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

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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..


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Re: [PATCH 07/10] sched: Migrate sched to use new tick dependency  mask model Peter Zijlstra <peterz@infradead.org> - 2015-08-03 16:10 +0200
  Re: [PATCH 07/10] sched: Migrate sched to use new tick dependency  mask model Frederic Weisbecker <fweisbec@gmail.com> - 2015-08-03 17:00 +0200
    Re: [PATCH 07/10] sched: Migrate sched to use new tick dependency  mask model Peter Zijlstra <peterz@infradead.org> - 2015-08-03 19:10 +0200
      Re: [PATCH 07/10] sched: Migrate sched to use new tick dependency  mask model Frederic Weisbecker <fweisbec@gmail.com> - 2015-08-03 19:40 +0200
        Re: [PATCH 07/10] sched: Migrate sched to use new tick dependency  mask model Peter Zijlstra <peterz@infradead.org> - 2015-08-04 09:50 +0200

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