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

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Date:	Tue, 31 Aug 2010 16:35:37 -0500 (CDT)
From:	Christoph Lameter <cl@...ux.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
cc:	Nitin Gupta <ngupta@...are.org>,
	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 Tue, 31 Aug 2010, Eric Dumazet wrote:

> > 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?
>
> Because we wanted to have fast 64bit counters, even on 32bit arches, and
> this has litle to do with 'atomic' on one entity, but a group of
> counters. (check drivers/net/loopback.c, lines 91-94). No lock prefix
> used in fast path.
>
> We also wanted readers to read correct values, not a value being changed
> by a writer, with inconsistent 32bit halves. SNMP applications want
> monotonically increasing counters.
>
> this_cpu_add()/this_cpu_read() doesnt fit.
>
> Even for single counter, this_cpu_read(64bit) is not using an RMW
> (cmpxchg8) instruction, so you can get very strange results when low
> order 32bit wraps.

How about fixing it so that everyone benefits?

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