Re: [PATCH 4/4] idr: Percpu ida

[Christoph Lameter]

On Mon, 17 Jun 2013, Kent Overstreet wrote:

> +static inline unsigned alloc_local_tag(struct percpu_ida *pool,
> +				       struct percpu_ida_cpu *tags)
> +{
> +	int tag = -ENOSPC;
> +
> +	spin_lock(&tags->lock);
> +	if (tags->nr_free)
> +			tag = tags->freelist[--tags->nr_free];
> +	spin_unlock(&tags->lock);
> +
> +	return tag;
> +}

This could be made much faster by avoiding real atomics (coming with
spinlocks) and using per cpu atomics instead. Slub f.e. uses a single
linked per cpu list managed via this_cpu_cmpxchg.

In order to avoid locking completely the state of the percpu tag list must
fit into one word that can be handled via the local cmpxchg (or two with
the cmpxchg_double). May require some changes to the data structure.



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