Re: [Xen-devel] [PATCH RFC v1 2/3] libxl: enable per-VCPU work conserving flag for RTDS

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Fri, Aug 04, 2017 at 02:53:51PM +0200, Dario Faggioli wrote:
> On Fri, 2017-08-04 at 13:10 +0100, Wei Liu wrote:
> > On Fri, Aug 04, 2017 at 10:13:18AM +0200, Dario Faggioli wrote:
> > > On Thu, 2017-08-03 at 17:39 -0400, Meng Xu wrote:
> > > > 
> > > *HOWEVER*, in this case, we do have that 'extratime' field already,
> > > as
> > > a leftover from SEDF, which is there taking space and cluttering
> > > the
> > > interface, so why don't make good use of it. Especially considering
> > > it
> > > was used for _exactly_ the same thing, and with _exactly_ the same
> > > meaning, and even for a very similar (i.e., SEDF was also real-
> > > time)
> > > kind of scheduler.
> > 
> > Correct me if I'm wrong:
> > 
> > 1. extratime is ever only used in SEDF
> > 2. SEDF is removed
> > 
> > That means we do have extratime to use in all other schedulers.
> > 
> I'm not sure what you mean with this last line.
> 
> IAC, this is how our the related data structures looks like, right now:
> 
> libxl_sched_params = Struct("sched_params",[
>     ("vcpuid",       integer, {'init_val': 
> 'LIBXL_SCHED_PARAM_VCPU_INDEX_DEFAULT'}),
>     ("weight",       integer, {'init_val': 
> 'LIBXL_DOMAIN_SCHED_PARAM_WEIGHT_DEFAULT'}),
>     ("cap",          integer, {'init_val': 
> 'LIBXL_DOMAIN_SCHED_PARAM_CAP_DEFAULT'}),
>     ("period",       integer, {'init_val': 
> 'LIBXL_DOMAIN_SCHED_PARAM_PERIOD_DEFAULT'}),
>     ("extratime",    integer, {'init_val': 
> 'LIBXL_DOMAIN_SCHED_PARAM_EXTRATIME_DEFAULT'}),
>     ("budget",       integer, {'init_val': 
> 'LIBXL_DOMAIN_SCHED_PARAM_BUDGET_DEFAULT'}),
>     ])
> 
> The extratime field is there. Any scheduler can use it, if it wants
> (and in the way it wants). Currently, no one of them does that.

Right, that's what I wanted to know.

> 
> libxl_domain_sched_params = Struct("domain_sched_params",[
>     ("sched",        libxl_scheduler),
>     ("weight",       integer, {'init_val': 
> 'LIBXL_DOMAIN_SCHED_PARAM_WEIGHT_DEFAULT'}),
>     ("cap",          integer, {'init_val': 
> 'LIBXL_DOMAIN_SCHED_PARAM_CAP_DEFAULT'}),
>     ("period",       integer, {'init_val': 
> 'LIBXL_DOMAIN_SCHED_PARAM_PERIOD_DEFAULT'}),
>     ("budget",       integer, {'init_val': 
> 'LIBXL_DOMAIN_SCHED_PARAM_BUDGET_DEFAULT'}),
> 
>     # The following three parameters ('slice', 'latency' and 'extratime') are 
> deprecated,
>     # and will have no effect if used, since the SEDF scheduler has been 
> removed.
>     # Note that 'period' was an SDF parameter too, but it is still effective 
> as it is
>     # now used (together with 'budget') by the RTDS scheduler.
>     ("slice",        integer, {'init_val': 
> 'LIBXL_DOMAIN_SCHED_PARAM_SLICE_DEFAULT'}),
>     ("latency",      integer, {'init_val': 
> 'LIBXL_DOMAIN_SCHED_PARAM_LATENCY_DEFAULT'}),
>     ("extratime",    integer, {'init_val': 
> 'LIBXL_DOMAIN_SCHED_PARAM_EXTRATIME_DEFAULT'}),
>     ])
> 
> Same here. 'slice', 'latency' and 'extratime' are there because we
> deprecate, but don't remove stuff. They're not used in any way. [*]
> 
> If, at some point, I'd decide to develop a feature for, say Credit2,
> that controll the latency (whatever that would mean, it's just an
> example! :-D) of domains, I think I'll use this 'latency' field, for
> its interface, instead of adding some other stuff.
> 
> > However, please consider the possibility of reintroducing SEDF in the
> > future. Suppose that would happen, does extratime still has the same
> > semantics?
> > 
> Well, I guess yes. But how does this matter? Each scheduler can, if it
> wants, use all these parameters in the way it actuallly prefers. So,
> the fact that RTDS will be using 'extratime' for letting vCPUs execute
> past their own real-time reservation, does not prevent the reintroduced
> SEDF --nor any other already existing or new scheduler-- to also use
> it, for similar (or maybe even not so similar) purposes.
> 
> Or am I missing something?

If extratime means different things to different schedulers, it's going
to be confusing. As a layperson I can't tell what extratime is or how it
is supposed to be used. I would like to have the field to have only one
meaning.

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