netdev - Re: [PATCH v2] l2tp: use per-cpu variables for u64

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Message-ID: <20120628082457.GA2507@raven>
Date:	Thu, 28 Jun 2012 09:24:58 +0100
From:	Tom Parkin <tparkin@...alix.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Rick Jones <rick.jones2@...com>,
	Ben Greear <greearb@...delatech.com>,
	Stephen Hemminger <shemminger@...tta.com>,
	netdev@...r.kernel.org, David.Laight@...LAB.COM,
	James Chapman <jchapman@...alix.com>
Subject: Re: [PATCH v2] l2tp: use per-cpu variables for u64_stats updates

On Thu, Jun 28, 2012 at 07:00:54AM +0200, Eric Dumazet wrote:
> [1] : LLTX drivers case 
>    since ndo_start_xmit() can be run concurrently by many cpus, safely
> updating an "unsigned long" requires additional hassle :
> 
>    1) Use of a spinlock to protect the update.
>    2) Use atomic_long_t instead of "unsigned long"
>    3) Use percpu data
> 
> 3) is overkill for devices with light traffic, because it consumes lot
> of RAM on machines with 2048 possible cpus, _and_ the reader must fold
> the data of all possible values.

Thanks Eric.

So am I right in thinking that a v3 patch which uses atomic_long_t for
the statistics would be the correct way forwards?

-- 
Tom Parkin
Katalix Systems Ltd
http://www.katalix.com
Catalysts for your Embedded Linux software development

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