linux-kernel - Re: [PATCH 4/4] idr: Percpu ida

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Date:	Tue, 18 Jun 2013 14:14:53 +0000
From:	Christoph Lameter <cl@...ux.com>
To:	Kent Overstreet <koverstreet@...gle.com>
cc:	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	tj@...nel.org, axboe@...nel.dk, nab@...ux-iscsi.org,
	Oleg Nesterov <oleg@...hat.com>,
	Ingo Molnar <mingo@...hat.com>,
	Andi Kleen <andi@...stfloor.org>
Subject: Re: [PATCH 4/4] idr: Percpu ida

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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