Re: [PATCH] time/tick-broadcast: Fix tick_broadcast_offline() lockdep complaint

[Paul E. McKenney]

On Tue, Jun 25, 2019 at 02:25:16PM +0200, Frederic Weisbecker wrote:
> On Tue, Jun 25, 2019 at 09:51:39AM +0200, Peter Zijlstra wrote:
> > On Tue, Jun 25, 2019 at 02:43:00AM +0200, Frederic Weisbecker wrote:
> > > Yeah, unfortunately there is no atomic_add_not_zero_return().
> > 
> > There is atomic_fetch_add_unless().
> 
> Ah, that could work!

And it allows dispensing with the initialization.  How about like
the following?

							Thanx, Paul

------------------------------------------------------------------------

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 102dfcf0a29a..d7be6d4b6a0a 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3050,8 +3050,36 @@ void scheduler_tick(void)
 
 struct tick_work {
 	int			cpu;
+	atomic_t		state;
 	struct delayed_work	work;
 };
+/* Values for ->state, see diagram below. */
+#define TICK_SCHED_REMOTE_OFFLINE	0
+#define TICK_SCHED_REMOTE_OFFLINING	1
+#define TICK_SCHED_REMOTE_RUNNING	2
+
+/*
+ * State diagram for ->state:
+ *
+ *
+ *          TICK_SCHED_REMOTE_OFFLINE
+ *                    |   ^
+ *                    |   |
+ *                    |   | sched_tick_remote()
+ *                    |   |
+ *                    |   |
+ *                    +--TICK_SCHED_REMOTE_OFFLINING
+ *                    |   ^
+ *                    |   |
+ * sched_tick_start() |   | sched_tick_stop()
+ *                    |   |
+ *                    V   |
+ *          TICK_SCHED_REMOTE_RUNNING
+ *
+ *
+ * Other transitions get WARN_ON_ONCE(), except that sched_tick_remote()
+ * and sched_tick_start() are happy to leave the state in RUNNING.
+ */
 
 static struct tick_work __percpu *tick_work_cpu;
 
@@ -3064,6 +3092,7 @@ static void sched_tick_remote(struct work_struct *work)
 	struct task_struct *curr;
 	struct rq_flags rf;
 	u64 delta;
+	int os;
 
 	/*
 	 * Handle the tick only if it appears the remote CPU is running in full
@@ -3077,7 +3106,7 @@ static void sched_tick_remote(struct work_struct *work)
 
 	rq_lock_irq(rq, &rf);
 	curr = rq->curr;
-	if (is_idle_task(curr))
+	if (is_idle_task(curr) || cpu_is_offline(cpu))
 		goto out_unlock;
 
 	update_rq_clock(rq);
@@ -3097,13 +3126,18 @@ static void sched_tick_remote(struct work_struct *work)
 	/*
 	 * Run the remote tick once per second (1Hz). This arbitrary
 	 * frequency is large enough to avoid overload but short enough
-	 * to keep scheduler internal stats reasonably up to date.
+	 * to keep scheduler internal stats reasonably up to date.  But
+	 * first update state to reflect hotplug activity if required.
 	 */
-	queue_delayed_work(system_unbound_wq, dwork, HZ);
+	os = atomic_fetch_add_unless(&twork->state, -1, TICK_SCHED_REMOTE_RUNNING);
+	WARN_ON_ONCE(os == TICK_SCHED_REMOTE_OFFLINE);
+	if (os == TICK_SCHED_REMOTE_RUNNING)
+		queue_delayed_work(system_unbound_wq, dwork, HZ);
 }
 
 static void sched_tick_start(int cpu)
 {
+	int os;
 	struct tick_work *twork;
 
 	if (housekeeping_cpu(cpu, HK_FLAG_TICK))
@@ -3112,15 +3146,20 @@ static void sched_tick_start(int cpu)
 	WARN_ON_ONCE(!tick_work_cpu);
 
 	twork = per_cpu_ptr(tick_work_cpu, cpu);
-	twork->cpu = cpu;
-	INIT_DELAYED_WORK(&twork->work, sched_tick_remote);
-	queue_delayed_work(system_unbound_wq, &twork->work, HZ);
+	os = atomic_xchg(&twork->state, TICK_SCHED_REMOTE_RUNNING);
+	WARN_ON_ONCE(os == TICK_SCHED_REMOTE_RUNNING);
+	if (os == TICK_SCHED_REMOTE_OFFLINE) {
+		twork->cpu = cpu;
+		INIT_DELAYED_WORK(&twork->work, sched_tick_remote);
+		queue_delayed_work(system_unbound_wq, &twork->work, HZ);
+	}
 }
 
 #ifdef CONFIG_HOTPLUG_CPU
 static void sched_tick_stop(int cpu)
 {
 	struct tick_work *twork;
+	int os;
 
 	if (housekeeping_cpu(cpu, HK_FLAG_TICK))
 		return;
@@ -3128,7 +3167,10 @@ static void sched_tick_stop(int cpu)
 	WARN_ON_ONCE(!tick_work_cpu);
 
 	twork = per_cpu_ptr(tick_work_cpu, cpu);
-	cancel_delayed_work_sync(&twork->work);
+	/* There cannot be competing actions, but don't rely on stop-machine. */
+	os = atomic_xchg(&twork->state, TICK_SCHED_REMOTE_OFFLINING);
+	WARN_ON_ONCE(os != TICK_SCHED_REMOTE_RUNNING);
+	/* Don't cancel, as this would mess up the state machine. */
 }
 #endif /* CONFIG_HOTPLUG_CPU */
 
@@ -3136,7 +3178,6 @@ int __init sched_tick_offload_init(void)
 {
 	tick_work_cpu = alloc_percpu(struct tick_work);
 	BUG_ON(!tick_work_cpu);
-
 	return 0;
 }