Re: [Xen-devel] Xen-unstable Linux 3.14-rc3 and 3.13 Network troubles "bisected"

[Sander Eikelenboom <linux@xxxxxxxxxxxxxx>]

Tuesday, March 11, 2014, 4:36:16 PM, you wrote:

> On Tue, Mar 11, 2014 at 02:00:41PM +0100, Sander Eikelenboom wrote:
> [...]
>> >> the issue when using 3.13.6 as a base and ..
>> >>   - pull all 3.14 patches from the  
>> >> git://git.kernel.org/pub/scm/linux/kernel/git/xen/tip.git tree
>> >>   - apply paul's commit "ca2f09f2b2c6c25047cfc545d057c4edfcfe561c 
>> >> xen-netback: improve guest-receive-side flow control"
>> >>   - applying annie's v2 patch
>> >>   - applying your patch
>> >> as dom0 and using a 3.14-rc5 as domU kernel.
>> >> 
>> >> Unfortunately i'm still getting the Bad grant references ..
>> >> 
>> 
>> > :-( That's bad news.
>> 
>> > I guess you always have the same DomU kernel when testing? That means we
>> > can narrow down the bug to netback only.
>> 
>> Yes my previous tests (from my previous mail):
>> 
>> - First testing a baseline that worked o.k. for several days (3.13.6 for 
>> both dom0 and domU)
>> - Testing domU 3.14-rc5 and dom0 3.13.6, this worked ok.
>> - Testing dom0 3.14-rc5 and domU 3.13.6, this failed.
>> - After that took 3.13.6 as base and first applied all the general xen 
>> related patches for the dom0 kernel, that works ok.
>> - After that started to apply the netback changes for 3.14 and that failed 
>> after the commit "ca2f09f2b2c6c25047cfc545d057c4edfcfe561c xen-netback: 
>> improve guest-receive-side flow control".
>> 
>> Also seem to indicate just that, although it could also be something in this 
>> netback commit that triggers a latent bug in netfront, can't rule that one 
>> out completly.
>> 
>> But the trigger is in that commit &&
>> annie's and your patch seem to have no effect at all( on this issue) &&
>> later commits in 3.14 do seems to mask it / make it less likely to trigger, 
>> but do not fix it.
>> 

> Unfortunately I've stared at the same piece of code for some time but
> don't have immediate clue. Later commits don't look suspecious either.

> I also looked at netfront code, but there's no slot couting change
> between 3.13 and 3.14.

> Do you have some straight setup instructions so that I can try to
> reproduce.

Not really .. since it is more easily triggerable without the later commits i 
will see if i can trigger it now by using
netperf.

My feeling says it is something like a off by one error (since it doesn't 
happen on all packets .. )
and that it was already in the code before but just wasn't triggered ..

Which makes it hard to spot .. especially since it seems quite difficult to 
trace were that bad grant reference comes from
(and if it is the cause .. or the effect).

Will see if i can reliably trigger it now with netperf and report back.

> Wei.

>> > Paul, do you have any idea what might go wrong?
>> 
>> > Wei.
>> 



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