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

[Jan Beulich <JBeulich@xxxxxxxx>]

On 02.07.2019 11:40, Dario Faggioli wrote:
> 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.

Well, okay, quite possible. But then for the example topology
you gave, what is going to be the effect of

xl vcpu-pin 0 0 0

? In particular for Dom0 and in particular for CPU 0, there may
be reasons to pin a particular vCPU to it.

Jan
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