linux-kernel - Re: [PATCH v3 2/2] sched/rt: Add check_preempt_equal_prio() logic in pick_next_task

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Message-ID: <CADcy93XLcdtLjamhFEr8oEV5uN1WgMrSFfog_172FcJZdibTyg@mail.gmail.com>
Date:	Fri, 13 Feb 2015 12:40:17 +0800
From:	Xunlei Pang <pang.xunlei@...aro.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Juri Lelli <juri.lelli@...il.com>
Subject: Re: [PATCH v3 2/2] sched/rt: Add check_preempt_equal_prio() logic in pick_next_task_rt()

Hi Steve,

On 13 February 2015 at 11:55, Xunlei Pang <pang.xunlei@...aro.org> wrote:
> Hi steve,
>
> On 13 February 2015 at 08:04, Steven Rostedt <rostedt@...dmis.org> wrote:
>> On Sun,  8 Feb 2015 23:51:26 +0800
>> Xunlei Pang <pang.xunlei@...aro.org> wrote:
>>
>>> check_preempt_curr() doesn't call sched_class::check_preempt_curr
>>> when the class of current is a higher level.
>>
>> The above sentence does not make sense.
>>
>>> So if there is a DL
>>> task running when doing this for RT, check_preempt_equal_prio()
>>
>> Doing what for RT?
>>
>>> will definitely miss, which may result in some response latency
>>
>> Miss what?
>
> Sorry, this may lack some information I need to further explain in detail.
>
>>
>>> for this RT task if it is pinned and there're some same-priority
>>> migratable rt tasks already queued.
>>>
>>> We should do the similar thing in select_task_rq_rt() when first
>>> picking rt tasks after running out of DL tasks.
>>>
>>> This patch tackles the issue by peeking the next rt task(RT1), and
>>> if find RT1 migratable, just requeue it to the tail of the rq using
>>> requeue_task_rt(rq, p, 0). In this way:
>>> - If there do have another rt task(RT2) with the same priority as
>>>   RT1, RT2 will finally be picked as the running task. While RT1
>>>   will be pushed onto another cpu via RT1's post_schedule(), as
>>>   RT1 is migratable. The difference from check_preempt_equal_prio()
>>>   here is that we just don't care whether RT2 is migratable.
>>>
>>> - Otherwise, if there's no rt task with the same priority as RT1,
>>>   RT1 will still be picked as the running task after the requeuing.
>>
>> What happens if there's three RT tasks of the same prio, RT1 is ready
>> to run and is migratable, RT2 is pinned, RT3 is migratable
>>
>> RT1 just got pushed behind RT3 and it is now not the next one to run.
>> RT2 will get this rq, RT3 will be pushed off, but say there's no more
>> rq's available to run RT1.
>>
>> You just broke FIFO.
>
> Yes, I've also thought of this point before.
>
> If this is a problem, we may have the same thing happening in
> current check_preempt_equal_prio() code:
> When a pinned waking task preempts the current successfully,
> because it thinks current is migratable via cpupri_find().
>
> But when resched happens, things may change, i.e. current
> becomes non-migratable, so the waking task gets running, while
> the previous running task gets stuck. See, it also broke FIFO.

Aside of this, please ignore this patch, the waking rt tasks will also be
pushed via task_woken_rt() when current is DL, which I missed before.

Thanks,
Xunlei
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