allow one held task through and make @target_cpu notice.
+	 * SCX_KICK_PREEMPT sends a reschedule IPI, which does not go through
+	 * irq_enter() and so does not run tick_irq_exit() itself: it only ends
+	 * the idle stretch whose bookkeeping the mis-gated interrupts dropped.
+	 */
+	release_now = true;
+	scx_bpf_kick_cpu(target_cpu, SCX_KICK_PREEMPT);
+
+rearm:
+
+	__sync_fetch_and_add(&nr_timers, 1);
+
+	ret = bpf_timer_start(timer, hold_ns,
+			      timer_pinned ? BPF_F_TIMER_CPU_PIN : 0);
+	if (ret)
+		scx_bpf_error("bpf_timer_start failed (%d)", ret);
+	return 0;
+}
+
+s32 BPF_STRUCT_OPS(tickbug_init_task, struct task_struct *p,
+		   struct scx_init_task_args *args)
+{
+	struct task_ctx *tctx;
+
+	/* Sleepable, so this allocation can use GFP_KERNEL. */
+	tctx = bpf_task_storage_get(&task_ctx_stor, p, 0,
+				    BPF_LOCAL_STORAGE_GET_F_CREATE);
+	if (!tctx)
+		return -ENOMEM;
+
+	tctx->held = false;
+
+	return 0;
+}
+
+int BPF_STRUCT_OPS_SLEEPABLE(tickbug_init)
+{
+	struct bpf_timer *timer;
+	u32 key = 0;
+	int ret;
+
+	if (target_cpu == timer_cpu) {
+		scx_bpf_error("target and timer CPU must differ (both %d)",
+			      target_cpu);
+		return -EINVAL;
+	}
+
+	ret = scx_bpf_create_dsq(SHARED_DSQ, -1);
+	if (ret) {
+		scx_bpf_error("scx_bpf_create_dsq failed (%d)", ret);
+		return ret;
+	}
+
+	timer = bpf_map_lookup_elem(&release_timer, &key);
+	if (!timer)
+		return -ESRCH;
+
+	bpf_timer_init(timer, &release_timer, CLOCK_MONOTONIC);
+	bpf_timer_set_callback(timer, release_timerfn);
+
+	/* Get ops.dispatch() to run on @timer_cpu so it arms the timer. */
+	scx_bpf_kick_cpu(timer_cpu, 0);
+
+	return 0;
+}
+
+void BPF_STRUCT_OPS(tickbug_exit, struct scx_exit_info *ei)
+{
+	UEI_RECORD(uei, ei);
+}
+
+SCX_OPS_DEFINE(tickbug_ops,
+	       /*
+		* The last runnable task on a CPU has to come through
+		* ops.enqueue() too, otherwise the tagged task would simply
+		* keep running on @target_cpu and never be held.
+		*/
+	       .flags			= SCX_OPS_ENQ_LAST,
+
+	       .select_cpu		= (void *)tickbug_select_cpu,
+	       .enqueue			= (void *)tickbug_enqueue,
+	       .dequeue			= (void *)tickbug_dequeue,
+	       .dispatch		= (void *)tickbug_dispatch,
+	       .init_task		= (void *)tickbug_init_task,
+	       .init			= (void *)tickbug_init,
+	       .exit			= (void *)tickbug_exit,
+	       /*
+		* A held task is runnable and not running, which is what the
+		* watchdog looks for. Keep the timeout well above any hold.
+		*/
+	       .timeout_ms		= 5000U,
+	       .name			= "tickbug");
diff --git a/tools/sched_ext/scx_tickbug.c b/tools/sched_ext/scx_tickbug.c
new file mode 100644
index 000000000000..1235611a33e1
--- /dev/null
+++ b/tools/sched_ext/scx_tickbug.c
@@ -0,0 +1,177 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Copyright (c) 2026 Radu Toader
+ */
+#define _GNU_SOURCE
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <inttypes.h>
+#include <signal.h>
+#include <assert.h>
+#include <libgen.h>
+#include <bpf/bpf.h>
+#include <scx/common.h>
+#include "scx_tickbug.bpf.skel.h"
+
+const char help_fmt[] =
+"Hold a task runnable on a CPU that is in the idle loop.\n"
+"\n"
+"Tasks named \"spin\" are queued on the target CPU's run queue and left\n"
+"there, so the CPU idles with a runnable task queued, until a periodic\n"
+"timer releases one. Every other task runs through a shared DSQ.\n"
+"\n"
+"See the top-level comment in .bpf.c for more details.\n"
+"\n"
+"Usage: %s [-c CPU] [-t CPU] [-H HOLD_MS] [-s SLICE_US] [-v]\n"
+"\n"
+"  -c CPU        Target CPU, the one that idles with a task queued (default: 0)\n"
+"  -t CPU        CPU the release timer is pinned to (default: 1)\n"
+"  -H HOLD_MS    How long a task is held before release (default: 10)\n"
+"  -s SLICE_US   Override slice duration\n"
+"  -v            Print libbpf debug messages\n"
+"  -h            Display this help and exit\n";
+
+static bool verbose;
+static volatile int exit_req;
+
+static int libbpf_print_fn(enum libbpf_print_level level, const char *format, va_list args)
+{
+	if (level == LIBBPF_DEBUG && !verbose)
+		return 0;
+	return vfprintf(stderr, format, args);
+}
+
+static void sigint_handler(int dummy)
