linux-kernel - Re: [PATCH 1/2] lib: Proportions with flexible period

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Date:	Thu, 17 May 2012 23:56:45 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Jan Kara <jack@...e.cz>
Cc:	Wu Fengguang <fengguang.wu@...el.com>,
	LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org
Subject: Re: [PATCH 1/2] lib: Proportions with flexible period

On Tue, 2012-05-15 at 17:43 +0200, Jan Kara wrote:
> +void fprop_fraction_percpu(struct fprop_global *p,
> +                          struct fprop_local_percpu *pl,
> +                          unsigned long *numerator, unsigned long *denominator)
> +{
> +       unsigned int seq;
> +       s64 den;
> +
> +       do {
> +               seq = read_seqcount_begin(&p->sequence);
> +               fprop_reflect_period_percpu(p, pl);
> +               *numerator = percpu_counter_read_positive(&pl->events);
> +               den = percpu_counter_read(&p->events);
> +               if (den <= 0)
> +                       den = percpu_counter_sum(&p->events);
> +               *denominator = den;
> +       } while (read_seqcount_retry(&p->sequence, seq));
> +} 


why not use percpu_counter_read_positive(&p->events) and ditch
percpu_counter_sum()? That sum can be terribly expensive..
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