linux-kernel - Re: [PATCH 03/10] Use percpu stats

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Date:	Mon, 9 Aug 2010 21:34:31 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Nitin Gupta <ngupta@...are.org>
Cc:	Pekka Enberg <penberg@...helsinki.fi>,
	Minchan Kim <minchan.kim@...il.com>, Greg KH <greg@...ah.com>,
	Linux Driver Project <devel@...verdev.osuosl.org>,
	linux-mm <linux-mm@...ck.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 03/10] Use percpu stats

On Mon,  9 Aug 2010 22:56:49 +0530 Nitin Gupta <ngupta@...are.org> wrote:

> +/*
> + * Individual percpu values can go negative but the sum across all CPUs
> + * must always be positive (we store various counts). So, return sum as
> + * unsigned value.
> + */
> +static u64 zram_get_stat(struct zram *zram, enum zram_stats_index idx)
>  {
> -	u64 val;
> -
> -	spin_lock(&zram->stat64_lock);
> -	val = *v;
> -	spin_unlock(&zram->stat64_lock);
> +	int cpu;
> +	s64 val = 0;
> +
> +	for_each_possible_cpu(cpu) {
> +		s64 temp;
> +		unsigned int start;
> +		struct zram_stats_cpu *stats;
> +
> +		stats = per_cpu_ptr(zram->stats, cpu);
> +		do {
> +			start = u64_stats_fetch_begin(&stats->syncp);
> +			temp = stats->count[idx];
> +		} while (u64_stats_fetch_retry(&stats->syncp, start));
> +		val += temp;
> +	}
>  
> +	WARN_ON(val < 0);
>  	return val;
>  }

That reimplements include/linux/percpu_counter.h, poorly.

Please see the June discussion "[PATCH v2 1/2] tmpfs: Quick token
library to allow scalable retrieval of tokens from token jar" for some
discussion.

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