Low level design description for _Scheduler_strong_APA_Get_lowest_scheduled

[Gedare Bloom]

Hi Richi,

I have a question about: "scenario where all the nodes that we go
through have a higher priority (less priority number) than our
filter_node(the node to schedule) in which case, we would be returning
ret: the minimum priority node encountered but there would be no
preemptions and filter_node would remain unassigned"

In this case, shouldn't the filter_node remain unscheduled?

On Thu, Aug 6, 2020 at 9:19 AM Richi Dubey <richidubey at gmail.com> wrote:
>
> The part 3 corresponding to the backtracking changes the filter_node pointer (the original node pointer passed to the function) and calls the Scheduler_SMP_Preempt, all of which is documented in the blog post. I feel like this might mean that the file is taking a higher-level role that it is supposed to and I cannot progress further without your views on this.
>

I understand the backtracking aims to follow the bipartite graph
backward, pushing tasks to cpus that were traversed. so one way to
achieve this is like you are trying, maintain a queue of the traversed
tasks/cpus, and just walk them both in reverse order (LIFO) offset by
one. Thinking like a stack, it is kind of like....

while (!done):
  c = pop(qCPU)
  assign(c, t)
  t = pop(qTasks)
suspend(t)
assign(cpu_to_preempt, new_task)

I don't see why it is necessary for you to be checking the priority
values on the backtrack path. You should build these LIFO queues in
such a way that they are correctly ordered.

At this point, you might want to see if you can find an implementation
of this algorithm (in LitmusRT or Linux) to gain some insights.

Gedare

>
> On Thu, Aug 6, 2020 at 8:45 PM Richi Dubey <richidubey at gmail.com> wrote:
>>
>> Hi,
>>
>> Please find the blog post- https://rtemswithrichi.wordpress.com/strong-apa-low-level-design-in-rtems/ that explains the low-level design for get_lowest_function, importance of which was explained in the earlier high-level design description mail.
>>
>> I really need your help in figuring out part 3 of the explanation corresponding to backtracking part of the algorithm in the blog post.
>>
>> If it helps, Please find the latest pull request here and the edited files schedulerstrongapa.c, schedulerstrongapa.h and scheduler.h.
>>
>> Thanks!
>> Richi.
>
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