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:	Tue, 1 Dec 2015 17:47:58 -0500
From:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:	Sander Eikelenboom <linux@...elenboom.it>
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 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.

-boris


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