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

[Sander Eikelenboom <linux@xxxxxxxxxxxxxx>]

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:
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

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

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

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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Attachment: dmesg-4.3.txt
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Attachment: dmesg-4.4.txt
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Attachment: dmesg-4.4-multi.txt
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