RE: [Xen-devel] cpuidle causing Dom0 soft lockups

["Yu, Ke" <ke.yu@xxxxxxxxx>]

>-----Original Message-----
>From: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx
>[mailto:xen-devel-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of Keir Fraser
>Sent: Monday, February 22, 2010 9:45 PM
>To: Jan Beulich; Yu, Ke
>Cc: Tian, Kevin; xen-devel@xxxxxxxxxxxxxxxxxxx
>Subject: Re: [Xen-devel] cpuidle causing Dom0 soft lockups
>
>On 22/02/2010 13:29, "Jan Beulich" <JBeulich@xxxxxxxxxx> wrote:
>
>>>>> Keir Fraser <keir.fraser@xxxxxxxxxxxxx> 15.02.10 18:33 >>>
>>> --- a/xen/common/sched_credit.c Mon Feb 15 08:19:07 2010 +0000
>>> +++ b/xen/common/sched_credit.c Mon Feb 15 17:25:29 2010 +0000
>>> @@ -1060,6 +1060,7 @@
>>>                 /* We got a candidate. Grab it! */
>>>                 CSCHED_VCPU_STAT_CRANK(speer, migrate_q);
>>>                 CSCHED_STAT_CRANK(migrate_queued);
>>> +                ASSERT(!vc->is_urgent);
>>>                 __runq_remove(speer);
>>>                 vc->processor = cpu;
>>>                 return speer;
>>
>> By what is this assertion motivated? I.e. why shouldn't an urgent vCPU
>> be getting here? We're seeing this (BUG_ON() in the checked in version)
>> trigger...
>
>The author's assertion was that vc must be runnable and hence cannot be
>polling and hence cannot be is_urgent. It sounded dodgy to me hence I
>upgarded it to a BUG_ON(), but couldn't actually eyeball a way in which
>vc->is_urgent could be true at that point in the code. We have the lock on
>the peer cpu's schedule_lock, so is_urgent cannot change under our feet, and
>vc is not running so it cannot be added to the domain's poll_mask under our
>feet.
>
> -- Keir

Right. According to the code, there should be no way to this BUG_ON. If it 
happens, that reveal either bugs of code or the necessity of adding code to 
migrate urgent vcpu count. Do you have more information on how this BUG_ON 
happens?

Regards
Ke

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