Re: [patch] rt: res_counter fix, v2

[KAMEZAWA Hiroyuki]

On Fri, 13 Feb 2009 08:56:40 +0900
KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> wrote:

> On Thu, 12 Feb 2009 22:28:33 +0530
> Balbir Singh <balbir@xxxxxxxxxxxxxxxxxx> wrote:
> 
> > * Ingo Molnar <mingo@xxxxxxx> [2009-02-12 12:28:54]:
> > 
> > > 
> > > * KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> wrote:
> > > 
> > > > On Thu, 12 Feb 2009 11:21:13 +0100
> > > > Ingo Molnar <mingo@xxxxxxx> wrote:
> > > The question is, are these local IRQ flags manipulations really needed
> > > in this code, and if yes, why?
> > 
> > We needed the local IRQ flags, since these counters are updated from
> > page fault context and from reclaim context with lru_lock held with
> > IRQ's disabled. I've been thinking about replacing the spin lock with
> > seq lock, but have not gotten to it yet.
> > 
> Hmm ?  I can't understand. Why we have to disable IRQ here again ?
> And,
>   - try_to_unmap() is called in shrink_page_list(),  there, no zone->lru_lock.
>   - page fault path doesn't hold zone->lru_lock.
> 
> My concern is only shmem. But I think it doesn't call charge() within lock, actually

Clarification :)
  res_counter_charge() is called from
    - page fault 
        => under down_read(mmap_sem), lock_page() may be held.  IRQ=ENABLED)
    - add_to_page_cache
        => under lock_page(), mapping->tree_lock is *not* held, IRQ=DISABLED
    - shmem
        => info->lock is held, we use __GFP_NOWAIT here. IRQ=ENABLED
    - shmem
        => info->lock is *not* held with GFP_KERNEL here, IRQ=ENABLED.
    - migration
        => under lock_page() and mmap_sem, IRQ=ENABLED

  res_counter_uncharge() is called from
    - page_remove_rmap()//(Only when ANON)
         => anon_vma->lock and pte_lock(),lock_page() can be held. IRQ=ENABLED?
    - remove_from_page_cache()
         => lock_page() and mapping->tree_lock is held, IRQ=DISABLED.

Summary:
   "Charge" is considered as heavy operation and the call path is placed where the
    thread can sleep, AMAP.
   "Uncharge" is considered as light operation and call path is under some # of
    spinlocks.


Bye,
-Kame

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