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

[Hans van Kranenburg]

On 02/28/2018 08:54 AM, Valentin Vidic wrote:
> On Tue, Feb 27, 2018 at 08:22:50PM +0100, Valentin Vidic wrote:
>> Since I can't reproduce it easily anymore I suspect something was
>> fixed in the meanwhile.  My original report was for 4.9.30-2+deb9u2
>> and since then there seems to be a number of fixes that could be
>> related to this:
> 
> Just rebooted both dom0 and domU with 4.9.30-2+deb9u2 and the the
> postgresql domU is having problems right away after boot:
> 
>   domid=1: nr_frames=32, max_nr_frames=32
> 
>   [  242.652100] INFO: task kworker/u90:0:6 blocked for more than 120 seconds.
> 
> Upgrading the kernels and I can't get it above 11 anymore:
> 
>   domid=1: nr_frames=11, max_nr_frames=32
> 
> So some of those many kernel fixes did the trick and things just
> work fine with the newer kernels without raising gnttab_max_frames.

During my testing I also couldn't quickly cause the nr_frames exhaustion
to happen with block devices, but I still can with a decent amount of
network interfaces inside the domU.

Anyway, I think the future proof solution here is to have clear
documentation about how to configure related settings, instead of trying
to find values that suit all users and that are not ridiculously high.

In Xen 4.10/4.11 the settings changed by the way. The default for in the
dom0 is 64 now, and the default for domUs can be set in xl.conf (which
is still 32), I have it at max_grant_frames=64 currently. It can also be
set per domU, but I like setting it system-wide more.

There's still a xen kernel option for this, which causes the dom0 value
to be set, and which determines the upper limit for the xl.conf option,
iirc.

Oh, and the setting for a domU can also be changed while it's running.
Mind blown.

So yeah, it's a bit complicated, like 4 or 6 knobs to turn which you all
need to get in the right direction, instead of only the old option.

I only don't know where to put the info pointing the user at the right
places to config this. NEWS.Debian? Somewhere else? There is reference
documentation about this in the man pages, but I don't think there's a
tutorial/howto kind of documentation.

Hans