linux-kernel - Re: [RFC][PATCH 04/13] sched/{rt,deadline}: Fix set_next_task vs pick_next

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Message-ID: <20190729130438.GE31398@hirez.programming.kicks-ass.net>
Date:   Mon, 29 Jul 2019 15:04:38 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Juri Lelli <juri.lelli@...hat.com>
Cc:     mingo@...nel.org, linux-kernel@...r.kernel.org,
        dietmar.eggemann@....com, luca.abeni@...tannapisa.it,
        bristot@...hat.com, balsini@...roid.com, dvyukov@...gle.com,
        tglx@...utronix.de, vpillai@...italocean.com, rostedt@...dmis.org
Subject: Re: [RFC][PATCH 04/13] sched/{rt,deadline}: Fix set_next_task vs
 pick_next_task

On Mon, Jul 29, 2019 at 01:27:02PM +0200, Juri Lelli wrote:
> On 29/07/19 13:15, Peter Zijlstra wrote:
> > On Mon, Jul 29, 2019 at 11:25:19AM +0200, Juri Lelli wrote:
> > > Hi,
> > > 
> > > On 26/07/19 16:54, Peter Zijlstra wrote:
> > > > Because pick_next_task() implies set_curr_task() and some of the
> > > > details haven't matter too much, some of what _should_ be in
> > > > set_curr_task() ended up in pick_next_task, correct this.
> > > > 
> > > > This prepares the way for a pick_next_task() variant that does not
> > > > affect the current state; allowing remote picking.
> > > > 
> > > > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > > > ---
> > > >  kernel/sched/deadline.c |   23 ++++++++++++-----------
> > > >  kernel/sched/rt.c       |   27 ++++++++++++++-------------
> > > >  2 files changed, 26 insertions(+), 24 deletions(-)
> > > > 
> > > > --- a/kernel/sched/deadline.c
> > > > +++ b/kernel/sched/deadline.c
> > > > @@ -1694,12 +1694,21 @@ static void start_hrtick_dl(struct rq *r
> > > >  }
> > > >  #endif
> > > >  
> > > > -static inline void set_next_task(struct rq *rq, struct task_struct *p)
> > > > +static void set_next_task_dl(struct rq *rq, struct task_struct *p)
> > > >  {
> > > >  	p->se.exec_start = rq_clock_task(rq);
> > > >  
> > > >  	/* You can't push away the running task */
> > > >  	dequeue_pushable_dl_task(rq, p);
> > > > +
> > > > +	if (hrtick_enabled(rq))
> > > > +		start_hrtick_dl(rq, p);
> > > > +
> > > > +	if (rq->curr->sched_class != &dl_sched_class)
> > > > +		update_dl_rq_load_avg(rq_clock_pelt(rq), rq, 0);
> > > > +
> > > > +	if (rq->curr != p)
> > > > +		deadline_queue_push_tasks(rq);
> > > 
> > > It's a minor thing, but I was wondering why you added the check on curr.
> > > deadline_queue_push_tasks() already checks if are there pushable tasks,
> > > plus curr can still be of a different class at this point?
> > 
> > Hmm, so by moving that code into set_next_task() it is exposed to the:
> > 
> >   if (queued)
> >     deuque_task();
> >   if (running)
> >     put_prev_task();
> > 
> >   /* do stuff */
> > 
> >   if (queued)
> >     enqueue_task();
> >   if (running)
> >     set_next_task();
> > 
> > patter from core.c; and in that case nothing changes. That said; I
> > might've gotten it wrong.
> 
> Right. But, I was wondering about the __schedule()->pick_next_task()
> case, where, say, prev (rq->curr) is RT/CFS and next (p) is DEADLINE.

So we do pick_next_task() first and then set rq->curr (obviously). So
the first set_next_task() will see rq->curr != p and we'll do the push
balance stuff.

Then the above pattern will always see rq->curr == p and we'll not
trigger push balancing.

Now, looking at it, this also doesn't do push balancing when we
re-select the same task, even though we really should be doing it. So I
suppose not adding the condition, and always doing the push balance,
while wasteful, is not wrong.

Hmm?