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From: Pavan Kondeti <>
To: Qais Yousef <>
Cc: Pavan Kondeti <>,
	Ingo Molnar <>,
	Peter Zijlstra <>,
	Steven Rostedt <>,
	Dietmar Eggemann <>,
	Juri Lelli <>,
	Vincent Guittot <>,
	Ben Segall <>, Mel Gorman <>,
	LKML <>
Subject: Re: [PATCH v2 5/6] sched/rt: Better manage pushing unfit tasks on wakeup
Date: Mon, 24 Feb 2020 21:34:41 +0530	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

Hi Qais,

On Mon, Feb 24, 2020 at 5:42 PM Qais Yousef <> 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
> 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.

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

  reply	other threads:[~2020-02-24 16:04 UTC|newest]

Thread overview: 21+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-02-23 18:39 [PATCH v2 0/6] RT Capacity Awareness Fixes & Improvements Qais Yousef
2020-02-23 18:39 ` [PATCH v2 1/6] sched/rt: cpupri_find: implement fallback mechanism for !fit case Qais Yousef
2020-02-23 18:39 ` [PATCH v2 2/6] sched/rt: Re-instate old behavior in select_task_rq_rt Qais Yousef
2020-02-25 15:21   ` Dietmar Eggemann
2020-02-26 11:34     ` Qais Yousef
2020-02-23 18:39 ` [PATCH v2 3/6] sched/rt: Optimize cpupri_find on non-heterogenous systems Qais Yousef
2020-02-23 18:39 ` [PATCH v2 4/6] sched/rt: allow pulling unfitting task Qais Yousef
2020-02-23 18:40 ` [PATCH v2 5/6] sched/rt: Better manage pushing unfit tasks on wakeup Qais Yousef
2020-02-24  6:10   ` Pavan Kondeti
2020-02-24 12:11     ` Qais Yousef
2020-02-24 16:04       ` Pavan Kondeti [this message]
2020-02-24 17:41         ` Qais Yousef
2020-02-25  3:55           ` Pavan Kondeti
2020-02-26 16:02             ` Qais Yousef
2020-02-27  3:36               ` Pavan Kondeti
2020-02-27 10:29                 ` Qais Yousef
2020-02-23 18:40 ` [PATCH v2 6/6] sched/rt: Remove unnecessary assignment in inc/dec_rt_migration Qais Yousef
2020-02-23 23:16   ` Dietmar Eggemann
2020-02-24 12:31     ` Qais Yousef
2020-02-24 13:03       ` Dietmar Eggemann
2020-02-24 13:47         ` Qais Yousef

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