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

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Message-ID: <20200807104618.GH2674@hirez.programming.kicks-ass.net>
Date:   Fri, 7 Aug 2020 12:46:18 +0200
From:   peterz@...radead.org
To:     Juri Lelli <juri.lelli@...hat.com>
Cc:     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,
        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

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.

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