linux-kernel - Re: [PATCH 2/3] sched/rt: allow pulling unfitting task

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Message-ID: <20200217112034.54yoockxvfe3fw6y@e107158-lin.cambridge.arm.com>
Date:   Mon, 17 Feb 2020 11:20:36 +0000
From:   Qais Yousef <qais.yousef@....com>
To:     Pavan Kondeti <pkondeti@...eaurora.org>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] sched/rt: allow pulling unfitting task

On 02/17/20 14:40, Pavan Kondeti wrote:
> Hi Qais,
> 
> On Fri, Feb 14, 2020 at 04:39:48PM +0000, Qais Yousef wrote:
> > When implemented RT Capacity Awareness; the logic was done such that if
> > a task was running on a fitting CPU, then it was sticky and we would try
> > our best to keep it there.
> > 
> > But as Steve suggested, to adhere to the strict priority rules of RT
> > class; allow pulling an RT task to unfitting CPU to ensure it gets a
> > chance to run ASAP. When doing so, mark the queue as overloaded to give
> > the system a chance to push the task to a better fitting CPU when a
> > chance arises.
> > 
> > Suggested-by: Steven Rostedt <rostedt@...dmis.org>
> > Signed-off-by: Qais Yousef <qais.yousef@....com>
> > ---
> >  kernel/sched/rt.c | 16 +++++++++++++---
> >  1 file changed, 13 insertions(+), 3 deletions(-)
> > 
> > diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> > index 4043abe45459..0c8bac134d3a 100644
> > --- a/kernel/sched/rt.c
> > +++ b/kernel/sched/rt.c
> > @@ -1646,10 +1646,20 @@ static void put_prev_task_rt(struct rq *rq, struct task_struct *p)
> >  
> >  static int pick_rt_task(struct rq *rq, struct task_struct *p, int cpu)
> >  {
> > -	if (!task_running(rq, p) &&
> > -	    cpumask_test_cpu(cpu, p->cpus_ptr) &&
> > -	    rt_task_fits_capacity(p, cpu))
> > +	if (!task_running(rq, p) && cpumask_test_cpu(cpu, p->cpus_ptr)) {
> > +
> > +		/*
> > +		 * If the CPU doesn't fit the task, allow pulling but mark the
> > +		 * rq as overloaded so that we can push it again to a more
> > +		 * suitable CPU ASAP.
> > +		 */
> > +		if (!rt_task_fits_capacity(p, cpu)) {
> > +			rt_set_overload(rq);
> > +			rq->rt.overloaded = 1;
> > +		}
> > +
> 
> Here rq is source rq from which the task is being pulled. I can't understand
> how marking overload condition on source_rq help. Because overload condition
> gets cleared in the task dequeue path. i.e dec_rt_tasks->dec_rt_migration->
> update_rt_migration().

Err yes indeed I rushed this patch. Let me try again.

Thanks

--
Qais Yousef