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

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

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On Tue, Oct 05, 2021 at 10:05:39AM +0200, Juergen Gross wrote:
> On 04.10.21 11:14, Marek Marczykowski-Górecki wrote:
> > 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.
> 
> Could you try the attached patch, please? I've tested it briefly with
> PV and PVH guests.

Yes, it helps, thanks!

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

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