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

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Date:	Wed, 02 Dec 2015 00:00:15 +0100
From:	Sander Eikelenboom <linux@...elenboom.it>
To:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
Cc:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	david.vrabel@...rix.com, linux-kernel@...r.kernel.org,
	xen-devel@...ts.xen.org
Subject: Re: [Xen-devel] linux 4.4 Regression: 100% cpu usage on idle pv
 guest under Xen with single vcpu.

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
>>>>>> [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.
>> 
>> 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:

arch/x86/include/asm/i8259.h
static inline int nr_legacy_irqs(void)
{
         return legacy_pic->nr_legacy_irqs;
}

returns something different in some circumstances ?

--
Sander

> 
> -boris
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