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

From: Dario Faggioli <dfaggioli@suse.com>
To: Jan Beulich <JBeulich@suse.com>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>,
	Dario Faggioli <dario.faggioli@citrix.com>,
	David Woodhouse <dwmw@amazon.co.uk>,
	Xen-devel <xen-devel@lists.xen.org>
Subject: Re: [PATCH v10 09/11] x86/ctxt: Issue a speculation barrier between vcpu contexts
Date: Fri, 26 Jan 2018 02:08:05 +0100	[thread overview]
Message-ID: <1516928885.15341.72.camel@suse.com> (raw)
In-Reply-To: <1516906149.15341.66.camel@suse.com>

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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?

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.

Thanks,
Dario
-- 
<<This happens because I choose it to happen!>> (Raistlin Majere)
-----------------------------------------------------------------
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Software Engineer @ SUSE https://www.suse.com/

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