2 files changed, 181 insertions, 7 deletions

diff --git a/tools/sched_ext/scx_central.bpf.c b/tools/sched_ext/scx_central.bpf.c
index 428b2262faa3..1d8fd570eaa7 100644
--- a/tools/sched_ext/scx_central.bpf.c
+++ b/tools/sched_ext/scx_central.bpf.c
@@ -13,7 +13,26 @@
  *    through per-CPU BPF queues. The current design is chosen to maximally
  *    utilize and verify various SCX mechanisms such as LOCAL_ON dispatching.
  *
- * b. Preemption
+ * b. Tickless operation
+ *
+ *    All tasks are dispatched with the infinite slice which allows stopping the
+ *    ticks on CONFIG_NO_HZ_FULL kernels running with the proper nohz_full
+ *    parameter. The tickless operation can be observed through
+ *    /proc/interrupts.
+ *
+ *    Periodic switching is enforced by a periodic timer checking all CPUs and
+ *    preempting them as necessary. Unfortunately, BPF timer currently doesn't
+ *    have a way to pin to a specific CPU, so the periodic timer isn't pinned to
+ *    the central CPU.
+ *
+ * c. Preemption
+ *
+ *    Kthreads are unconditionally queued to the head of a matching local dsq
+ *    and dispatched with SCX_DSQ_PREEMPT. This ensures that a kthread is always
+ *    prioritized over user threads, which is required for ensuring forward
+ *    progress as e.g. the periodic timer may run on a ksoftirqd and if the
+ *    ksoftirqd gets starved by a user thread, there may not be anything else to
+ *    vacate that user thread.
  *
  *    SCX_KICK_PREEMPT is used to trigger scheduling and CPUs to move to the
  *    next tasks.
@@ -32,14 +51,17 @@ char _license[] SEC("license") = "GPL";
 
 enum {
 	FALLBACK_DSQ_ID		= 0,
+	MS_TO_NS		= 1000LLU * 1000,
+	TIMER_INTERVAL_NS	= 1 * MS_TO_NS,
 };
 
 const volatile s32 central_cpu;
 const volatile u32 nr_cpu_ids = 1;	/* !0 for veristat, set during init */
 const volatile u64 slice_ns = SCX_SLICE_DFL;
 
+bool timer_pinned = true;
 u64 nr_total, nr_locals, nr_queued, nr_lost_pids;
-u64 nr_dispatches, nr_mismatches, nr_retries;
+u64 nr_timers, nr_dispatches, nr_mismatches, nr_retries;
 u64 nr_overflows;
 
 UEI_DEFINE(uei);
@@ -52,6 +74,23 @@ struct {
 
 /* can't use percpu map due to bad lookups */
 bool RESIZABLE_ARRAY(data, cpu_gimme_task);
+u64 RESIZABLE_ARRAY(data, cpu_started_at);
+
+struct central_timer {
+	struct bpf_timer timer;
+};
+
+struct {
+	__uint(type, BPF_MAP_TYPE_ARRAY);
+	__uint(max_entries, 1);
+	__type(key, u32);
+	__type(value, struct central_timer);
+} central_timer SEC(".maps");
+
+static bool vtime_before(u64 a, u64 b)
+{
+	return (s64)(a - b) < 0;
+}
 
 s32 BPF_STRUCT_OPS(central_select_cpu, struct task_struct *p,
 		   s32 prev_cpu, u64 wake_flags)
@@ -71,9 +110,22 @@ void BPF_STRUCT_OPS(central_enqueue, struct task_struct *p, u64 enq_flags)
 
 	__sync_fetch_and_add(&nr_total, 1);
 
+	/*
+	 * Push per-cpu kthreads at the head of local dsq's and preempt the
+	 * corresponding CPU. This ensures that e.g. ksoftirqd isn't blocked
+	 * behind other threads which is necessary for forward progress
+	 * guarantee as we depend on the BPF timer which may run from ksoftirqd.
+	 */
+	if ((p->flags & PF_KTHREAD) && p->nr_cpus_allowed == 1) {
+		__sync_fetch_and_add(&nr_locals, 1);
+		scx_bpf_dispatch(p, SCX_DSQ_LOCAL, SCX_SLICE_INF,
+				 enq_flags | SCX_ENQ_PREEMPT);
+		return;
+	}
+
 	if (bpf_map_push_elem(&central_q, &pid, 0)) {
 		__sync_fetch_and_add(&nr_overflows, 1);
-		scx_bpf_dispatch(p, FALLBACK_DSQ_ID, SCX_SLICE_DFL, enq_flags);
+		scx_bpf_dispatch(p, FALLBACK_DSQ_ID, SCX_SLICE_INF, enq_flags);
 		return;
 	}
 
