Subject: Re: [Xen-devel] linux 4.4 Regression: 100% cpu usage on idle pv guest
 under Xen with single vcpu.
To: Sander Eikelenboom <linux@eikelenboom.it>
References: <80dcf47eb772c65a62652d7a56c8ed26@eikelenboom.it>
 <20151130214513.GF14317@char.us.oracle.com>
 <16a638d3c5e24963599e621f265181f8@eikelenboom.it>
 <565CD3A7.1060006@oracle.com>
 <1454b921884da94b7c1e9f434c13eb4a@eikelenboom.it>
 <565E239E.8070507@oracle.com>
 <c79709518e722725981236aa82981ce4@eikelenboom.it>
Cc: linux-kernel@vger.kernel.org, xen-devel@lists.xen.org,
        david.vrabel@citrix.com
From: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Message-ID: <565E2AF1.7090902@oracle.com>
Date: Tue, 1 Dec 2015 18:19:13 -0500
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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
>>>>>>> [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 ?

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
line.

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


-boris

>
> -- 
> Sander
>
>>
>> -boris
>
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@lists.xen.org
> http://lists.xen.org/xen-devel

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