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

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Date:	Sat, 28 Nov 2015 16:47:43 +0100
From:	Sander Eikelenboom <linux@...elenboom.it>
To:	boris.ostrovsky@...cle.com, david.vrabel@...rix.com
Cc:	linux-kernel@...r.kernel.org, xen-devel@...ts.xen.org
Subject: linux 4.4 Regression: 100% cpu usage on idle pv guest under Xen
 with single vcpu.

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

View attachment "dotconfig" of type "text/plain" (105273 bytes)

View attachment "dmesg-4.3.txt" of type "text/plain" (15420 bytes)

View attachment "dmesg-4.4.txt" of type "text/plain" (15361 bytes)

View attachment "dmesg-4.4-multi.txt" of type "text/plain" (15327 bytes)