Messages in this thread | | | Date | Wed, 26 Feb 2020 16:02:48 +0000 | From | Qais Yousef <> | Subject | Re: [PATCH v2 5/6] sched/rt: Better manage pushing unfit tasks on wakeup |
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On 02/25/20 09:25, Pavan Kondeti wrote: > > I haven't been staring at this code for as long as you, but since we have > > logic at wakeup to do a push, I think we need something here anyway for unfit > > tasks. > > > > Fixing select_task_rq_rt() to better balance tasks will help a lot in general, > > but if that was enough already then why do we need to consider a push at the > > wakeup at all then? > > > > AFAIU, in SMP the whole push-pull mechanism is racy and we introduce redundancy > > at taking the decision on various points to ensure we minimize this racy nature > > of SMP systems. Anything could have happened between the time we called > > select_task_rq_rt() and the wakeup, so we double check again before we finally > > go and run. That's how I interpret it. > > > > I am open to hear about other alternatives first anyway. Your help has been > > much appreciated so far. > > > > The search inside find_lowest_rq() happens without any locks so I believe it > is expected to have races like this. In fact there is a comment in the code > saying "This test is optimistic, if we get it wrong the load-balancer > will have to sort it out" in select_task_rq_rt(). However, the push logic > as of today works only for overloaded case. In that sense, your patch fixes > this race for b.L systems. At the same time, I feel like tracking nonfit tasks > just to fix this race seems to be too much. I will leave this to Steve and > others to take a decision.
I do think without this tasks can end up on the wrong CPU longer than they should. Keep in mind that if a task is boosted to run on a big core, it still have to compete with non-boosted tasks who can run on a any cpu. So this opportunistic push might be necessary.
For 5.6 though, I'll send an updated series that removes the fitness check from task_woken_rt() && switched_to_rt() and carry on with this discussion for 5.7.
> > I thought of suggesting to remove the below check from select_task_rq_rt() > > p->prio < cpu_rq(target)->rt.highest_prio.curr > > which would then make the target CPU overloaded and the push logic would > spread the tasks. That works for a b.L system too. However there seems to > be a very good reason for doing this. see > https://lore.kernel.org/patchwork/patch/539137/ > > The fact that a CPU is part of lowest_mask but running a higher prio RT > task means there is a race. Should we retry one more time to see if we find > another CPU?
Isn't this what I did in v1?
https://lore.kernel.org/lkml/20200214163949.27850-4-qais.yousef@arm.com/
Thanks
-- Qais Yousef
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