Re: [PATCH 03/10] Use percpu stats

[Andrew Morton <akpm@linux-foundation.org>]

On Mon,  9 Aug 2010 22:56:49 +0530 Nitin Gupta <ngupta@vflare.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.