[PATCH] set_pgdat_percpu_threshold() don't use for_each_online_cpu

[KOSAKI Motohiro]

> > @@ -159,6 +165,44 @@ static void refresh_zone_stat_thresholds(void)
> >  	}
> >  }
> >  
> > +void reduce_pgdat_percpu_threshold(pg_data_t *pgdat)
> > +{
> > +	struct zone *zone;
> > +	int cpu;
> > +	int threshold;
> > +	int i;
> > +
> 
> get_online_cpus();


This caused following runtime warnings. but I don't think here is
real lock inversion. 

=================================
[ INFO: inconsistent lock state ]
2.6.37-rc1-mm1+ #150
---------------------------------
inconsistent {RECLAIM_FS-ON-W} -> {IN-RECLAIM_FS-W} usage.
kswapd0/419 [HC0[0]:SC0[0]:HE1:SE1] takes:
 (cpu_hotplug.lock){+.+.?.}, at: [<ffffffff810520d1>] get_online_cpus+0x41/0x60
{RECLAIM_FS-ON-W} state was registered at:
  [<ffffffff8108a1a3>] mark_held_locks+0x73/0xa0
  [<ffffffff8108a296>] lockdep_trace_alloc+0xc6/0x100
  [<ffffffff8113fba9>] kmem_cache_alloc+0x39/0x2b0
  [<ffffffff812eea10>] idr_pre_get+0x60/0x90
  [<ffffffff812ef5b7>] ida_pre_get+0x27/0xf0
  [<ffffffff8106ebf5>] create_worker+0x55/0x190
  [<ffffffff814fb4f4>] workqueue_cpu_callback+0xbc/0x235
  [<ffffffff8151934c>] notifier_call_chain+0x8c/0xe0
  [<ffffffff8107a34e>] __raw_notifier_call_chain+0xe/0x10
  [<ffffffff81051f30>] __cpu_notify+0x20/0x40
  [<ffffffff8150bff7>] _cpu_up+0x73/0x113
  [<ffffffff8150c175>] cpu_up+0xde/0xf1
  [<ffffffff81dcc81d>] kernel_init+0x21b/0x342
  [<ffffffff81003724>] kernel_thread_helper+0x4/0x10
irq event stamp: 27
hardirqs last  enabled at (27): [<ffffffff815152c0>] _raw_spin_unlock_irqrestore+0x40/0x80
hardirqs last disabled at (26): [<ffffffff81514982>] _raw_spin_lock_irqsave+0x32/0xa0
softirqs last  enabled at (20): [<ffffffff810614c4>] del_timer_sync+0x54/0xa0
softirqs last disabled at (18): [<ffffffff8106148c>] del_timer_sync+0x1c/0xa0

other info that might help us debug this:
no locks held by kswapd0/419.

stack backtrace:
Pid: 419, comm: kswapd0 Not tainted 2.6.37-rc1-mm1+ #150
Call Trace:
 [<ffffffff810890b1>] print_usage_bug+0x171/0x180
 [<ffffffff8108a057>] mark_lock+0x377/0x450
 [<ffffffff8108ab67>] __lock_acquire+0x267/0x15e0
 [<ffffffff8107af0f>] ? local_clock+0x6f/0x80
 [<ffffffff81086789>] ? trace_hardirqs_off_caller+0x29/0x150
 [<ffffffff8108bf94>] lock_acquire+0xb4/0x150
 [<ffffffff810520d1>] ? get_online_cpus+0x41/0x60
 [<ffffffff81512cf4>] __mutex_lock_common+0x44/0x3f0
 [<ffffffff810520d1>] ? get_online_cpus+0x41/0x60
 [<ffffffff810744f0>] ? prepare_to_wait+0x60/0x90
 [<ffffffff81086789>] ? trace_hardirqs_off_caller+0x29/0x150
 [<ffffffff810520d1>] ? get_online_cpus+0x41/0x60
 [<ffffffff810868bd>] ? trace_hardirqs_off+0xd/0x10
 [<ffffffff8107af0f>] ? local_clock+0x6f/0x80
 [<ffffffff815131a8>] mutex_lock_nested+0x48/0x60
 [<ffffffff810520d1>] get_online_cpus+0x41/0x60
 [<ffffffff811138b2>] set_pgdat_percpu_threshold+0x22/0xe0
 [<ffffffff81113970>] ? calculate_normal_threshold+0x0/0x60
 [<ffffffff8110b552>] kswapd+0x1f2/0x360
 [<ffffffff81074180>] ? autoremove_wake_function+0x0/0x40
 [<ffffffff8110b360>] ? kswapd+0x0/0x360
 [<ffffffff81073ae6>] kthread+0xa6/0xb0
 [<ffffffff81003724>] kernel_thread_helper+0x4/0x10
 [<ffffffff81515710>] ? restore_args+0x0/0x30
 [<ffffffff81073a40>] ? kthread+0x0/0xb0
 [<ffffffff81003720>] ? kernel_thread_helper+0x0/0x10


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. 


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