Re: [Xen-users] whether xen scheduler supports preemption

[Christy Business <christy4business@xxxxxxxxx>]

Hello

Please, i want to continue in this topic!

How does the xen schedule ensures that the cap(not null) value  of a VM is respected? Firstly in a case of a single VCPU VM and in the case of the multiple  VPCU VM?

2013/6/26 张伟 <zhangwqh@xxxxxxx>

Thank you very much for your reply! See below.

在 2013-06-26 07:27:26，"Dario Faggioli" <dario.faggioli@xxxxxxxxxx> 写道：
>On mar, 2013-06-25 at 22:29 +0800, 张伟 wrote:
>> Hi, all
>> 
>Hi,
>
>> I want to know whether xen scheduler supports preemption? 
>>
>Yes, it definitely does.
>
>> If it supports,  in the source code, which part decides it has the
>> preemption function? 
>> 
>All the important bits about Xen scheduling can be found, in the source
>code, in the following files:
>
>xen/common/schedule.c
>xen/common/sched_credit.c
>xen/common/sched_credit2.c
>xen/common/sched_sedf.c
>xen/common/sched_arinc653.c
>
>Look particularly carefully at schedule.c, which hosts the generic
>scheduling frameweork, common to all the scheduling algorithms we
>support, and to sched_credit.c, which is where the scheduling algorithm
>that is used by default is implemented.
>
>Regarding what you're saying about the "preemption function", I'm sorry,
>but I cannot parse that part of the sentence... What do you mean by
>"which part decides it has the preemption function"?

My meaning is that which code decides the xen scheduler has the preemption ability. In the sched_credit.c file, there is a function, csched_vcpu_wake()->__runq_tickle(), in the function, __runq_tickle(), at the end, there is the following code:

if ( !cpumask_empty(&mask) )
        cpumask_raise_softirq(&mask, SCHEDULE_SOFTIRQ);

It will raise SCHEDULE_SOFTIRQ interrupt, whether here decides it has the preemption ability, or other parts? 

If it raise a SCHEDULE_SOFTIRQ interrupt, when will deal with this software interrupt? In time or the current vcpu gives up the physical cpu?  

>
>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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