On Tue, Apr 20, 2021 at 10:46:33AM +0100, Vincent Donnefort wrote:
> Found the issue:
>
> $ cat hotplug/states:
> 219: sched:active
> 220: online
>
> CPU0:
>
> $ echo 219 > hotplug/fail
> $ echo 0 > online
>
> => cpu_active = 1 cpu_dying = 1
>
> which means that later on, for another CPU hotunplug, in
> __balance_push_cpu_stop(), the fallback rq for a kthread can select that
> CPU0, but __migrate_task() would fail and we end-up in an infinite loop,
> trying to migrate that task to CPU0.
>
> The problem is that for a failure in sched:active, as "online" has no
> callback,
> there will be no call to cpuhp_invoke_callback(). Hence, the cpu_dying bit
> would
> not be reset.
Urgh! Good find.
> Maybe cpuhp_reset_state() and cpuhp_set_state() would then be a better place
> to
> switch the dying bit?
Yes, except now cpuhp_invoke_ap_callback() makes my head hurt, that runs
the callbacks out of order. I _think_ we can ignore it, but ....
Something like the below, let me see if I can reproduce and test.
---
diff --git a/kernel/cpu.c b/kernel/cpu.c
index 838dcf238f92..05272bae953d 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -160,9 +160,6 @@ static int cpuhp_invoke_callback(unsigned int cpu, enum
cpuhp_state state,
int (*cb)(unsigned int cpu);
int ret, cnt;
- if (cpu_dying(cpu) != !bringup)
- set_cpu_dying(cpu, !bringup);
-
if (st->fail == state) {
st->fail = CPUHP_INVALID;
return -EAGAIN;
@@ -467,13 +464,16 @@ static inline enum cpuhp_state
cpuhp_set_state(struct cpuhp_cpu_state *st, enum cpuhp_state target)
{
enum cpuhp_state prev_state = st->state;
+ bool bringup = st->state < target;
st->rollback = false;
st->last = NULL;
st->target = target;
st->single = false;
- st->bringup = st->state < target;
+ st->bringup = bringup;
+ if (cpu_dying(cpu) != !bringup)
+ set_cpu_dying(cpu, !bringup);
return prev_state;
}
@@ -481,6 +481,8 @@ cpuhp_set_state(struct cpuhp_cpu_state *st, enum
cpuhp_state target)
static inline void
cpuhp_reset_state(struct cpuhp_cpu_state *st, enum cpuhp_state prev_state)
{
+ bool bringup = !st->bringup;
+
st->target = prev_state;
/*
@@ -503,7 +505,9 @@ cpuhp_reset_state(struct cpuhp_cpu_state *st, enum
cpuhp_state prev_state)
st->state++;
}
- st->bringup = !st->bringup;
+ st->bringup = bringup;
+ if (cpu_dying(cpu) != !bringup)
+ set_cpu_dying(cpu, !bringup);
}
/* Regular hotplug invocation of the AP hotplug thread */
