[PATCHSET sched_ext/for-7.2-fixes] sched_ext: Fix finite-slice ticks on nohz_full

[PATCHSET sched_ext/for-7.2-fixes] sched_ext: Fix finite-slice ticks on nohz_full

[Andrea Righi]

When an EXT task with a finite slice is scheduled on a nohz_full CPU, the
scheduler may incorrectly keep the tick disabled because it still observes the
outgoing task (e.g., the idle task) during the context switch. As a result, the
incoming task can run past its assigned slice.

Fix this by always enabling the scheduler tick for finite-slice EXT tasks and
provide a kselftest that reproduces the scenario and verifies that scheduler
ticks are delivered correctly. Test is skipped in kernels without available
nohz_full CPUS.

Test case (using virtme-ng):

  $ vng -vb --config tools/testing/selftests/sched_ext/config \
        --configitem CONFIG_NO_HZ_FULL=y
  $ vng --append "nohz_full=all" -- \
        tools/testing/selftests/sched_ext/runner -t nohz_tick

Before:

  $ vng --append "nohz_full=all" -- tools/testing/selftests/sched_ext/runner -t nohz_tick
  ===== START =====
  TEST: nohz_tick
  DESCRIPTION: Verify finite EXT slices restart the NOHZ_FULL tick
  OUTPUT:
  ERR: nohz_tick.c:270
  Finite-slice worker received only 1 scheduler ticks
  not ok 1 nohz_tick #
  =====  END  =====


  =============================

  RESULTS:

  PASSED:  0
  SKIPPED: 0
  FAILED:  1

  Failed tests:
    - nohz_tick

After:

  $ vng --append "nohz_full=all" -- tools/testing/selftests/sched_ext/runner -t nohz_tick
  ===== START =====
  TEST: nohz_tick
  DESCRIPTION: Verify finite EXT slices restart the NOHZ_FULL tick
  OUTPUT:
  CPU 1 received 4 finite-slice ticks
  ok 1 nohz_tick #
  =====  END  =====


  =============================

  RESULTS:

  PASSED:  1
  SKIPPED: 0
  FAILED:  0

Andrea Righi (2):
  sched_ext: Enable tick for finite slices on nohz_full
  selftests/sched_ext: Verify nohz_full tick behavior

 kernel/sched/ext/ext.c                            |  14 +-
 tools/testing/selftests/sched_ext/Makefile        |   1 +
 tools/testing/selftests/sched_ext/nohz_tick.bpf.c |  65 +++++
 tools/testing/selftests/sched_ext/nohz_tick.c     | 309 ++++++++++++++++++++++
 4 files changed, 386 insertions(+), 3 deletions(-)

[PATCH 1/2] sched_ext: Enable tick for finite slices on nohz_full

[Andrea Righi]

set_next_task_scx() updates the tick dependency before __schedule()
updates rq->curr. When switching from a non-EXT task, such as idle, to
an EXT task with a finite slice, sched_update_tick_dependency() checks
the outgoing task and can allow the tick to remain stopped. The incoming
task can then fail to receive the ticks needed to expire its slice.

A finite slice is sufficient by itself to require the tick, so set
TICK_DEP_BIT_SCHED directly when transitioning to a finite slice. Keep
using sched_update_tick_dependency() when transitioning to an infinite
slice so that the scheduler evaluates all runqueue requirements before
clearing the dependency.

Fixes: 22a920209ab6 ("sched_ext: Implement tickless support")
Signed-off-by: Andrea Righi <arighi@nvidia.com>
---
 kernel/sched/ext/ext.c | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/ext/ext.c b/kernel/sched/ext/ext.c
index e75e2fd5ab7e3..dc20a8eed8f6c 100644
--- a/kernel/sched/ext/ext.c
+++ b/kernel/sched/ext/ext.c
@@ -2982,12 +2982,20 @@ static void set_next_task_scx(struct rq *rq, struct task_struct *p, bool first)
 	 */
 	if ((p->scx.slice == SCX_SLICE_INF) !=
 	    (bool)(rq->scx.flags & SCX_RQ_CAN_STOP_TICK)) {
-		if (p->scx.slice == SCX_SLICE_INF)
+		if (p->scx.slice == SCX_SLICE_INF) {
 			rq->scx.flags |= SCX_RQ_CAN_STOP_TICK;
-		else
+			sched_update_tick_dependency(rq);
+		} else {
 			rq->scx.flags &= ~SCX_RQ_CAN_STOP_TICK;
 
