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

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Message-ID: <4FEB7DC0.9090404@candelatech.com>
Date:	Wed, 27 Jun 2012 14:40:16 -0700
From:	Ben Greear <greearb@...delatech.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	Stephen Hemminger <shemminger@...tta.com>,
	Rick Jones <rick.jones2@...com>,
	Tom Parkin <tparkin@...alix.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 06/27/2012 02:32 PM, Eric Dumazet wrote:
> On Wed, 2012-06-27 at 23:20 +0200, Eric Dumazet wrote:
>> On Wed, 2012-06-27 at 13:58 -0700, Ben Greear wrote:
>>
>>> It's worse than that:  Even on 64-bit kernels, counters that are returned by
>>> netlink and /proc/net/dev as 64-bit may still wrap themselves at 32-bit
>>> intervals.
>>
>> Really ?
>>
>> Thats incredible you dont send a bug report then.
>
> A bug report to the application author, not the kernel.

Yeah, application author was me.

> /proc/net/dev is an ASCII file, and nothing gives the width of a field.
>
> Therefore, an application should cope with all cases (counter being a
> 32bit or 64bit integer), wrapping included.
>
> Note that this has little to do with the application or kernel being
> 32/64 bit code.

Notice that the netlink stats are claiming 64-bit and they are not
(always) 64-bit.

That is a nice binary API that is still wrapping before its time
in many cases.  There may be good performance reasons for keeping
some counters at 32-bit, but it plays merry hell with anyone wanting
to optimize an application to poll less often for stats that are
supposedly 64-bit.

Thanks,
Ben

-- 
Ben Greear <greearb@...delatech.com>
Candela Technologies Inc  http://www.candelatech.com



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