Messages in this thread | | | From | Pavan Kondeti <> | Date | Mon, 24 Feb 2020 21:34:41 +0530 | Subject | Re: [PATCH v2 5/6] sched/rt: Better manage pushing unfit tasks on wakeup |
| |
Hi Qais,
On Mon, Feb 24, 2020 at 5:42 PM Qais Yousef <qais.yousef@arm.com> wrote: [...] > We could do, temporarily, to get these fixes into 5.6. But I do think > select_task_rq_rt() doesn't do a good enough job into pushing unfit tasks to > the right CPUs. > > I don't understand the reasons behind your objection. It seems you think that > select_task_rq_rt() should be enough, but not AFAICS. Can you be a bit more > detailed please? > > FWIW, here's a screenshot of what I see > > https://imgur.com/a/peV27nE > > After the first activation, select_task_rq_rt() fails to find the right CPU > (due to the same move all tasks to the cpumask_fist()) - but when the task > wakes up on 4, the logic I put causes it to migrate to CPU2, which is the 2nd > big core. CPU1 and CPU2 are the big cores on Juno. > > Now maybe we should fix select_task_rq_rt() to better balance tasks, but not > sure how easy is that. >
Thanks for the trace. Now things are clear to me. Two RT tasks woke up simultaneously and the first task got its previous CPU i.e CPU#1. The next task goes through find_lowest_rq() and got the same CPU#1. Since this task priority is not more than the just queued task (already queued on CPU#1), it is sent to its previous CPU i.e CPU#4 in your case.
From task_woken_rt() path, CPU#4 attempts push_rt_tasks(). CPU#4 is not overloaded, but we have rt_task_fits_capacity() check which forces the push. Since the CPU is not overloaded, your has_unfit_tasks() comes to rescue and push the task. Since the task has not scheduled in yet, it is eligible for push. You added checks to skip resched_curr() in push_rt_tasks() otherwise the push won't happen.
Finally, I understood your patch. Obviously this is not clear to me before. I am not sure if this patch is the right approach to solve this race. I will think a bit more.
Thanks, Pavan -- Qualcomm India Private Limited, on behalf of Qualcomm Innovation Center, Inc. Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project
| |