-		sched_update_tick_dependency(rq);
+			/*
+			 * The rq still references the outgoing scheduling context.
+			 * A finite slice is sufficient by itself to require the tick.
+			 */
+			if (tick_nohz_full_cpu(cpu_of(rq)))
+				tick_nohz_dep_set_cpu(cpu_of(rq),
+							TICK_DEP_BIT_SCHED);
+		}
 
 		/*
 		 * For now, let's refresh the load_avgs just when transitioning
-- 
2.55.0

[PATCH 2/2] selftests/sched_ext: Verify nohz_full tick behavior

[Andrea Righi]

A finite-slice EXT task needs the periodic scheduler tick to expire its
slice even when nohz_full is enabled.

Add a regression test to validate the proper nohz_full behavior:
 - the test selects a nohz_full CPU and first runs an EXT task with an
   infinite slice,
 - the task then is stopped, leaving the CPU idle with the tick stopped,
 - then run an EXT task with a finite slice and verify that its
   ops.tick() callback is invoked.

Skip the test when an allowed nohz_full CPU and a separate housekeeping
CPU are not available.

Signed-off-by: Andrea Righi <arighi@nvidia.com>
---
 tools/testing/selftests/sched_ext/Makefile    |   1 +
 .../selftests/sched_ext/nohz_tick.bpf.c       |  65 ++++
 tools/testing/selftests/sched_ext/nohz_tick.c | 309 ++++++++++++++++++
 3 files changed, 375 insertions(+)
 create mode 100644 tools/testing/selftests/sched_ext/nohz_tick.bpf.c
 create mode 100644 tools/testing/selftests/sched_ext/nohz_tick.c

