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

On 2015-12-02 00:41, Boris Ostrovsky wrote:
On 12/01/2015 06:30 PM, Sander Eikelenboom wrote:
On 2015-12-02 00:19, Boris Ostrovsky wrote:
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

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

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:

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

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


That was already underway compiling :)

And it does reveal that reverting both fixes the issue, no stuck kworker thread .. and no: genirq: Flags mismatch irq 8. 00000000 (hvc_console) vs. 00000000 (rtc0)
   hvc_open: request_irq failed with rc -16.

Let me try it again tomorrow. Can you post your guest config file, Xen
version and host HW (Intel or AMD)? 'xl info' maybe?


Hi Boris,

A fresh new day .. a fresh new thought.
If i look at the /proc/interrupts from a broken and a kernel with both commits the thing that catches the eye is irq8, just as the dmesg message was telling.

In my PV guest rtc0 now seems to try and take irq8 that was already assigned to HVC ? Sounds like some assumptions around the legacy range are broken somewhere.

What is the benefit of not just reserving the legacy range ?

Attached the /proc/interrupts from both boots.


What i did get was an conflict reverting b4ff8389ed14b849354b59ce9b360bdefcdbf99c: arch/arm64/include/asm/irq.h, although that shouldn't matter because we are on x86 and not on arm.

-- Sander

-- Sander


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Attachment: interrupts-after-reverts.txt
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Attachment: interrupts-broken.txt
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