Re: [PATCH v2 for Xen 4.6 4/4] xl: enabling XL to set per-VCPU parameters of a domain for RTDS scheduler

[Dario Faggioli <dario.faggioli@citrix.com>]

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On Tue, 2015-06-09 at 10:12 -0500, Chong Li wrote:
> On Tue, Jun 9, 2015 at 3:01 AM, Dario Faggioli
> <dario.faggioli@citrix.com> wrote:

> How about making
> 
> # xl sched-rtds -d vm1
> 
> output the per-dom parameters (of vm1), and meanwhile let
>
> # xl sched-rtds -d vm1 -v all
> 
> output the per-vcpu parameters (of vm1).
> 
> So, in this way, all the "per-vcpu" commands need to consistently specify "-v".
> 
I see what you mean, but what I don't like is the distinction between
per-dom and per-vcpu parameters and command... I don't think it make
sense (see below).

> > So, IIRC, in patch 3 you are just not touching
> > libxl_domain_sched_params_get(), nor sched_rtds_domain_get() and
> > (perhaps in another patch) xc_sched_rtds_domain_get(). What I'm asking
> > is, what does it mean "users who only do per-dom settings"? What happens
> > if I, for instance, do a bunch of:
> >
> >  libxl_vcpu_sched_params_set(...)
> >
> > for various vcpus, each one with different parameters, and then call:
> >
> >  libxl_domain_sched_params_get(...)
> >
> > Have you tested this case? What's the output?
> 
> I described this use case in the cover letter.
> 
Ok, yes, now I remember seeing it, and wanting to comment. Then, because
of the bad threading of the series, the cover letter is somewhere lost
in my xen-devel folder, and I forgot to revisit it when reviewing the
single patches.

It's certainly my fault, but that's one of the many reasons why the
proper threading is important.

> "This command should only be used when all vcpus of a domain have the
> same parameters, otherwise the output
> is pointless. The period and budget shown in the output are equal to
> the parameters of the first
> VCPU of each domain."
> 
> Or, I think we can just output a warning message in this case.
> 
Neither would be optimal, IMO. If I set the parameters of the vcpus of a
VM, then my shift finishes and I leave, how does the sysadmin that does
the next shift (or even me, after 1 month) whether I gave to all the
vcpus the same parameters, and hence whether the output of `xl
sched-rtds -o' (or whatever) can be trusted?

What do you mean with "output a warning message"? Do you mean printing a
warning if someone tries to retrieve domain scheduling parameters of the
RTDS scheduling? Or do you mean print a warning if the vcpus have
different parameters?

As said, I don't like much this option either, but doing the former may
be the only option we have (the latter looks just silly).

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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