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

[Boris Ostrovsky <boris.ostrovsky@oracle.com>]

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
>>>
>>>