Re: [Xen-devel] [PATCH v10 09/11] x86/ctxt: Issue a speculation barrier between vcpu contexts

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 26.01.18 at 02:08, <dfaggioli@xxxxxxxx> wrote:
> On Thu, 2018-01-25 at 19:49 +0100, Dario Faggioli wrote:
>> On Thu, 2018-01-25 at 16:09 +0000, Andrew Cooper wrote:
>> > On 25/01/18 15:57, Jan Beulich wrote:
>> > >
>> > > For the record, the overwhelming majority of calls to
>> > > __sync_local_execstate() being responsible for the behavior
>> > > come from invalidate_interrupt(), which suggests to me that
>> > > there's a meaningful number of cases where a vCPU is migrated
>> > > to another CPU and then back, without another vCPU having
>> > > run on the original CPU in between. If I'm not wrong with this,
>> > > I have to question why the vCPU is migrated then in the first
>> > > place.
>> > 
>> > Dario made a different fix to Credit1 upstream which was supposed
>> > to
>> > resolve this behaviour (although I can't locate the patch by a list
>> > of
>> > titles), but based on these observations, I'd say the misfeature
>> > hasn't
>> > been fixed.
>> > 
>> Yes, it's 341450eaf753 ("xen: credit1: increase efficiency and
>> scalability of load balancing."), and that commit and the XenServer
>> patch are functionally equivalent.
>> 
>> So, I'm not convinced about that being the actual cause of the
>> described behaviour. Tracing would be the way to tell (hopefully) for
>> sure.
>> 
> And in order to go and investigate this a bit further, Jan, what is it
> that you were doing when you saw what you described above? AFAIUI,
> that's booting an HVM guest, isn't it?

Yes, plus then run some arbitrary work inside it.

> How many vCPUs on how many pCPUs? Basically, I would just need a
> confirmation that the system was not oversubscribed, but if, while
> we're here, you can tell me the details, I'll put together an as much
> as possible similar scenario.

12 pCPU-s, HVM guest has 8 vCPU-s, Dom0 has 12 pCPU-s (but
no other load than that caused by the guest). If you want, I could
bring the code I've used for monitoring into patch form and hand
it to you.

Jan


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