linux-kernel - Re: [RFC PATCH 3/3] sched,rt: break out of load balancing if an RT task appears

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Message-ID: <fa406883f0eace37fe7f658814e29f82a4f0addf.camel@redhat.com>
Date:   Tue, 28 Apr 2020 17:33:36 -0500
From:   Scott Wood <swood@...hat.com>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Rik van Riel <riel@...riel.com>,
        Mel Gorman <mgorman@...e.de>, linux-kernel@...r.kernel.org,
        linux-rt-users <linux-rt-users@...r.kernel.org>
Subject: Re: [RFC PATCH 3/3] sched,rt: break out of load balancing if an RT
 task appears

On Tue, 2020-04-28 at 22:56 +0100, Valentin Schneider wrote:
> On 28/04/20 06:02, Scott Wood wrote:
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index dfde7f0ce3db..e7437e4e40b4 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -9394,6 +9400,10 @@ static int should_we_balance(struct lb_env *env)
> >       struct sched_group *sg = env->sd->groups;
> >       int cpu, balance_cpu = -1;
> > 
> > +	/* Run the realtime task now; load balance later. */
> > +	if (rq_has_runnable_rt_task(env->dst_rq))
> > +		return 0;
> > +
> 
> I have a feeling this isn't very nice to CFS tasks, since we would now
> "waste" load-balance attempts if they happen to coincide with an RT task
> being runnable.
>
> On your 72 CPUs machine, the system-wide balance happens (at best) every
> 72ms if you have idle time, every ~2300ms otherwise (every balance
> CPU gets to try to balance however, so it's not as horrible as I'm making
> it sound). This is totally worst-case scenario territory, and you'd hope
> newidle_balance() could help here and there (as it isn't gated by any
> balance interval).
> 
> Still, even for a single rq, postponing a system-wide balance for a
> full balance interval (i.e. ~2 secs worst case here) just because we had a
> single RT task running when we tried to balance seems a bit much.
> 
> It may be possible to hack something to detect those cases and reset the
> interval to "now" when e.g. dequeuing the last RT task (& after having
> previously aborted a load-balance due to RT/DL/foobar).

Yeah, some way to retry at an appropriate time after aborting a rebalance
would be good.


> > +
> > +/* Is there a task of a high priority class? */
> > +static inline bool rq_has_runnable_rt_task(struct rq *rq)
> > +{
> > +	return unlikely(rq->nr_running != rq->cfs.h_nr_running);
> 
> Seeing as that can be RT, DL or stopper, that name is somewhat misleading.

rq_has_runnable_rt_dl_task()?  Or is there some term that unambiguously
encompasses both?

-Scott