@@ -106,7 +158,7 @@ static bool dispatch_to_cpu(s32 cpu)
 		 */
 		if (!bpf_cpumask_test_cpu(cpu, p->cpus_ptr)) {
 			__sync_fetch_and_add(&nr_mismatches, 1);
-			scx_bpf_dispatch(p, FALLBACK_DSQ_ID, SCX_SLICE_DFL, 0);
+			scx_bpf_dispatch(p, FALLBACK_DSQ_ID, SCX_SLICE_INF, 0);
 			bpf_task_release(p);
 			/*
 			 * We might run out of dispatch buffer slots if we continue dispatching
@@ -120,7 +172,7 @@ static bool dispatch_to_cpu(s32 cpu)
 		}
 
 		/* dispatch to local and mark that @cpu doesn't need more */
-		scx_bpf_dispatch(p, SCX_DSQ_LOCAL_ON | cpu, SCX_SLICE_DFL, 0);
+		scx_bpf_dispatch(p, SCX_DSQ_LOCAL_ON | cpu, SCX_SLICE_INF, 0);
 
 		if (cpu != central_cpu)
 			scx_bpf_kick_cpu(cpu, SCX_KICK_IDLE);
@@ -188,9 +240,102 @@ void BPF_STRUCT_OPS(central_dispatch, s32 cpu, struct task_struct *prev)
 	}
 }
 
+void BPF_STRUCT_OPS(central_running, struct task_struct *p)
+{
+	s32 cpu = scx_bpf_task_cpu(p);
+	u64 *started_at = ARRAY_ELEM_PTR(cpu_started_at, cpu, nr_cpu_ids);
+	if (started_at)
+		*started_at = bpf_ktime_get_ns() ?: 1;	/* 0 indicates idle */
+}
+
+void BPF_STRUCT_OPS(central_stopping, struct task_struct *p, bool runnable)
+{
+	s32 cpu = scx_bpf_task_cpu(p);
+	u64 *started_at = ARRAY_ELEM_PTR(cpu_started_at, cpu, nr_cpu_ids);
+	if (started_at)
+		*started_at = 0;
+}
+
+static int central_timerfn(void *map, int *key, struct bpf_timer *timer)
+{
+	u64 now = bpf_ktime_get_ns();
+	u64 nr_to_kick = nr_queued;
+	s32 i, curr_cpu;
+
+	curr_cpu = bpf_get_smp_processor_id();
+	if (timer_pinned && (curr_cpu != central_cpu)) {
+		scx_bpf_error("Central timer ran on CPU %d, not central CPU %d",
+			      curr_cpu, central_cpu);
+		return 0;
+	}
+
+	bpf_for(i, 0, nr_cpu_ids) {
+		s32 cpu = (nr_timers + i) % nr_cpu_ids;
+		u64 *started_at;
+
+		if (cpu == central_cpu)
+			continue;
+
+		/* kick iff the current one exhausted its slice */
+		started_at = ARRAY_ELEM_PTR(cpu_started_at, cpu, nr_cpu_ids);
+		if (started_at && *started_at &&
+		    vtime_before(now, *started_at + slice_ns))
+			continue;
+
+		/* and there's something pending */
+		if (scx_bpf_dsq_nr_queued(FALLBACK_DSQ_ID) ||
+		    scx_bpf_dsq_nr_queued(SCX_DSQ_LOCAL_ON | cpu))
+			;
+		else if (nr_to_kick)
+			nr_to_kick--;
+		else
+			continue;
+
+		scx_bpf_kick_cpu(cpu, SCX_KICK_PREEMPT);
+	}
+
+	bpf_timer_start(timer, TIMER_INTERVAL_NS, BPF_F_TIMER_CPU_PIN);
+	__sync_fetch_and_add(&nr_timers, 1);
+	return 0;
+}
+
 int BPF_STRUCT_OPS_SLEEPABLE(central_init)
 {
-	return scx_bpf_create_dsq(FALLBACK_DSQ_ID, -1);
+	u32 key = 0;
+	struct bpf_timer *timer;
+	int ret;
+
+	ret = scx_bpf_create_dsq(FALLBACK_DSQ_ID, -1);
+	if (ret)
+		return ret;
+
+	timer = bpf_map_lookup_elem(&central_timer, &key);
+	if (!timer)
+		return -ESRCH;
+
+	if (bpf_get_smp_processor_id() != central_cpu) {
+		scx_bpf_error("init from non-central CPU");
+		return -EINVAL;
+	}
+
+	bpf_timer_init(timer, &central_timer, CLOCK_MONOTONIC);
+	bpf_timer_set_callback(timer, central_timerfn);
+
+	ret = bpf_timer_start(timer, TIMER_INTERVAL_NS, BPF_F_TIMER_CPU_PIN);
+	/*
+	 * BPF_F_TIMER_CPU_PIN is pretty new (>=6.7). If we're running in a
+	 * kernel which doesn't have it, bpf_timer_start() will return -EINVAL.
+	 * Retry without the PIN. This would be the perfect use case for
+	 * bpf_core_enum_value_exists() but the enum type doesn't have a name
+	 * and can't be used with bpf_core_enum_value_exists(). Oh well...
+	 */
+	if (ret == -EINVAL) {
+		timer_pinned = false;
+		ret = bpf_timer_start(timer, TIMER_INTERVAL_NS, 0);
+	}
+	if (ret)
+		scx_bpf_error("bpf_timer_start failed (%d)", ret);
+	return ret;
 }
 
