Re: [Xen-devel] linux 4.4 Regression: 100% cpu usage on idle pv guest under Xen with single vcpu.

[Boris Ostrovsky]

On 12/01/2015 06:00 PM, Sander Eikelenboom wrote:
> On 2015-12-01 23:47, Boris Ostrovsky wrote:
> > On 11/30/2015 05:55 PM, Sander Eikelenboom wrote:
> > > On 2015-11-30 23:54, Boris Ostrovsky wrote:
> > > > On 11/30/2015 04:46 PM, Sander Eikelenboom wrote:
> > > > > On 2015-11-30 22:45, Konrad Rzeszutek Wilk wrote:
> > > > > > On Sat, Nov 28, 2015 at 04:47:43PM +0100, Sander Eikelenboom wrote:
> > > > > > > Hi all,
> > > > > > >
> > > > > > > I have just tested a 4.4-rc2 kernel (current linus tree) + the 
> > > > > > > tip tree
> > > > > > > pulled on top.
> > > > > > >
> > > > > > > Running this kernel under Xen on PV-guests with multiple vcpus 
> > > > > > > goes well (on
> > > > > > > idle < 10% cpu usage),
> > > > > > > but a guest with only a single vcpu doesn't idle at all, it 
> > > > > > > seems a kworker
> > > > > > > thread is stuck:
> > > > > > > root       569 98.0  0.0      0     0 ?        R 16:02 12:47
> > > > > > > [kworker/0:1]
> > > > > > >
> > > > > > > Running a 4.3 kernel works fine with a single vpcu, bisecting 
> > > > > > > would probably
> > > > > > > quite painful since there were some breakages this merge window 
> > > > > > > with respect
> > > > > > > to Xen pv-guests.
> > > > > > >
> > > > > > > There are some differences in the diff's from booting a 4.3, 
> > > > > > > 4.4-single,
> > > > > > > 4.4-multi cpu boot:
> > > > > >
> > > > > > Boris has been tracking a bunch of them. I am attaching the 
> > > > > > latest set of
> > > > > > patches I've to carry on top of v4.4-rc3.
> > > > >
> > > > > Hi Konrad,
> > > > >
> > > > > i will test those, see if it fixes all my issues and report back
> > > >
> > > > They shouldn't help you ;-( (and I just saw a message from you 
> > > > confirming this)
> > > >
> > > > The first one fixes a 32-bit bug (on bare metal too). The second fixes
> > > > a fatal bug for 32-bit PV guests. The other two are code
> > > > improvements/cleanup.
> > >
> > > One of these patches also fixes a bug i was having with a 
> > > pci-passthrough device in
> > > a HVM that wasn't working (depending on which dom0-kernel i was 
> > > using (4.3 or 4.4)),
> > > but didn't report yet.
> > >
> > > Fingers crossed but i think this pv-guest single vcpu issue is the 
> > > last i'm troubled by for now ;)
> >
> > I could not reproduce this, including with your kernel config file.
>
> Hmm that's unpleasant :-\
>
> Hmm other strange thing is it doesn't seem to affect dom0 (which is 
> also a PV guest), but only unprivileged ones
> All unprivileged pv-guests seem to have the irq issue, but only with a 
> single vcpu i see to get the stuck kworker thread that got my 
> attention, with a 2 vcpu that doesn't seem to happen, but you still 
> get the dmesg output and warnings about hvc)
>
> Could it be that:
>
> arch/x86/include/asm/i8259.h
> static inline int nr_legacy_irqs(void)
> {
>         return legacy_pic->nr_legacy_irqs;
> }
>
> returns something different in some circumstances ?

It should return 16 pre-8c058b0b9c34d8c8d7912880956543769323e2d8 and 0 
after that commit.

This is the last number that you see in
    NR_IRQS:4352 nr_irqs:48 0
line.

I think you should be able to safely revert both 
b4ff8389ed14b849354b59ce9b360bdefcdbf99c and 
8c058b0b9c34d8c8d7912880956543769323e2d8 and see if it makes any 
difference.


-boris

> --
> Sander
>
> > -boris
>
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