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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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