 void BPF_STRUCT_OPS(central_exit, struct scx_exit_info *ei)
@@ -209,6 +354,8 @@ SCX_OPS_DEFINE(central_ops,
 	       .select_cpu		= (void *)central_select_cpu,
 	       .enqueue			= (void *)central_enqueue,
 	       .dispatch		= (void *)central_dispatch,
+	       .running			= (void *)central_running,
+	       .stopping		= (void *)central_stopping,
 	       .init			= (void *)central_init,
 	       .exit			= (void *)central_exit,
 	       .name			= "central");
diff --git a/tools/sched_ext/scx_central.c b/tools/sched_ext/scx_central.c
index 5f09fc666a63..fb3f50886552 100644
--- a/tools/sched_ext/scx_central.c
+++ b/tools/sched_ext/scx_central.c
@@ -48,6 +48,7 @@ int main(int argc, char **argv)
 	struct bpf_link *link;
 	__u64 seq = 0;
 	__s32 opt;
+	cpu_set_t *cpuset;
 
 	libbpf_set_print(libbpf_print_fn);
 	signal(SIGINT, sigint_handler);
@@ -77,10 +78,35 @@ int main(int argc, char **argv)
 
 	/* Resize arrays so their element count is equal to cpu count. */
 	RESIZE_ARRAY(skel, data, cpu_gimme_task, skel->rodata->nr_cpu_ids);
+	RESIZE_ARRAY(skel, data, cpu_started_at, skel->rodata->nr_cpu_ids);
 
 	SCX_OPS_LOAD(skel, central_ops, scx_central, uei);
+
+	/*
+	 * Affinitize the loading thread to the central CPU, as:
+	 * - That's where the BPF timer is first invoked in the BPF program.
+	 * - We probably don't want this user space component to take up a core
+	 *   from a task that would benefit from avoiding preemption on one of
+	 *   the tickless cores.
+	 *
+	 * Until BPF supports pinning the timer, it's not guaranteed that it
+	 * will always be invoked on the central CPU. In practice, this
+	 * suffices the majority of the time.
+	 */
+	cpuset = CPU_ALLOC(skel->rodata->nr_cpu_ids);
+	SCX_BUG_ON(!cpuset, "Failed to allocate cpuset");
+	CPU_ZERO(cpuset);
+	CPU_SET(skel->rodata->central_cpu, cpuset);
+	SCX_BUG_ON(sched_setaffinity(0, sizeof(cpuset), cpuset),
+		   "Failed to affinitize to central CPU %d (max %d)",
+		   skel->rodata->central_cpu, skel->rodata->nr_cpu_ids - 1);
+	CPU_FREE(cpuset);
+
 	link = SCX_OPS_ATTACH(skel, central_ops, scx_central);
 
+	if (!skel->data->timer_pinned)
+		printf("WARNING : BPF_F_TIMER_CPU_PIN not available, timer not pinned to central\n");
+
 	while (!exit_req && !UEI_EXITED(skel, uei)) {
 		printf("[SEQ %llu]\n", seq++);
 		printf("total   :%10" PRIu64 "    local:%10" PRIu64 "   queued:%10" PRIu64 "  lost:%10" PRIu64 "\n",
@@ -88,7 +114,8 @@ int main(int argc, char **argv)
 		       skel->bss->nr_locals,
 		       skel->bss->nr_queued,
 		       skel->bss->nr_lost_pids);
-		printf("                    dispatch:%10" PRIu64 " mismatch:%10" PRIu64 " retry:%10" PRIu64 "\n",
+		printf("timer   :%10" PRIu64 " dispatch:%10" PRIu64 " mismatch:%10" PRIu64 " retry:%10" PRIu64 "\n",
+		       skel->bss->nr_timers,
 		       skel->bss->nr_dispatches,
 		       skel->bss->nr_mismatches,
 		       skel->bss->nr_retries);