Re: [PATCH v7]xen: sched: convert RTDS from time to event driven model

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

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On Wed, 2016-03-09 at 23:00 -0500, Meng Xu wrote:
> On Wed, Mar 9, 2016 at 10:46 AM, Dario Faggioli
> <dario.faggioli@citrix.com> wrote:
> > 
> > Basically, by doing all the replenishments (which includes updating
> > all
> > the deadlines) upfront, we should be able to prevent the above
> > situation.
> > 
> > So, again, thoughts?
> I think we need to decide which vcpu is on depleted q before update
> deadline and we also need to record which vcpus should be updated. 
>
I think you are right about the need of a depleted flag, or something
that will have the same effect (but I really do like using a flag for
that! :-D ).

I don't think we really need to count anything. In fact, what I had in
mind and tried to put down in pseudocode is that we traverse the list
of replenishment events twice. During the first traversal, we do not
remove the elements that we replenish (i.e., the ones that we call
rt_update_deadline() on). Therefore, we can just do the second
traversal, find them all in there, handle the tickling, and --in this
case-- remove and re-insert them. Wouldn't this work?

> So
> I added some code into your code:
> 
> #define __RTDS_is_depleted     3
> #define RTDS_is_depleted  (1<<__RTDS_is_depleted)
> 
As said, I like this. However...

> int num_repl_vcpus = 0;
>  for_each_to_be_replenished_vcpu(v)
>  {
>    if (v->cur_budget <= 0)
>        set_bit(__RTDS_is_depleted, &v->flags);
> 
... I think we can do this in burn_budget(), where we have this check
in place already.

>     rt_update_deadline(v);
>     num_repl_vcpus++;
>  }
> 
>   for_each_to_be_replenished_vcpu(v)
>   {
>     deadline_queue_remove(replq, v);
> 
>     if ( curr_on_cpu(v->processor) == v)) //running
>     {
>         if ( v->cur_deadline >= runq[0]->cur_deadline )
>           tickle(runq[0]); /* runq[0] => first vcpu in the runq */
>     }
>     else if ( __vcpu_on_q(v) )
>     {
>         if ( v->flags & RTDS_is_depleted )         //depleted
>         {
>           clear_bit(__RTDS_is_depleted, &v->flags);
>
 if ( test_and_clear(xxx) )

Or __test_and_clear(xxx).

>           tickle(v);
>         }
>         else                              //runnable
>           /* do nothing */
>     }
>     deadline_queue_insert(v, replq);
> 
>     /* stop at the vcpu that does not need replenishment */
>     num_repl_vcpus--;
>     if (!num_repl_vcpus)
>         break;
>
If we really need to record/mark this, I want to think at how that
would be best done, as I'm not a fan of this counting... But hopefully,
we just don't need to do anything like that, do we?

Thanks and 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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