Re: [Xen-devel] [PATCH 13/60] xen/sched: move some per-vcpu items to struct sched_unit

From: Dario Faggioli <dfaggioli@suse.com>
To: Jan Beulich <JBeulich@suse.com>, Juergen Gross <JGross@suse.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>, Wei Liu <wl@xen.org>,
	Konrad Wilk <konrad.wilk@oracle.com>,
	George Dunlap <George.Dunlap@eu.citrix.com>,
	Andrew Cooper <andrew.cooper3@citrix.com>,
	Tim Deegan <tim@xen.org>, Julien Grall <julien.grall@arm.com>,
	Meng Xu <mengxu@cis.upenn.edu>,
	"xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>,
	Ian Jackson <ian.jackson@citrix.com>,
	Roger Pau Monne <roger.pau@citrix.com>
Subject: Re: [Xen-devel] [PATCH 13/60] xen/sched: move some per-vcpu items to struct sched_unit
Date: Tue, 2 Jul 2019 08:21:53 +0000	[thread overview]
Message-ID: <ace01bb733d0dd624b9e9ee07c5da4f5567a8a8a.camel@suse.com> (raw)
In-Reply-To: <ee6a055b-2a47-e6ee-91d4-f23bfce5243d@suse.com>

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On Tue, 2019-07-02 at 07:54 +0000, Jan Beulich wrote:
> On 02.07.2019 08:30, Juergen Gross wrote:
> > On 01.07.19 17:46, Jan Beulich wrote:
> > > 
> > > Hmm, that's indeed what I was deducing, but how will we sell this
> > > to people actually fiddling with vCPU affinities? I foresee
> > > getting
> > > bug reports that the respective xl command(s) do(es)n't do
> > > anymore
> > > what it used to do.
> > 
> > The new behavior must be documented, sure.
> 
> Documentation is just one aspect. Often enough people only read docs
> when wanting to introduce new functionality, which I consider a fair
> model. Such people will be caught by surprise that the pinning
> behavior does not work the same way anymore.
> 
That is indeed the case, and we need to think about how to address it,
I agree.

> And again - if someone pins every vCPU to a single pCPU, that last
> such pinning operation will be what takes long term effect. Aiui all
> vCPU-s in the unit will then be pinned to that one pCPU, i.e.
> they'll either all compete for the one pCPU's time, or only one of
> them will ever get scheduled.
> 
I'm not sure I'm getting this. On an, say, SMT system, with 4 threads
per core, a unit is 4 vCPUs and a pCPU is 4 threads.

If we pin all the 4 vCPUs of a unit to one 4 thread pCPU, each vCPU
will get a thread.

Isn't it so?

Regards
-- 
Dario Faggioli, Ph.D
http://about.me/dario.faggioli
Virtualization Software Engineer
SUSE Labs, SUSE https://www.suse.com/
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