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

[Juergen Gross <jgross@suse.com>]

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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?


Juergen

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