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Subject: Re: [PATCH 03/10] Use percpu stats
From: Eric Dumazet <eric.dumazet@gmail.com>
To: Christoph Lameter <cl@linux.com>
Cc: Nitin Gupta <ngupta@vflare.org>, Pekka Enberg <penberg@cs.helsinki.fi>,
        Minchan Kim <minchan.kim@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>, Greg KH <greg@kroah.com>,
        Linux Driver Project <devel@driverdev.osuosl.org>,
        linux-mm <linux-mm@kvack.org>,
        linux-kernel <linux-kernel@vger.kernel.org>
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Date: Tue, 31 Aug 2010 23:41:18 +0200
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Le mardi 31 août 2010 à 16:35 -0500, Christoph Lameter a écrit :
> 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?
> 

IMHO, this_cpu_read() is fine as is : a _read_ operation.

Dont pretend it can be used in every context, its not true.


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