On Tue, Apr 20, 2021 at 04:58:00PM +0200, Peter Zijlstra wrote:
> On Tue, Apr 20, 2021 at 04:39:04PM +0200, Peter Zijlstra wrote:
> > On Tue, Apr 20, 2021 at 04:20:56PM +0200, Peter Zijlstra wrote:
> > > 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.
>
> > I seem to have triggered the BUG() in select_fallback_rq() with your
> > recipie.
> > Have cpu0 fail on sched:active, then offline all other CPUs.
> >
> > Now lemme add that patch.
>
> (which obviously didn't actually build) seems to fix it.
>
> ---
> diff --git a/kernel/cpu.c b/kernel/cpu.c
> index 838dcf238f92..e538518556f4 100644
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -63,6 +63,7 @@ struct cpuhp_cpu_state {
> bool rollback;
> bool single;
> bool bringup;
> + int cpu;
> struct hlist_node *node;
> struct hlist_node *last;
> enum cpuhp_state cb_state;
> @@ -160,9 +161,6 @@ 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 +465,16 @@ 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(st->cpu) != !bringup)
> + set_cpu_dying(st->cpu, !bringup);
>
> return prev_state;
> }
> @@ -481,6 +482,8 @@ 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 +506,9 @@ 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(st->cpu) != !bringup)
> + set_cpu_dying(st->cpu, !bringup);
> }
>
> /* Regular hotplug invocation of the AP hotplug thread */
> @@ -693,6 +698,7 @@ static void cpuhp_create(unsigned int cpu)
>
> init_completion(&st->done_up);
> init_completion(&st->done_down);
> + st->cpu = cpu;
> }
>
> static int cpuhp_should_run(unsigned int cpu)
All good with that snippet on my end.
I wonder if balance_push() shouldn't use the cpu_of() accessor
instead of rq->cpu.
Otherwise,
+ Reviewed-by: Vincent Donnefort <vincent.donnef...@arm.com>
