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

[Peter Zijlstra <peterz@infradead.org>]

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..