Messages in this thread | | | From | Valentin Schneider <> | Subject | Re: [PATCH -v2 15/17] sched: Fix migrate_disable() vs rt/dl balancing | Date | Tue, 06 Oct 2020 12:20:43 +0100 |
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On 05/10/20 15:57, Peter Zijlstra wrote: > In order to minimize the interference of migrate_disable() on lower > priority tasks, which can be deprived of runtime due to being stuck > below a higher priority task. Teach the RT/DL balancers to push away > these higher priority tasks when a lower priority task gets selected > to run on a freshly demoted CPU (pull). > > This adds migration interference to the higher priority task, but > restores bandwidth to system that would otherwise be irrevocably lost. > Without this it would be possible to have all tasks on the system > stuck on a single CPU, each task preempted in a migrate_disable() > section with a single high priority task running. > > This way we can still approximate running the M highest priority tasks > on the system. >
Ah, so IIUC that's the important bit that makes it we can't just say go through the pushable_tasks list and skip migrate_disable() tasks.
Once the highest-prio task exits its migrate_disable() region, your patch pushes it away. If we ended up with a single busy CPU, it'll spread the tasks around one migrate_enable() at a time.
That time where the top task is migrate_disable() is still a crappy time, and as you pointed out earlier today if it is a genuine pcpu task then the whole thing is -EBORKED...
An alternative I could see would be to prevent those piles from forming altogether, say by issuing a similar push_cpu_stop() on migrate_disable() if the next pushable task is already migrate_disable(); but that's a proactive approach whereas yours is reactive, so I'm pretty sure that's bound to perform worse.
> Migrating the top task away is (ofcourse) still subject to > migrate_disable() too, which means the lower task is subject to an > interference equivalent to the worst case migrate_disable() section. > > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
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