diff --git a/tools/testing/selftests/sched_ext/Makefile b/tools/testing/selftests/sched_ext/Makefile
index 5d2dffca0e918..3cfe90e0f34fa 100644
--- a/tools/testing/selftests/sched_ext/Makefile
+++ b/tools/testing/selftests/sched_ext/Makefile
@@ -176,6 +176,7 @@ auto-test-targets :=			\
 	maybe_null			\
 	minimal				\
 	non_scx_kfunc_deny		\
+	nohz_tick			\
 	numa				\
 	allowed_cpus			\
 	peek_dsq			\
diff --git a/tools/testing/selftests/sched_ext/nohz_tick.bpf.c b/tools/testing/selftests/sched_ext/nohz_tick.bpf.c
new file mode 100644
index 0000000000000..6998c5dd6bcb9
--- /dev/null
+++ b/tools/testing/selftests/sched_ext/nohz_tick.bpf.c
@@ -0,0 +1,65 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (c) 2026 NVIDIA CORPORATION & AFFILIATES
+ *
+ * Exercise tick dependency transitions between infinite and finite slices.
+ */
+#include <scx/common.bpf.h>
+
+char _license[] SEC("license") = "GPL";
+
+const volatile s32 test_cpu;
+bool finite_phase;
+u64 nr_inf_running;
+u64 nr_finite_running;
+u64 nr_finite_ticks;
+
+UEI_DEFINE(uei);
+
+s32 BPF_STRUCT_OPS(nohz_tick_select_cpu, struct task_struct *p, s32 prev_cpu,
+		   u64 wake_flags)
+{
+	return prev_cpu;
+}
+
+void BPF_STRUCT_OPS(nohz_tick_enqueue, struct task_struct *p, u64 enq_flags)
+{
+	u64 slice = finite_phase ? 1000000ULL : SCX_SLICE_INF;
+
+	scx_bpf_dsq_insert(p, SCX_DSQ_GLOBAL, slice, enq_flags);
+	if (enq_flags & SCX_ENQ_LAST)
+		scx_bpf_kick_cpu(test_cpu, SCX_KICK_IDLE);
+}
+
+void BPF_STRUCT_OPS(nohz_tick_running, struct task_struct *p)
+{
+	if (bpf_get_smp_processor_id() != test_cpu)
+		return;
+
+	if (finite_phase)
+		__sync_fetch_and_add(&nr_finite_running, 1);
+	else
+		__sync_fetch_and_add(&nr_inf_running, 1);
+}
+
+void BPF_STRUCT_OPS(nohz_tick_tick, struct task_struct *p)
+{
+	if (bpf_get_smp_processor_id() == test_cpu && finite_phase)
+		__sync_fetch_and_add(&nr_finite_ticks, 1);
+}
+
+void BPF_STRUCT_OPS(nohz_tick_exit, struct scx_exit_info *ei)
+{
+	UEI_RECORD(uei, ei);
+}
+
+SEC(".struct_ops.link")
+struct sched_ext_ops nohz_tick_ops = {
+	.select_cpu		= (void *)nohz_tick_select_cpu,
+	.enqueue		= (void *)nohz_tick_enqueue,
+	.running		= (void *)nohz_tick_running,
+	.tick			= (void *)nohz_tick_tick,
+	.exit			= (void *)nohz_tick_exit,
+	.name			= "nohz_tick",
+	.timeout_ms		= 1000U,
+};
diff --git a/tools/testing/selftests/sched_ext/nohz_tick.c b/tools/testing/selftests/sched_ext/nohz_tick.c
new file mode 100644
index 0000000000000..5921bb9b9e83a
--- /dev/null
+++ b/tools/testing/selftests/sched_ext/nohz_tick.c
@@ -0,0 +1,309 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (c) 2026 NVIDIA CORPORATION & AFFILIATES
+ *
+ * Validate that a finite-slice EXT task restarts the scheduler tick when it
+ * follows an infinite-slice EXT task and an idle interval on a NOHZ_FULL CPU.
+ */
+#define _GNU_SOURCE
+
+#include <bpf/bpf.h>
+#include <errno.h>
+#include <sched.h>
+#include <signal.h>
+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/wait.h>
+#include <unistd.h>
+
+#include <scx/common.h>
+
+#include "nohz_tick.bpf.skel.h"
+#include "scx_test.h"
+
+#ifndef SCHED_EXT
+#define SCHED_EXT 7
+#endif
+
+#define MIN_FINITE_TICKS 3
+#define PHASE_TIMEOUT_MS 1000
+
+struct nohz_tick_ctx {
+	struct nohz_tick *skel;
+	cpu_set_t original_mask;
+	int test_cpu;
+};
+
+static int first_allowed_cpu(const cpu_set_t *mask, int first, int last)
+{
+	int cpu;
+
+	for (cpu = first; cpu <= last && cpu < CPU_SETSIZE; cpu++)
+		if (CPU_ISSET(cpu, mask))
+			return cpu;
+
+	return -1;
+}
+
+static int find_nohz_full_cpu(const cpu_set_t *allowed)
