linux-kernel - Re: [RFC PATCH v2 0/6] SCHED_DEADLINE server infrastructure

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Date:   Fri, 7 Aug 2020 15:30:41 +0200
From:   Juri Lelli <juri.lelli@...hat.com>
To:     luca abeni <luca.abeni@...tannapisa.it>
Cc:     peterz@...radead.org, mingo@...hat.com, rostedt@...dmis.org,
        tglx@...utronix.de, linux-kernel@...r.kernel.org,
        tommaso.cucinotta@...tannapisa.it, alessio.balsini@...il.com,
        bristot@...hat.com, dietmar.eggemann@....com,
        linux-rt-users@...r.kernel.org, mtosatti@...hat.com,
        williams@...hat.com, valentin.schneider@....com
Subject: Re: [RFC PATCH v2 0/6] SCHED_DEADLINE server infrastructure

Hi Luca,

On 07/08/20 15:16, luca abeni wrote:
> Hi Juri,
> 
> thanks for sharing the v2 patchset!
> 
> In the next days I'll have a look at it, and try some tests...

Thanks!

> In the meanwhile, I have some questions/comments after a first quick
> look.
> 
> If I understand well, the patchset does not apply deadline servers to
> FIFO and RR tasks, right? How does this patchset interact with RT
> throttling?

Well, it's something like the dual of it, in that RT Throttling directly
throttles RT tasks to make spare CPU cycles available to fair tasks
while this patchset introduces deadline servers to schedule fair tasks,
thus still reserving CPU time for those (when needed).

> If I understand well, patch 6/6 does something like "use deadline
> servers for SCHED_OTHER only if FIFO/RR tasks risk to starve
> SCHED_OTHER tasks"... Right?

That's the basic idea, yes.

> I understand this is because you do not
> want to delay RT tasks if they are not starving other tasks. But then,
> maybe what you want is not deadline-based scheduling. Maybe a
> reservation-based scheduler based on fixed priorities is what you want?
> (with SCHED_DEADLINE, you could provide exact performance guarantees to
> SCHED_OTHER tasks, but I suspect patch 6/6 breaks these guarantees?)

I agree that we are not interested in exact guarantees in this case, but
why not using something that it's already there and would give us the
functionality we need (fix starvation for fair)? It would also work well
in presence of "real" deadline tasks I think, in that you could account
for these fair servers while performing admission control.

Best,

Juri