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    SubjectRe: [RFC][PATCH] lib: generic percpu counter array
    On Thu, 5 Nov 2009, KAMEZAWA Hiroyuki wrote:

    > +static inline void
    > +counter_array_add(struct counter_array *ca, int idx, int val)
    > +{
    > + ca->counters[idx] += val;
    > +}

    This is not a per cpu operation and therefore expensive. The new percpu
    this_cpu_inc f.e. generates a single x86 instruction for an increment.

    > +void __counter_array_add(struct counter_array *ca, int idx, int val, int batch)
    > +{
    > + long count, *pcount;
    > +
    > + preempt_disable();
    > +
    > + pcount = this_cpu_ptr(ca->v.array);
    > + count = pcount[idx] + val;
    > + if (!ca->v.nosync && ((count > batch) || (count < -batch))) {
    > + atomic_long_add(count, &ca->counters[idx]);
    > + pcount[idx] = 0;
    > + } else
    > + pcount[idx] = count;
    > + preempt_enable();
    > +}

    Too expensive to use in critical VM paths. The percpu operations generate
    a single instruction instead of the code above. No need for preempt etc.


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