sorry for the delay.
> Well what's actually happening here? Where is the alleged deadlock?
>
> In the kernel_init() case we have a GFP_KERNEL allocation inside
> get_online_cpus(). In the other case we simply have kswapd calling
> get_online_cpus(), yes?
Yes.
>
> Does lockdep consider all kswapd actions to be "in reclaim context"?
> If so, why?
kswapd call lockdep_set_current_reclaim_state() at thread starting time.
see below.
----------------------------------------------------------------------
static int kswapd(void *p)
{
unsigned long order;
pg_data_t *pgdat = (pg_data_t*)p;
struct task_struct *tsk = current;
struct reclaim_state reclaim_state = {
.reclaimed_slab = 0,
};
const struct cpumask *cpumask = cpumask_of_node(pgdat->node_id);
lockdep_set_current_reclaim_state(GFP_KERNEL);
......
----------------------------------------------------------------------
> > I think we have two option 1) call lockdep_clear_current_reclaim_state()
> > every time 2) use for_each_possible_cpu instead for_each_online_cpu.
> >
> > Following patch use (2) beucase removing get_online_cpus() makes good
> > side effect. It reduce potentially cpu-hotplug vs memory-shortage deadlock
> > risk.
>
> Well. Being able to run for_each_online_cpu() is a pretty low-level
> and fundamental thing. It's something we're likely to want to do more
> and more of as time passes. It seems a bad thing to tell ourselves
> that we cannot use it in reclaim context. That blots out large chunks
> of filesystem and IO-layer code as well!
>
> > --- a/mm/vmstat.c
> > +++ b/mm/vmstat.c
> > @@ -193,18 +193,16 @@ void set_pgdat_percpu_threshold(pg_data_t *pgdat,
> > int threshold;
> > int i;
> >
> > - get_online_cpus();
> > for (i = 0; i < pgdat->nr_zones; i++) {
> > zone = &pgdat->node_zones[i];
> > if (!zone->percpu_drift_mark)
> > continue;
> >
> > threshold = (*calculate_pressure)(zone);
> > - for_each_online_cpu(cpu)
> > + for_each_possible_cpu(cpu)
> > per_cpu_ptr(zone->pageset, cpu)->stat_threshold
> > = threshold;
> > }
> > - put_online_cpus();
> > }
>
> That's a pretty sad change IMO, especially of num_possible_cpus is much
> larger than num_online_cpus.
As far as I know, CPU hotplug is used server area and almost server have
ACPI or similar flexible firmware interface. then, num_possible_cpus is
not so much big than actual numbers of socket.
IOW, I haven't hear embedded people use cpu hotplug. If you've hear, please
let me know.
> What do we need to do to make get_online_cpus() safe to use in reclaim
> context? (And in kswapd context, if that's really equivalent to
> "reclaim context").
Hmm... It's too hard.
kmalloc() is called from everywhere and cpu hotplug is happen any time.
then, any lock design break your requested rule. ;)
And again, _now_ I don't think for_each_possible_cpu() is very costly.
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