Re: [Xen-devel] [PATCH v1 3/4] libxl: add rt scheduler

[George Dunlap <George.Dunlap@xxxxxxxxxxxxx>]

On Thu, Sep 4, 2014 at 4:44 PM, Dario Faggioli
<dario.faggioli@xxxxxxxxxx> wrote:
> On gio, 2014-09-04 at 11:07 -0400, Meng Xu wrote:
>
>> 2014-09-04 10:51 GMT-04:00 George Dunlap
>> <george.dunlap@xxxxxxxxxxxxx>:
>>         On 09/04/2014 03:47 PM, Meng Xu wrote:
>>
>>         > Hi George,
>>         >
>>         >
>>         > 2014-09-04 10:27 GMT-04:00 George Dunlap
>>         > <George.Dunlap@xxxxxxxxxxxxx>:
>>         >         On Wed, Sep 3, 2014 at 4:33 PM, George Dunlap
>>         >         <George.Dunlap@xxxxxxxxxxxxx> wrote:
>>         >         >
>>         >         > While the domctl interface is not stable, the
>>         >         libxl interface *is*
>>         >         > stable, so we definitely need to think carefully
>>         >         about what we want
>>         >         > this to look like.
>>         >         >
>>         >         > Let me give that a think. :-)
>>         >
>>         >
>>         >         OK, so we had a chat about this at our team meeting
>>         >         today, and here is
>>         >         what we came up with.
>>         >
>>         >         The feature freeze for 4.5 is next Wednesday.
>>         >
>>         >         The core scheduler is in good enough shape to be
>>         >         checked in as an
>>         >         "experimental" mode, so it would be really nice to
>>         >         be able to get this
>>         >         checked in.
>>         >
>>         >         The DOMCTL interface isn't stable so we can change
>>         >         that if we need to;
>>         >         however, the libxl interface *is* stable.
>>         >
>>         >         The current libxl scheduler parameter interface
>>         >         assumes one set of
>>         >         parameters per domain; it's not yet setup for
>>         >         per-vcpu parameters.  It
>>         >         is unlikely that we would be able to converge on a
>>         >         new interface by
>>         >         next week.
>>         >
>>         >         So the suggestion was this: For the moment, use the
>>         >         existing libxl
>>         >         interface on a per-domain basis.  Internally, this
>>         >         will set all vcpus
>>         >         to the same values.  This will allow us to check in
>>         >         a useable version
>>         >         of the scheduler for people to test and improve.
>>         >         Then for 4.6 we can
>>         >         start working on a suitable libxl interface for
>>         >         setting per-vcpu
>>         >         scheduling parameters.
>>         >
>>         >         Dario / Ian, did I miss anything?
>>         >
>>         >         Meng / &c, does that sound reasonable?
>>         >
>>         >
>>         > I have a question as to the user interface.
>>         > For 4.5, we only allow users to set all vcpus to the same
>>         > values (I'm totally fine with it.);
>>         > But how about the get function? When users issue the command
>>         > "xl sched-rt", how should we display the parameters of
>>         > vcpus? We just give the "period", "budget" and "#VCPU" for a
>>         > domain? I'm fine with this display for 4.5.
>>         >
>>         >
>>         > However ,my concerns is: In 4.6, when we allow vcpus to have
>>         > different parameters and need to display every vcpu's
>>         > parameters, how should we display when users use command "xl
>>         > sched-rt"? When vcpus have different period and budget, we
>>         > cannot display like what we did in 4.5 then. :-(
>>         >
>>         >
>>         > It's just my thought, just in case we neglect it. :-)
>>
>>
>>         I think the xl interface doesn't have quite the same
>>         consistency guarantees as the libxl interface.  For now, I
>>         think just make it print one budget / period for the domain;
>>         and we can change it later.
>>
>>
>>
>>
>> I see. I'm totally ok with the decision! :-)
>> So I will only use the existing libxl interface without adding an
>> array to it, to set/get the vcpus' parameters of a domain. Am I right?
>>
> Yep, no array. You just add a 'period' and a 'budget' fields _inside_
> libxl_domain_sched_params, without putting them inside any wrapping
> struct, union or array.

Except that you don't need to add a "period" field, since there's
already one there (for the SEDF scheduler).

We could re-use the "slice" field instead of adding "budget", but I
think probably for clarity adding "budget" is better (although I'm
open to other opinions on that one).

 -George

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