Re: xen-balloon thread using 100% of CPU, regression in 5.4.150

["Marek Marczykowski-Górecki" <marmarek@invisiblethingslab.com>]

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On Mon, Oct 04, 2021 at 07:31:40AM +0200, Juergen Gross wrote:
> On 03.10.21 06:47, Marek Marczykowski-Górecki wrote:
> > Hi,
> > 
> > After updating a PVH domU to 5.4.150, I see xen-balloon thread using
> > 100% CPU (one thread).
> > This is a domain started with memory=maxmem=716800KiB (via libvirt). Then,
> > inside, I see:
> > 
> > # cat /sys/devices/system/xen_memory/xen_memory0/target_kb
> > 716924
> > # cat /sys/devices/system/xen_memory/xen_memory0/info/current_kb
> > 716400
> > 
> > Doing `cat info/current_kb > target_kb` "fixes" the issue. But still,
> > something is wrong - on earlier kernel (5.4.143 to be precise), it
> > wasn't spinning, with exactly the same values reported in sysfs. It
> > shouldn't run in circles if it can't get that much memory it wants. I
> > strongly suspect "xen/balloon: use a kernel thread instead a workqueue"
> > or related commit being responsible, but I haven't verified it.
> 
> I think you are right. I need to handle the BP_ECANCELED case similar to
> BP_EAGAIN in the kernel thread (wait until target size changes again).
> 
> One further question: do you see any kernel message in the guest related
> to the looping balloon thread?

Nothing, only the usual "xen:balloon: Initialising balloon driver", and
nothing related to balloon after that.


-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab

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