linux-kernel - Re: [BUG] packet loss with PROVE

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Date:	Tue, 22 Mar 2016 09:18:16 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Borislav Petkov <bp@...en8.de>
Cc:	Chris Bainbridge <chris.bainbridge@...il.com>,
	linux-edac@...r.kernel.org, linux-kernel@...r.kernel.org,
	mchehab@...radead.org, markus@...ppelsdorf.de, tony.luck@...el.com
Subject: Re: [BUG] packet loss with PROVE_LOCKING, bisected to EDAC fix

On Tue, Mar 22, 2016 at 06:31:54AM +0100, Borislav Petkov wrote:
> On Mon, Mar 21, 2016 at 09:42:09PM +0000, Chris Bainbridge wrote:
> > Hi,
> > 
> > I was testing something on an old server (Dell T105 opteron) and noticed
> > packet loss after updating the kernel from 3.10 to 4.5. The test was:
> > 
> > On Dell run: iperf -s
> > On another system: iperf3 -c dell -u -b 20M -l 1k -t 1000
> > 
> > This sends a 20mbit UDP stream to the Dell. It works fine normally (0%
> > packet loss), but when CONFIG_PROVE_LOCKING is enabled there is high
> > (35%) packet loss. (DEBUG_LOCKDEP also seems to cause packet loss)
> > 
> > The packet loss bisected back to:
> > 
> > commit 88d84ac97378c2f1d5fec9af1e8b7d9a662d6b00
> > Author: Borislav Petkov <bp@...e.de>
> > Date:   Fri Jul 19 12:28:25 2013 +0200
> > 
> >     EDAC: Fix lockdep splat
> 
> Hmm, how would that cause a packet loss?!

The previous bug would disable lockdep and thereby avoid much of the
normal overhead associated with lockdep. I suspect the packet loss is a
result of increased overhead.

IOW, everything works as expected.