linux-kernel - Re: [Xen-devel] [PATCH RFC] xen: if on Xen, "flatten" the scheduling domain hierarchy

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Date:	Tue, 15 Sep 2015 19:16:58 +0200
From:	Dario Faggioli <dario.faggioli@...rix.com>
To:	Juergen Gross <jgross@...e.com>
CC:	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	"xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
	"Andrew Cooper" <Andrew.Cooper3@...rix.com>,
	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	George Dunlap <George.Dunlap@...rix.com>,
	David Vrabel <david.vrabel@...rix.com>,
	"Luis R. Rodriguez" <mcgrof@...not-panic.com>
Subject: Re: [Xen-devel] [PATCH RFC] xen: if on Xen, "flatten" the
 scheduling domain hierarchy

On Wed, 2015-09-02 at 16:30 +0200, Juergen Gross wrote:
> On 09/02/2015 04:08 PM, Boris Ostrovsky wrote:
> > On 09/02/2015 07:58 AM, Juergen Gross wrote:
> >> On 08/31/2015 06:12 PM, Boris Ostrovsky wrote:

> >>> If set_cpu_sibling_map()'s has_mp is false, wouldn't we effectively have
> >>> both of your patches?
> >>
> >> Hmm, sort of.
> >>
> >> OTOH this would it make hard to make use of some of the topology
> >> information in case of e.g. pinned vcpus (as George pointed out).
> >
> >
> > I didn't mean to just set has_mp to zero unconditionally (for Xen, or
> > any other, guest). We'd need to have some logic as to when to set it to
> > false.
> 
> In case we want to be able to use some of the topology information this
> would mean we'd have two different mechanisms to either disable all
> topology usage or only parts of it. I'd rather have a way to specify
> which levels of the topology information (numa nodes, cache siblings,
> core siblings) are to be used. Using none is just one possibility with
> all levels disabled.
> 
I agree, indeed, acting on has_mp seems overkill/not ideal to me too
(I'm not even sure I fully understand how it's used in
set_cpu_sibling_map()... I'll dig more).

However...

> >>
> >>> Also, it seems to me that Xen guests would not be the only ones having
> >>> to deal with topology inconsistencies due to migrating VCPUs. Don't KVM
> >>> guests, for example, have the same problem? And if yes, perhaps we
> >>> should try solving it in non-Xen-specific way (especially given that
> >>> both of those patches look pretty simple and thus are presumably easy to
> >>> integrate into common code).
> >>
> >> Indeed. I'll have a try.
> >>
...yes, this is an interesting point, and it's worth try looking at how
to implement things that way.

Thanks and Regards,
Dario
-- 
<<This happens because I choose it to happen!>> (Raistlin Majere)
-----------------------------------------------------------------
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)

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