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

[Meng Xu <xumengpanda@xxxxxxxxx>]

On Tue, Mar 15, 2016 at 11:32 PM, Chong Li <lichong659@xxxxxxxxx> wrote:
> On Tue, Mar 15, 2016 at 10:14 PM, Meng Xu <mengxu@xxxxxxxxxxxxx> wrote:
>> On Tue, Mar 15, 2016 at 1:22 PM, Chong Li <lichong659@xxxxxxxxx> wrote:
>>> On Tue, Mar 15, 2016 at 11:41 AM, Dario Faggioli
>>> <dario.faggioli@xxxxxxxxxx> 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@xxxxxxxxxx> 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.
>

We need initializing any variable we are going to use, of course. We
should not reply on the compiler to give an initialized value. :-)

Meng
-- 
-----------
Meng Xu
PhD Student in Computer and Information Science
University of Pennsylvania
http://www.cis.upenn.edu/~mengxu/

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