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



On 02.07.19 12:01, Jan Beulich wrote:
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

dom0 vcpu0 will be pinned to cpu0.
dom0 vcpu1 will be pinned to cpu1.
dom0 vcpu2 will be pinned to cpu2.
dom0 vcpu3 will be pinned to cpu3.

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

In reality only pinning vcpu0 to cpu0 should be needed. And this is done
e.g. in dcdbas_smi_request().


Juergen

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