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

[Boris Ostrovsky]

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.


> Thanks :)
>
> --
> Sander
>
> > > Between 4.3 and 4.4-single:
> > >
> > > -NR_IRQS:4352 nr_irqs:32 16
> > > +Using NULL legacy PIC
> > > +NR_IRQS:4352 nr_irqs:32 0

This is fine, as long as you have b4ff8389ed14b849354b59ce9b360bdefcdbf99c.

> > > -cpu 0 spinlock event irq 17
> > > +cpu 0 spinlock event irq 1

This is strange. I wouldn't expect spinlocks to use legacy irqs.

> > > and later on:
> > >
> > > -hctosys: unable to open rtc device (rtc0)
> > > +rtc_cmos rtc_cmos: hctosys: unable to read the hardware clock
> > >
> > > +genirq: Flags mismatch irq 8. 00000000 (hvc_console) vs. 00000000 
> > > (rtc0)
> > > +hvc_open: request_irq failed with rc -16.
> > > +Warning: unable to open an initial console.
> > >
> > >
> > > between 4.4-single and 4.4-multi:
> > >
> > >  Using NULL legacy PIC
> > > -NR_IRQS:4352 nr_irqs:32 0
> > > +NR_IRQS:4352 nr_irqs:48 0

This is probably OK too since nr_irqs depend on number of CPUs.

I think something is messed up with IRQ. I saw last week something from 
setup_irq() generating a stack dump (warninig) for rtc_cmos but it 
appeared harmless at that time and now I don't see it anymore.

-boris


> > > and later on:
> > >
> > > -rtc_cmos rtc_cmos: hctosys: unable to read the hardware clock
> > > +hctosys: unable to open rtc device (rtc0)
> > >
> > > -genirq: Flags mismatch irq 8. 00000000 (hvc_console) vs. 00000000 
> > > (rtc0)
> > > -hvc_open: request_irq failed with rc -16.
> > > -Warning: unable to open an initial console.
> > >
> > > attached:
> > >     - dmesg with 4.3 kernel with 1 vcpu
> > >     - dmesg with 4.4 kernel with 1 vpcu
> > >     - dmesg with 4.4 kernel with 2 vpcus
> > >     - .config of the 4.4 kernel is attached.
> > >
> > > --
> > > Sander

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