+{
+	char buf[4096], *cur, *end;
+	FILE *file;
+
+	file = fopen("/sys/devices/system/cpu/nohz_full", "r");
+	if (!file)
+		return -1;
+	if (!fgets(buf, sizeof(buf), file)) {
+		fclose(file);
+		return -1;
+	}
+	fclose(file);
+
+	cur = buf;
+	while (*cur) {
+		long first, last;
+		int cpu;
+
+		while (*cur == ' ' || *cur == '\t' || *cur == ',')
+			cur++;
+		if (*cur < '0' || *cur > '9')
+			break;
+
+		errno = 0;
+		first = strtol(cur, &end, 10);
+		if (errno || end == cur || first < 0 || first >= CPU_SETSIZE)
+			return -1;
+		cur = end;
+		last = first;
+		if (*cur == '-') {
+			cur++;
+			errno = 0;
+			last = strtol(cur, &end, 10);
+			if (errno || end == cur || last < first)
+				return -1;
+			cur = end;
+		}
+
+		cpu = first_allowed_cpu(allowed, first, last);
+		if (cpu >= 0)
+			return cpu;
+	}
+
+	return -1;
+}
+
+static pid_t start_worker(int cpu)
+{
+	struct sched_param param = {};
+	cpu_set_t mask;
+	pid_t pid;
+
+	pid = fork();
+	if (pid != 0)
+		return pid;
+
+	/*
+	 * Become EXT before touching the target so it stays idle until wakeup.
+	 */
+	if (sched_setscheduler(0, SCHED_EXT, &param))
+		_exit(1);
+
+	CPU_ZERO(&mask);
+	CPU_SET(cpu, &mask);
+	if (sched_setaffinity(0, sizeof(mask), &mask))
+		_exit(1);
+
+	for (;;)
+		asm volatile("" ::: "memory");
+}
+
+static void stop_worker(pid_t pid)
+{
+	if (pid <= 0)
+		return;
+
+	kill(pid, SIGKILL);
+	waitpid(pid, NULL, 0);
+}
+
+static int pause_worker(pid_t pid)
+{
+	int status;
+
+	if (kill(pid, SIGSTOP))
+		return -errno;
+	if (waitpid(pid, &status, WUNTRACED) != pid)
+		return -errno;
+	if (!WIFSTOPPED(status))
+		return -ECHILD;
+
+	return 0;
+}
+
+static bool wait_for_counter(const u64 *counter, u64 value, int timeout_ms)
+{
+	int elapsed;
+
+	for (elapsed = 0; elapsed < timeout_ms; elapsed++) {
+		if (__atomic_load_n(counter, __ATOMIC_RELAXED) >= value)
+			return true;
+		usleep(1000);
+	}
+
+	return false;
+}
+
+static enum scx_test_status setup(void **ctx_ptr)
+{
+	struct nohz_tick_ctx *ctx;
+	cpu_set_t controller_mask;
+	int cpu;
+
+	ctx = calloc(1, sizeof(*ctx));
+	SCX_FAIL_IF(!ctx, "Failed to allocate context");
+	if (sched_getaffinity(0, sizeof(ctx->original_mask),
+			      &ctx->original_mask)) {
+		free(ctx);
+		SCX_FAIL("Failed to get affinity (%d)", errno);
+	}
+
+	cpu = find_nohz_full_cpu(&ctx->original_mask);
+	if (cpu < 0) {
+		fprintf(stderr, "SKIP: no allowed NOHZ_FULL CPU\n");
+		free(ctx);
+		return SCX_TEST_SKIP;
+	}
+
+	controller_mask = ctx->original_mask;
+	CPU_CLR(cpu, &controller_mask);
+	if (CPU_COUNT(&controller_mask) == 0) {
+		fprintf(stderr, "SKIP: no housekeeping CPU available\n");
+		free(ctx);
+		return SCX_TEST_SKIP;
+	}
+
+	ctx->test_cpu = cpu;
+	ctx->skel = nohz_tick__open();
+	if (!ctx->skel) {
+		free(ctx);
+		SCX_FAIL("Failed to open skeleton");
+	}
+
+	SCX_ENUM_INIT(ctx->skel);
+	ctx->skel->rodata->test_cpu = cpu;
+	ctx->skel->struct_ops.nohz_tick_ops->flags |= SCX_OPS_SWITCH_PARTIAL |
+							   SCX_OPS_ENQ_LAST;
+	if (nohz_tick__load(ctx->skel)) {
+		nohz_tick__destroy(ctx->skel);
+		free(ctx);
+		SCX_FAIL("Failed to load skeleton");
+	}
+
+	if (sched_setaffinity(0, sizeof(controller_mask), &controller_mask)) {
+		nohz_tick__destroy(ctx->skel);
+		free(ctx);
+		SCX_FAIL("Failed to move controller off CPU %d (%d)", cpu, errno);
+	}
+
+	*ctx_ptr = ctx;
+	return SCX_TEST_PASS;
+}
+
+static enum scx_test_status run(void *ctx_ptr)
+{
+	struct nohz_tick_ctx *ctx = ctx_ptr;
+	struct nohz_tick *skel = ctx->skel;
+	struct bpf_link *link = NULL;
