Re: [Xen-devel] [PATCH v3 for Xen 4.6 3/4] libxl: enable per-VCPU parameter settings for RTDS scheduler

[Chong Li <lichong659@xxxxxxxxx>]

On Tue, Jun 30, 2015 at 11:19 AM, Ian Campbell <ian.campbell@xxxxxxxxxx> wrote:
> On Tue, 2015-06-30 at 11:10 -0500, Chong Li wrote:
>> On Tue, Jun 30, 2015 at 10:57 AM, Ian Campbell <ian.campbell@xxxxxxxxxx> 
>> wrote:
>> > On Tue, 2015-06-30 at 10:42 -0500, Chong Li wrote:
>> >> >> @@ -1554,10 +1564,17 @@ int libxl_sched_credit_params_set(libxl_ctx 
>> >> >> *ctx, uint32_t poolid,
>> >> >>  #define LIBXL_DOMAIN_SCHED_PARAM_EXTRATIME_DEFAULT -1
>> >> >>  #define LIBXL_DOMAIN_SCHED_PARAM_BUDGET_DEFAULT    -1
>> >> >>
>> >> >> +/* Per-VCPU parameters*/
>> >> >> +#define LIBXL_DOMAIN_SCHED_PARAM_VCPU_INDEX_DEFAULT -1
>> >> >
>> >> > What is the effect of passing vcpuid == -1 to this interface?
>> >>
>> >> I think the right way should be:
>> >>
>> >> #define LIBXL_SCHED_PARAM_VCPU_INDEX_DEFAULT -1
>> >>
>> >> which is the default value for the vcpuid in libxl_sched_params. Even
>> >> though libxl_sched_params is used for per-vcpu settings, all the other
>> >> fields (except vcpuid) would still use
>> >> LIBXL_DOMAIN_SCHED_PARAM_*_DEFAULT values, just for convenience.
>> >
>> > My question is what does the concept of vcpu==-1 mean in this interface?
>>
>> We need a default value for vcpuid when initiating libxl_sched_params,
>> just like other fields (e.g., budget, weight ...) in the struct. But
>> the default value could be others, maybe "0". I'm not sure.
>
> What will you do if you come across a -1 in this field? What are the
> semantics of the interface WRT this field.
>
> Note that this field is not the same as the others in this struct, it is
> in effect part of the "key" while the others are the "values".
>
For example, if vm1 has 4 VCPUs, and the user types in:

"xl rtds -d vm1 -v 1 -v 3"

to get (output) the per-vcpu information of VCPU 1 and VCPU 3. In
libxl.c, we create an array (say vcpus), which has two elements (each
one is a libxl_sched_params). Then we do:

vcpus[0].vcpuid = 1; vcpus[1].vcpuid = 3;

and all the other fields in each libxl_sched_params keep empty, but
will be filled in by the hypercall handler.

So vcpuid == -1 only exists during initiation, and it will be
definitely given a valid value later.

Chong

> Ian.
>



-- 
Chong Li
Department of Computer Science and Engineering
Washington University in St.louis

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