Re: [PATCH 07/10] sched: Migrate sched to use new tick dependency mask model

[Frederic Weisbecker]

On Mon, Aug 03, 2015 at 04:00:46PM +0200, Peter Zijlstra wrote:
> On Thu, Jul 23, 2015 at 06:42:12PM +0200, Frederic Weisbecker wrote:
> > Instead of providing asynchronous checks for the nohz subsystem to verify
> > sched tick dependency, migrate sched to the new mask.
> > 
> > The easiest is to recycle the current asynchronous tick dependency check
> > which verifies the class of the current task and its requirements for
> > periodic preemption checks.
> > 
> > We need to evaluate this tick dependency on three places:
> > 
> > 1) Task enqueue: One or more tasks have been enqueued, we must check
> >    if those are competing with the current task.
> > 
> > 2) Task dequeue: A possibly competing task has been dequeued, clear the
> >    tick dependency if needed.
> > 
> > 3) schedule(): we might be switching to a task of another scheduler
> >    class. Each class has its preemption rules, we must re-evaluate it.
> 
> This is insane.. You add a whole bunch of work per wakeup/sleep/context
> switch to avoid some work at tick time. That's a broken trade-off.
> 
> We can context switch _waaaay_ more than we have ticks.
> 
> Furthermore, you do tons of pointless work, we call add_nr_running()
> from the individual classes, and then your routine goes and checks what
> class we're in etc..

I think I could remove the context switch part. But then I need to find a
way to perform these checks on enqueue and dequeue task time:

  sched_update_dependency(cpu)
  {
      if (SCHED_FIFO task on the cpu runqueue) {
          tick_nohz_clear_dep(cpu)
	  return;
      }

      if (SCHED_RR task on the cpu runqueue) {
          if (more than one such task) {
              tick_nohz_set_dep(cpu)
	      return;
	  }
      }

      if (SCHED_NORMAL task on the cpu runqueue) {
          if (more than one such task) {
	      tick_nohz_set_dep(cpu)
	      return;
	  }
      }

      tick_nohz_clear_dep();
   }

That's still heavyweight because enqueue and dequeue can be very frequent
but we get rid of the sched_switch hook because we don't care about the
current task at all.

Now, consider that we could cut all this checks into parts and optimize
that per sched class::enqueue/dequeue.

So we can divide the dependency into:

         struct rq {
	     ...
	     int nr_fifo;
	     int nr_rr;
	     int nr_normal;
	}


	int rq_update_tick_dep(struct rq *rq)
	{
	     if (rq->nr_fifo && (rq->nr_rr > 1 || rq->nr_normal > 1))
	         tick_nohz_set_dep(SCHED_TICK_DEP);
             else
	         tick_nohz_set_dep(SCHED_TICK_DEP)
        }

Then we add or dec the relevant counter fields from the various sched_class::enqueue/dequeue.
I think I saw some of these counters already exist but perhaps not all of them. There are
per class rqs but rt_nr_running counts tasks without distinction of policies.
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