netdev - Re: [PATCH 1/2] [IPV4] UDP: Always checksum even if without socket filter

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Message-Id: <20071119.142313.63549156.davem@davemloft.net>
Date:	Mon, 19 Nov 2007 14:23:13 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	andi@...stfloor.org
Cc:	wangchen@...fujitsu.com, herbert@...dor.apana.org.au,
	netdev@...r.kernel.org
Subject: Re: [PATCH 1/2] [IPV4] UDP: Always checksum even if without socket
 filter

From: Andi Kleen <andi@...stfloor.org>
Date: Mon, 19 Nov 2007 16:29:33 +0100

> > > > 
> > > > All of our options suck, we just have to choose the least sucking one
> > > > and right now to me that's decrementing the counter as much as I
> > > > empathize with the SNMP application overflow detection issue.
> > > 
> > > If the SNMP monitor detects an false overflow the error it reports 
> > > will be much worse than a single missing packet. So you would replace 
> > > one error with a worse error.
> > 
> > This can be fixed, the above cannot.
> 
> I don't see how, short of breaking the interface
> (e.g. reporting 64bit or separate overflow counts)

As someone who just spent an entire weekend working on
cpu performance counter code, I know it's possible.

When you overflow, the new value is "a lot" less than
the last sampled one.  When the value backtracks like
we're discussing it could here, it only decrease
a very little bit.
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