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Re: [Xen-devel] Re: domU console buffer behaviour

  • To: Ferenc Wagner <wferi@xxxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxx>
  • From: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
  • Date: Mon, 29 Dec 2008 22:01:57 +0000
  • Cc:
  • Delivery-date: Mon, 29 Dec 2008 14:02:27 -0800
  • List-id: Xen developer discussion <xen-devel.lists.xensource.com>
  • Thread-index: AclqAREHubHafGrLzUuZS6ccZJuS/A==
  • Thread-topic: [Xen-devel] Re: domU console buffer behaviour

On 29/12/2008 21:54, "Ferenc Wagner" <wferi@xxxxxxx> wrote:

>> I think the console daemon tries to discard contiguous chunks of
>> data, rather than odd characters here and there. How effective it
>> really is I'm not sure, but certainly you can expect the discards to
>> be in reasonable-sized chunks and also to be pretty random.
> Huh, now I'm no closer to have an idea about the expected behaviour.
> What does the console daemon try to achieve?  Does the randomness stem
> from the scheduling irregularity?  On which side of the daemon is the
> 1 MB buffer?

The daemon greedily takes characters from a small ring buffer shared with
the guest, and places them in the much bigger 1MB buffer. The guest can
expect characters to not sit around in the shared ring, and it is possible
that a guest could lock up if that were to happen (although in your example
only the user process writing to the console should hang in that
[impossible] case). Rather than stopping reading characters when the 1MB
buffer fills, the daemon instead discards character sequences from the 1MB

>> Why do you think this has something to do with pv_ops lockups?
> That's just the only trace I can start with.  On kernel lockups, I
> usually look for clues in the console output.  Now I found it garbled.
> Either it is normal and I should look elsewhere, or it is a buffer
> handling bug, possibly overwriting some memory and causing havoc
> later.  I know that's a long shot...  But even SysRq didn't work, so I
> have nothing more to work with.  I'm looking at tools/console/daemon/io.c
> now.

It does sound like a long shot! Did you expect those VMs to be producing a
lot of console output?

 -- Keir

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