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