+	enum scx_test_status status = SCX_TEST_FAIL;
+	pid_t finite_worker = -1;
+	pid_t inf_worker = -1;
+	int ret;
+
+	link = bpf_map__attach_struct_ops(skel->maps.nohz_tick_ops);
+	if (!link) {
+		SCX_ERR("Failed to attach scheduler");
+		goto out;
+	}
+
+	/*
+	 * Establish SCX_RQ_CAN_STOP_TICK with an infinite-slice task.
+	 */
+	inf_worker = start_worker(ctx->test_cpu);
+	if (inf_worker < 0) {
+		SCX_ERR("Failed to start infinite-slice worker (%d)", errno);
+		goto out;
+	}
+	if (!wait_for_counter(&skel->bss->nr_inf_running, 1,
+			      PHASE_TIMEOUT_MS)) {
+		SCX_ERR("Infinite-slice worker was not scheduled");
+		goto out;
+	}
+
+	/* Block without exiting so the rq retains the infinite-slice state. */
+	ret = pause_worker(inf_worker);
+	if (ret) {
+		SCX_ERR("Failed to stop infinite-slice worker (%d)", ret);
+		goto out;
+	}
+
+	/* Let the target enter idle with its tick stopped. */
+	usleep(100000);
+
+	/*
+	 * The next EXT task receives a finite slice and must restart the tick.
+	 */
+	__atomic_store_n(&skel->bss->finite_phase, true, __ATOMIC_RELEASE);
+	finite_worker = start_worker(ctx->test_cpu);
+	if (finite_worker < 0) {
+		SCX_ERR("Failed to start finite-slice worker (%d)", errno);
+		goto out;
+	}
+	if (!wait_for_counter(&skel->bss->nr_finite_running, 1,
+			      PHASE_TIMEOUT_MS)) {
+		SCX_ERR("Finite-slice worker was not scheduled");
+		goto out;
+	}
+	if (!wait_for_counter(&skel->bss->nr_finite_ticks, MIN_FINITE_TICKS,
+			      PHASE_TIMEOUT_MS)) {
+		SCX_ERR("Finite-slice worker received only %llu scheduler ticks",
+			(unsigned long long)skel->bss->nr_finite_ticks);
+		goto out;
+	}
+
+	if (skel->data->uei.kind != EXIT_KIND(SCX_EXIT_NONE)) {
+		SCX_ERR("Scheduler exited unexpectedly (kind=%llu code=%lld)",
+			(unsigned long long)skel->data->uei.kind,
+			(long long)skel->data->uei.exit_code);
+		goto out;
+	}
+
+	fprintf(stderr, "CPU %d received %llu finite-slice ticks\n",
+		ctx->test_cpu,
+		(unsigned long long)skel->bss->nr_finite_ticks);
+	status = SCX_TEST_PASS;
+out:
+	stop_worker(finite_worker);
+	stop_worker(inf_worker);
+	if (link)
+		bpf_link__destroy(link);
+	return status;
+}
+
+static void cleanup(void *ctx_ptr)
+{
+	struct nohz_tick_ctx *ctx = ctx_ptr;
+
+	sched_setaffinity(0, sizeof(ctx->original_mask), &ctx->original_mask);
+	nohz_tick__destroy(ctx->skel);
+	free(ctx);
+}
+
+struct scx_test nohz_tick = {
+	.name = "nohz_tick",
+	.description = "Verify finite EXT slices restart the NOHZ_FULL tick",
+	.setup = setup,
+	.run = run,
+	.cleanup = cleanup,
+};
+REGISTER_SCX_TEST(&nohz_tick)
-- 
2.55.0

Re: [PATCH 1/2] sched_ext: Enable tick for finite slices on nohz_full

[Tejun Heo]

Hello, Andrea.

Sashiko's first point looks real to me. The fix covers infinite->finite,
but a finite slice landing on a nohz_full CPU after idle has the same
issue whenever the previous run wasn't infinite (finite->idle->finite):
the enqueue path already cleared TICK_DEP_BIT_SCHED against the idle
rq->curr, and set_next_task_scx() re-asserts only on a slice-type
transition, so with SCX_RQ_CAN_STOP_TICK unchanged the tick stays
stopped.

set_next_task_scx() knows the incoming task, so it can assert directly
rather than keying off the transition, like sched_fair_update_stop_tick()
does for CFS. Moving the finite assertion out of the transition branch
should do it:

	if (p->scx.slice != SCX_SLICE_INF && tick_nohz_full_cpu(cpu_of(rq)))
		tick_nohz_dep_set_cpu(cpu_of(rq), TICK_DEP_BIT_SCHED);

Can you fold that in and add a finite->idle->finite selftest case for v2?

Thanks.

--
tejun