Re: [PATCH 01/14] Per zone counter functionality

[Christoph Lameter <clameter@sgi.com>]

On Fri, 9 Jun 2006, Andi Kleen wrote:

> It would be nicer to use some variant of local_t - then you could do that
> without turning off interrupts (which some CPUs like P4 don't like)
> 
> There currently is not 1 byte local_t but it could be added.
> 
> Mind you it would only make sense when most of the calls are not already
> with interrupts disabled.

We have discussed this before and there is a comment in the patch:

+ *
+ * Some processors have inc/dec instructions that are atomic vs an interrupt.
+ * However, the code must first determine the differential location in a zone
+ * based on the processor number and then inc/dec the counter. There is no
+ * guarantee without disabling preemption that the processor will not change
+ * in between and therefore the atomicity vs. interrupt cannot be exploited
+ * in a useful way here.
+ */
+void __inc_zone_page_state(struct page *page, enum zone_stat_item item)
+{
+       struct zone *zone = page_zone(page);
+       s8 *p = diff_pointer(zone, item);
+
+       (*p)++;
+
+       if (unlikely(*p > STAT_THRESHOLD)) {
+               zone_page_state_add(*p, zone, item);
+               *p = 0;
+       }
+}

AFAIK the restrictions on local_t use are such that is barely usable.
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