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

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Message-ID: <AANLkTikdhnr12uU8Wp60BygZwH770RBfxyfLNMzUsQje@mail.gmail.com>
Date:	Tue, 31 Aug 2010 16:31:08 -0400
From:	Nitin Gupta <ngupta@...are.org>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Pekka Enberg <penberg@...helsinki.fi>,
	Minchan Kim <minchan.kim@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	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, Aug 30, 2010 at 12:20 PM, Christoph Lameter <cl@...ux.com> wrote:
> On Mon, 9 Aug 2010, Nitin Gupta wrote:
>
>> -static void zram_stat_inc(u32 *v)
>> +static void zram_add_stat(struct zram *zram,
>> +                     enum zram_stats_index idx, s64 val)
>>  {
>> -     *v = *v + 1;
>> +     struct zram_stats_cpu *stats;
>> +
>> +     preempt_disable();
>> +     stats = __this_cpu_ptr(zram->stats);
>> +     u64_stats_update_begin(&stats->syncp);
>> +     stats->count[idx] += val;
>> +     u64_stats_update_end(&stats->syncp);
>> +     preempt_enable();
>
> Maybe do
>
> #define zram_add_stat(zram, index, val)
>                this_cpu_add(zram->stats->count[index], val)
>
> instead? It creates an add in a single "atomic" per cpu instruction and
> deals with the fallback scenarios for processors that cannot handle 64
> bit adds.
>
>

Yes, this_cpu_add() seems sufficient. I can't recall why I used u64_stats_*
but if it's not required for atomic access to 64-bit then why was it added to
the mainline in the first place?

Thanks,
Nitin
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