Re: [PATCH v6 for Xen 4.7 1/4] xen: enable per-VCPU parameter settings for RTDS scheduler

[Chong Li <lichong659@gmail.com>]

On Tue, Mar 15, 2016 at 10:14 PM, Meng Xu <mengxu@cis.upenn.edu> wrote:
> On Tue, Mar 15, 2016 at 1:22 PM, Chong Li <lichong659@gmail.com> wrote:
>> On Tue, Mar 15, 2016 at 11:41 AM, Dario Faggioli
>> <dario.faggioli@citrix.com> wrote:
>>> On Tue, 2016-03-15 at 11:22 -0500, Chong Li wrote:
>>>> On Mon, Mar 14, 2016 at 5:05 AM, Dario Faggioli
>>>> <dario.faggioli@citrix.com> wrote:
>>>> >
>>>
>>> We said 'once' and then 'once per domain', but something I'd be fine
>>> with (coupled with keeping G_WARNING) would be 'once per operation'.
>>> Basically, if a domain has 128 vcpus, and an hypercall tries to set all
>>> of them to period=100, budget=50, we just print the warning once. Then,
>>> if after a while the sysadmin tries the same again, we again just log
>>> once, etc.
>>>
>>> Doing this seems much easier, as the 'warned' flag could just be a
>>> local variable of the hypercall implementation. I'm quite sure that
>>> would work if there is not any continuation/re-issueing mechanism in
>>> the hypercall in question. BUT in our case there is, so things may be
>>> more complicated... :-/
>>>
>>> Had you thought about a solution like this already? If no, can you see
>>> whether there is a nice and easy way to make something like what I just
>>> described above to work in our case?
>>>
>> How about:
>>
>> We create a global variable in sched_rt.c:
>>     /* This variable holds its value through hyerpcall re-issueing.
>>      * When finding vcpu settings with too low budget or period (e.g,
>> 100 us), we print a warning
>>      * and set this variable "true". No more warnings are printed
>> until this variable
>>      * becomes false.
>>      */
>>     static bool warned;
>> Initialize it as "false" in rt_init().
>> In your example,
>> we "warned = true" when we find the first vcpu has budget less than
>> 100 us. Outside
>> of the while loop, we do:
>>     if ( index == op->u.v.nr_vcpus ) /* no more hypercall re-issueing */
>>         warned = false;
>>
> Hi Chong,
>
> I don't think creating a global variable just for the warning thing is
> a better idea. Even if we do want such a variable, it should only
> occur in rt_dom_cntl() function, since it is only used in
> rt_dom_cntl().
> Global variable should be used "globally", isn't it. ;-)
>
You're right.

If we define

   static bool warned;

at the beginning of rt_dom_cntl(), do we need to initialize it? If without
initialization, I think its default value is "false", which is just
what we need.

Chong


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

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