Re: [PATCH V7 0/9] SCHED_DEADLINE server infrastructure

[Juri Lelli]

Hi Daniel,

On 27/05/24 14:06, Daniel Bristot de Oliveira wrote:
> This is v7 of Peter's SCHED_DEADLINE server infrastructure
> implementation [1].

I finally managed to give this a go and can report that it works great
for what I've seen. :)

So, please consider this reply a

Tested-by: Juri Lelli <juri.lelli@xxxxxxxxxx>

> SCHED_DEADLINE servers can help fixing starvation issues of low priority
> tasks (e.g., SCHED_OTHER) when higher priority tasks monopolize CPU
> cycles. Today we have RT Throttling; DEADLINE servers should be able to
> replace and improve that.

...

> The problem with DL server only implementation is that FIFO tasks might
> suffer preemption from NORMAL even when spare CPU cycles are available.
> In fact, fair deadline server is enqueued right away when NORMAL tasks
> wake up and they are first scheduled by the server, thus potentially
> preempting a well behaving FIFO task. This is of course not ideal.
> 
> We had discussions about it, and one of the possibilities would be
> using a different scheduling algorithm for this. But IMHO that is
> an overkill.
> 
> Juri and I discussed this and though about delaying the server
> activation for the (period - runtime), thus enabling the server
> only if the fair scheduler is about to starve. We called it
> the defer server.
> 
> The defer the server start to the (absolute deadline - runtime)
> point in time. This is achieved by starting the dl server throttled,
> with a next replenishing time set to activate the server at
> (absolute deadline - runtime).
> 
> The server is enqueued with the runtime replenished. As the fair
> scheduler runs without boost, its runtime is consumed. If the
> fair server has its runtime before the runtime - deadline time,
> the a new period is set, and the timer armed for the new
> deadline.

I also wanted to pay particular attention to this part implementing the
deferred server, but failed to find enough focus time for now. I will
keep trying. One thing that I wondered though is if this change (and the
move towards this replacing current RT throttling) would call for a Doc
update. What do you think?

Thanks!
Juri