linux-kernel - Re: [RFC][PATCH] lib: generic percpu counter array

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Date:	Thu, 5 Nov 2009 10:15:36 -0500 (EST)
From:	Christoph Lameter <cl@...ux-foundation.org>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"hugh.dickins@...cali.co.uk" <hugh.dickins@...cali.co.uk>,
	akpm@...ux-foundation.org,
	"kosaki.motohiro@...fujitsu.com" <kosaki.motohiro@...fujitsu.com>
Subject: Re: [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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