Messages in this thread | | | Date | Mon, 3 Aug 2015 16:00:46 +0200 | From | Peter Zijlstra <> | Subject | 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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