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

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Date:   Fri, 7 Aug 2020 16:04:11 +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

On 07/08/20 15:41, luca abeni wrote:
> Hi Juri,
> 
> On Fri, 7 Aug 2020 15:30:41 +0200
> Juri Lelli <juri.lelli@...hat.com> wrote:
> [...]
> > > 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).
> 
> Ah, OK... I was thinking about using deadline servers for both RT and
> non-RT tasks. And to use them not only to throttle, but also to provide
> some kind of performance guarantees (to all the scheduling classes).
> Think about what can be done when combining this mechanism with
> cgroups/containers :)
> 
> [...]
> > > 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)?
> 
> Ok, if performance guarantees to non-RT tasks are not the goal, then I
> agree. I was thinking that since the patchset provides a mechanism to
> schedule various classes of tasks through deadline servers, then
> using these servers to provide some kinds of guarantees could be
> interesting ;-)

Not saying that the wider scope approach is not worth pursuing, you know
I would be very much interested into that as well :-), but I'd leave it
for a later time. This proposal looks reasonably achieaveble in somewhat
short times and it should provide provable benefits to production today.
Plus, you are again right, foundations would be there already when we'll
be ready for the wider solution.