Re: [PATCH v2 for Xen 4.6 3/4] libxl: enabling XL to set per-VCPU parameters of a domain for RTDS scheduler

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

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On Mon, 2015-06-08 at 15:55 -0500, Chong Li wrote:
> On Mon, Jun 8, 2015 at 10:56 AM, Dario Faggioli

> > So, Thoughts? What do you think the best way forward could be?
> 
> I like option 2 more. But I think we may also need a 'vcpuid' field in
> libxl_sched_params.
> 
For sparse array support, yes. At which point, I would flip the names as
well, i.e., something like this:

libxl_vcpu_sched_params = Struct("vcpu_sched_params",[
    ("vcpuid",       integer, { xxx some init val xxx}),
    ("weight",       integer, {'init_val': 'LIBXL_PARAM_WEIGHT_DEFAULT'}),
    ("cap",          integer, {'init_val': 'LIBXL_PARAM_CAP_DEFAULT'}),
    ("period",       integer, {'init_val': 'LIBXL_PARAM_PERIOD_DEFAULT'}),
    ("slice",        integer, {'init_val': 'LIBXL_PARAM_SLICE_DEFAULT'}),
    ("latency",      integer, {'init_val': 'LIBXL_PARAM_LATENCY_DEFAULT'}),
    ("extratime",    integer, {'init_val': 'LIBXL_PARAM_EXTRATIME_DEFAULT'}),
    ("budget",       integer, {'init_val': 'LIBXL_PARAM_BUDGET_DEFAULT'}),
    ])

libxl_sched_params = Struct("sched_params",[
    ("sched",        libxl_scheduler),
    ("vcpus",        Array(libxl_sched_params, "num_vcpus")),
    ])

With the possibility of naming the latter 'libxl_vcpus_sched_params',
which is more descriptive, but perhaps is too similar to
libxl_vcpu_sched_params.

Ian, George, what do you think?

While we're here, another thing we would appreciate some feedback on is
what should happen to libxl_domain_sched_params_get(). This occurred to
my mind while reviewing patch 4 of this series. Actually, I think we've
discussed this before, but can't find the reference now.

Anyway, my view is that, for a scheduler that uses per-vcpu parameters,
libxl_domain_sched_params_set() should set the same parameters for all
the vcpus.
When it comes to _get(), however, I'm not sure. To match the _set()
case, we'd need to return the parameters of all the vcpus, but we can't,
because the function takes a libxl_domain_sched_params argument, which
just holds 1 tuple.

Should we just WARN and ask, when on that specific scheduler, to use the
per-vcpu variant being introduced in this patch
(libxl_vcpu_sched_params_get())?

This does not look ideal, but without changing the prototype of
libxl_domain_sched_params_get(), I don't see what else sensible we could
do... :-/

Should we change it, and do the LIBXL_API_VERSION "trick"?

So, again, thoughts?

Regards,
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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