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

[Dario Faggioli <dfaggioli@xxxxxxxx>]

On Tue, 2019-07-02 at 08:29 +0000, Jan Beulich wrote:
> On 02.07.2019 10:21, Dario Faggioli wrote:
> > On Tue, 2019-07-02 at 07:54 +0000, Jan Beulich wrote:
> > > 
> > > 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.
> 
> No, the meaning of pCPU is a single thread of a single core. I.e.
> what is represented by a single cpumask_t bit.
>
Fine, let's continue to call that a pCPU. Then, when core-scheduling is
enabled, there's no <<multiple vCPUs of a unit being pinned to the same
pCPU and all competing for jut its CPU time>>.

There's units of 4 vCPUs being pinned to 4 pCPUs (the 4 pCPUs of a
core, not 4 random, nor arbitrary, ones).

That is the point, AFAIUI.

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