[Pkg-xen-devel] Bug#880554: xen domu freezes with kernel linux-image-4.9.0-4-amd64

[Hans van Kranenburg]

Hi Christian and everyone else,

Ack on reassign to Xen.

On 01/06/2018 04:11 PM, Yves-Alexis Perez wrote:
> control: reassign -1 xen-hypervisor-4.8-amd64
> 
> On Sat, 2018-01-06 at 15:23 +0100, Valentin Vidic wrote:
>> On Sat, Jan 06, 2018 at 03:08:26PM +0100, Yves-Alexis Perez wrote:
>>> According to that link, the fix seems to be configuration rather than
>>> code.
>>> Does this mean this bug against the kernel should be closed?
>>
>> Yes, the problem seems to be in the Xen hypervisor and not the Linux
>> kernel itself.  The default value for the gnttab_max_frames parameter
>> needs to be increased to avoid domU disk IO hangs, for example:
>>
>>   GRUB_CMDLINE_XEN="dom0_mem=10240M gnttab_max_frames=256"
>>
>> So either close the bug or reassign it to xen-hypervisor package so
>> they can increase the default value for this parameter in the
>> hypervisor code.
>>
> Ok, I'll reassign and let the Xen maintainers handle that (maybe in a stable
> update).
> 
> @Xen maintainers: see the complete bug log for more information, but basically
> it seems that a domu freezes happens with the “new” multi-queue xen blk
> driver, and the fix is to increase a configuration value. Valentin suggests
> adding that to the default.

The dom0 gnttab_max_frames boot setting is about how many pages are
allocated to fill with 'grants'. The grant concept is related to sharing
information between the dom0 and domU.

It allows memory pages to be shared back and forth, so that e.g. a domU
can fill a page with outgoing network packets or disk writes. Then the
dom0 can take over ownership of the memory page and read the contents
and do its trick with it. In this way, zero-copy IO is implemented.

When running xen domUs, the total amount of network interfaces and block
devices that are attached to all of the domUs that are running (and,
apparently, how heavy they are used) cause the usage of these grant guys
to increase. At some point you run out of grants because all of the
pages are filled.

I agree that the upstream default, 32 is quite low. This is indeed a
configuration issue. I myself ran into this years ago with a growing
number of domUs and network interfaces in use. We have been using
gnttab_max_nr_frames=128 for a long time already instead.

I was tempted to reassign src:xen, but in the meantime, this option has
already been removed again, so this bug does not apply to unstable
(well, as soon as we get something new in there) any more (as far as I
can see quickly now).

https://xenbits.xen.org/gitweb/?p=xen.git;a=commit;h=18b1be5e324bcbe2f10898b116db641d404b3d30

Including a better default for gnttab_max_nr_frames in the grub config
in the debian xen package in stable sounds reasonable from a best
practices point of view.

But, I would be interested in learning more about the relation with
block mq although. Does using newer linux kernels (like from
stretch-backports) for the domU always put a bigger strain on this? Or,
is it just related to the overall number of network devices and block
devices you are adding to your domUs in your specific own situation, and
did you just trip over the default limit?

In any case, the grub option thing is a conffile, so any user upgrading
has to accept/merge the change, so we won't cause a stable user to just
run out of memory because of a few extra kilobytes of memory usage
without notice.

Hans van Kranenburg

P.S. Debian Xen team is in the process of being "rebooted" while the
current shitstorm about meltdown/spectre is going on, so don't hold your
breath. :)

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