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

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Date:   Fri, 7 Aug 2020 15:49:41 +0200
From:   luca abeni <luca.abeni@...tannapisa.it>
To:     peterz@...radead.org
Cc:     Juri Lelli <juri.lelli@...hat.com>, 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 6/6] sched/fair: Implement starvation monitor

Hi Peter,

On Fri, 7 Aug 2020 12:46:18 +0200
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

Are you thinking about scheduling both RT and non-RT tasks through
deadline servers? If yes, then I think that using something like
laxity-based scheduling for the SCHED_OTHER server can be a good idea
(but then we need to understand how to combine deadline-based
scheduling with laxity-based scheduling, etc...)

Or are you thinking about keeping the SCHED_OTHER server throttled
until its laxity is 0 (or until its laxity is lower than some small
value)? In this second case, the approach would work even if RT tasks
are not scheduled through a server (but I do not know which kind of
performance guarantee we could provide).


> -- and start the server when
> rq->nr_running != rq->cfs.h_nr_running, IOW when there's !fair tasks
> around.

Yes, this could be a good optimization.



			Luca
> 
> Not saying we should do it like that, but that's perhaps more
> deterministic than this.