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SubjectRe: [PATCH] time/tick-broadcast: Fix tick_broadcast_offline() lockdep complaint
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;
}

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