+{
+	exit_req = 1;
+}
+
+int main(int argc, char **argv)
+{
+	struct scx_tickbug *skel;
+	struct bpf_link *link;
+	__u64 seq = 0, ecode;
+	__u32 nr_cpu_ids;
+	__s32 opt;
+
+	libbpf_set_print(libbpf_print_fn);
+	signal(SIGINT, sigint_handler);
+	signal(SIGTERM, sigint_handler);
+restart:
+	optind = 1;
+	skel = SCX_OPS_OPEN(tickbug_ops, scx_tickbug);
+
+	nr_cpu_ids = libbpf_num_possible_cpus();
+	assert(nr_cpu_ids > 0);
+	assert(nr_cpu_ids <= INT32_MAX);
+
+	skel->rodata->target_cpu = 0;
+	skel->rodata->timer_cpu = 1;
+	skel->rodata->hold_ns = 10ULL * 1000 * 1000;
+	skel->rodata->slice_ns = __COMPAT_ENUM_OR_ZERO("scx_public_consts", "SCX_SLICE_DFL");
+
+	while ((opt = getopt(argc, argv, "c:t:H:s:vh")) != -1) {
+		switch (opt) {
+		case 'c':
+		case 't': {
+			unsigned long cpu = strtoul(optarg, NULL, 0);
+
+			if (cpu >= nr_cpu_ids) {
+				fprintf(stderr, "invalid CPU id %lu (%u max)\n",
+					cpu, nr_cpu_ids - 1);
+				scx_tickbug__destroy(skel);
+				return 1;
+			}
+			if (opt == 'c')
+				skel->rodata->target_cpu = (s32)cpu;
+			else
+				skel->rodata->timer_cpu = (s32)cpu;
+			break;
+		}
+		case 'H':
+			skel->rodata->hold_ns = strtoull(optarg, NULL, 0) * 1000 * 1000;
+			break;
+		case 's':
+			skel->rodata->slice_ns = strtoull(optarg, NULL, 0) * 1000;
+			break;
+		case 'v':
+			verbose = true;
+			break;
+		default:
+			fprintf(stderr, help_fmt, basename(argv[0]));
+			scx_tickbug__destroy(skel);
+			return opt != 'h';
+		}
+	}
+
+	if (skel->rodata->target_cpu >= (s32)nr_cpu_ids ||
+	    skel->rodata->timer_cpu >= (s32)nr_cpu_ids) {
+		fprintf(stderr, "need CPUs 0 and 1 to exist: target %d, timer %d, %u online\n",
+			skel->rodata->target_cpu, skel->rodata->timer_cpu, nr_cpu_ids);
+		scx_tickbug__destroy(skel);
+		return 1;
+	}
+	if (skel->rodata->target_cpu == skel->rodata->timer_cpu) {
+		fprintf(stderr, "the target CPU and the timer CPU must differ (both %d)\n",
+			skel->rodata->target_cpu);
+		scx_tickbug__destroy(skel);
+		return 1;
+	}
+	if (!skel->rodata->hold_ns) {
+		fprintf(stderr, "a hold of 0 would not hold anything\n");
+		scx_tickbug__destroy(skel);
+		return 1;
+	}
+
+	SCX_OPS_LOAD(skel, tickbug_ops, scx_tickbug, uei);
+	link = SCX_OPS_ATTACH(skel, tickbug_ops, scx_tickbug);
+
+	printf("target_cpu=%d timer_cpu=%d hold_ms=%llu slice_us=%llu\n",
+	       skel->rodata->target_cpu, skel->rodata->timer_cpu,
+	       (unsigned long long)(skel->rodata->hold_ns / 1000000),
+	       (unsigned long long)(skel->rodata->slice_ns / 1000));
+
+	if (!skel->data->timer_pinned)
+		printf("WARNING: BPF_F_TIMER_CPU_PIN unavailable, release timer is not pinned\n");
+
+	while (!exit_req && !UEI_EXITED(skel, uei)) {
+		__u64 released = skel->bss->nr_released;
+
+		printf("[SEQ %llu]\n", seq++);
+		printf("enq    :%10" PRIu64 "   held:%10" PRIu64
+		       " released:%10" PRIu64 " timers:%10" PRIu64 "\n",
+		       skel->bss->nr_enq, skel->bss->nr_held,
+		       skel->bss->nr_released, skel->bss->nr_timers);
+		printf("pass   :%10" PRIu64 "  local:%10" PRIu64
+		       "     lost:%10" PRIu64 " othrcpu:%10" PRIu64
+		       " overflow:%10" PRIu64 " requeue:%10" PRIu64 "\n",
+		       skel->bss->nr_passthru, skel->bss->nr_locals,
+		       skel->bss->nr_lost, skel->bss->nr_other_cpu,
+		       skel->bss->nr_overflow, skel->bss->nr_requeue);
+		printf("last   :%10" PRIu64 "    dup:%10" PRIu64
+		       "    stale:%10" PRIu64 "  undone:%10" PRIu64
+		       "      now:%10" PRIu64 "\n",
+		       skel->bss->nr_last, skel->bss->nr_dup,
+		       skel->bss->nr_stale, skel->bss->nr_undone,
+		       skel->bss->nr_held_now);
+		/*
+		 * The mean and the worst hold are the independent variable of
+		 * the whole experiment: without them the run proves nothing.
+		 */
+		printf("hold_us: mean %8llu   max %8llu\n",
+		       (unsigned long long)(released ?
+			       (skel->bss->total_held_ns / released) / 1000 : 0),
+		       (unsigned long long)(skel->bss->max_held_ns / 1000));
+		fflush(stdout);
+		sleep(1);
+	}
+
+	bpf_link__destroy(link);
+	ecode = UEI_REPORT(skel, uei);
+	scx_tickbug__destroy(skel);
+
+	if (!exit_req && UEI_ECODE_RESTART(ecode))
+		goto restart;
+	return 0;
+}
-- 
2.43.0