linux-kernel - Re: [RFC PATCH v2 6/6] sched/fair: Implement starvation monitor

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Date:   Fri, 7 Aug 2020 14:50:27 +0200
From:   Juri Lelli <juri.lelli@...hat.com>
To:     Daniel Bristot de Oliveira <bristot@...hat.com>
Cc:     peterz@...radead.org, mingo@...hat.com, rostedt@...dmis.org,
        tglx@...utronix.de, linux-kernel@...r.kernel.org,
        luca.abeni@...tannapisa.it, tommaso.cucinotta@...tannapisa.it,
        alessio.balsini@...il.com, dietmar.eggemann@....com,
        linux-rt-users@...r.kernel.org, mtosatti@...hat.com,
        williams@...hat.com, valentin.schneider@....com
Subject: Re: [RFC PATCH v2 6/6] sched/fair: Implement starvation monitor

On 07/08/20 13:30, Daniel Bristot de Oliveira wrote:
> On 8/7/20 12:46 PM, peterz@...radead.org wrote:
> > On Fri, Aug 07, 2020 at 11:56:04AM +0200, Juri Lelli wrote:
> >> Starting deadline server for lower priority classes right away when
> >> first task is enqueued might break guarantees, as tasks belonging to
> >> intermediate priority classes could be uselessly preempted. E.g., a well
> >> behaving (non hog) FIFO task can be preempted by NORMAL tasks even if
> >> there are still CPU cycles available for NORMAL tasks to run, as they'll
> >> be running inside the fair deadline server for some period of time.
> >>
> >> To prevent this issue, implement a starvation monitor mechanism that
> >> starts the deadline server only if a (fair in this case) task hasn't
> >> been scheduled for some interval of time after it has been enqueued.
> >> Use pick/put functions to manage starvation monitor status.
> > One thing I considerd was scheduling this as a least-laxity entity --
> > such that it runs late, not early -- and start the server when
> > rq->nr_running != rq->cfs.h_nr_running, IOW when there's !fair tasks
> > around.

IIUC, this would still require programming a timer to fire when laxity
is 0, but doing that only when there are !fair tasks around (so when
enqueuing the first !fair or if there are !fair already when first fair
is enqueued) would probably save us some overhead, I agree (as no timer
and no enqueue of deadline server would be needed in the common "only
fair" case).

> > 
> > Not saying we should do it like that, but that's perhaps more
> > deterministic than this.
> > 
> 
> I agree, what we want here is something that schedules the server if it still
> retains some runtime when the laxity is 0. But this is easier said than done, as
> this would require another scheduler (other pros and cons and analysis (and
> hours of work)...).
> 
> But, for the starvation monitor purpose, the goal is not (necessarily) to
> provide a deterministic guarantee for the starving task, but to avoid system
> issues while minimizing the damage to the "real" real-time workload. With that
> in mind, we could relax our ambitions...
> 
> Thoughts?

I agree that we don't probably want to develop an additional scheduler/
policy for this, but I'll think a bit more about Peter's idea. Maybe
it's already a viable optimization w/o changing EDF/CBS.