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

[Christian Schwamborn]

Hi Hans,

I can try, but the only system I can really test this is a productive 
system, as this 'reliable' shows this issue (and I don't want to crash 
it on purpose on a regular basis). Since I set gnttab_max_frame to a 
higher value it runs smooth. If you're confident this will work I can 
try this in the eventing, when all users logged off.

Best regards,
Chritian


On 23.02.2018 16:18, Hans van Kranenburg wrote:
> Hi Valentin, Christian,
> Finally getting back to you about the max grant frames issue.
> 
> We discussed this with upstream Xen developers, and a different fix was
> proposed. I would really appreciate if you could test it and confirm it
> also solves the issue. Testing does not involve recompiling the
> hypervisor with patches etc.
> 
> The deadline for changes for the 9.4 Stretch point release is end next
> week, so we aim to get it in then.
> 
> The cause of the problem is, like earlier discused, the "blkback
> multipage ring" changes a.k.a. "multi-queue xen blk driver" which eats
> grant frame resources way too fast.
> 
> As shown in the reports, this issue already exists while using the
> normal stretch kernel (not only newer backports) in combination with Xen
> 4.8.
> 
> The upstream change we found earlier that doubles the max number to 64
> is part of a bigger change that touches more of the inner workings,
> making Xen better able to handle the domU kernel behavior. This whole
> change is not going to be backported to Xen 4.8.
> 
> 
> Can you please test the following, instead of setting the
> gnttab_max_frames value:
> 
> Create the file
>      /etc/modprobe.d/xen-blkback-fewer-gnttab-frames
> with contents...
> 
> # apropos of #880554
> # workaround is not required for Xen 4.9 and later
> options xen_blkback max_ring_page_order=0
> options xen_blkback max_queues=1
> 
> ...and reboot.
> 
> This will cause the domU kernels to behave more in a way that Xen 4.8
> can cope with.
> 
> Regards,
> Hans
>