Re: [ltt-dev] [PATCH] Fix dirty page accounting in redirty_page_for_writepage()

[Mathieu Desnoyers <compudj@krystal.dyndns.org>]

* Christoph Lameter (cl@linux.com) wrote:
> On Fri, 1 May 2009, Mathieu Desnoyers wrote:
> 
> > What I do here is to let those u8 counters increment as free-running
> > counters. Yes, they will periodically overflow the 8 bits. But I don't
> > rely on this for counting the number of increments we need between
> > global counter updates : I use the bitmask taken from the threshold
> > value (which is now required to be a power of two) to detect 0, 1, 2, 3,
> > 4, 5, 6 or 7-bit counter overflow. Therefore we can still have the kind
> > of granularity currently provided. The only limitation is that we have
> > to use powers of two for the threshold, so we end up counting in power
> > of two modulo, which will be unaffected by the u8 overflow.
> 
> Ack. Got it. Looks good.
> 

Super ! :)

So, back to my original point : do you agree on the usefulness of
separating fallback irq-disabling from the per-cpu atomic construct ?

e.g. :

__inc_zone_state
  percpu_add_return_irq(var);

inc_zone_state
  percpu_irqsave(flags);
  __inc_zone_state()
  percpu_irqrestore(flags);

This would require that percpu_add_return_irq should always be called
either in :
  - irq disabled code paths
  - in code paths surrounded by percpu_irqsave/restore.

In this example :

x86 would map :

percpu_irqsave/restore to "nothing".
percpu_add_return_irq to xadd instruction. It is irq-safe by design.

Other architectures (fallback) would map

percpu_irqsave/restore to local_irq_save/restore.
percpu_add_return_irq to var += value; return var;

Mathieu

-- 
Mathieu Desnoyers
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