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

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Date:	Wed, 4 Feb 2015 21:05:14 +0800
From:	Xunlei Pang <pang.xunlei@...aro.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Xunlei Pang <xlpang@....com>, lkml <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Juri Lelli <juri.lelli@...il.com>
Subject: Re: [PATCH RESEND 2/2] sched/rt: Add check_preempt_equal_prio() logic
 in pick_next_task_rt()

Hi Steve,

On 4 February 2015 at 11:17, Steven Rostedt <rostedt@...dmis.org> wrote:
> On Wed,  4 Feb 2015 09:12:21 +0800
> Xunlei Pang <xlpang@....com> wrote:
>
>> From: Xunlei Pang <pang.xunlei@...aro.org>
>>
>> check_preempt_curr() doesn't call sched_class::check_preempt_curr
>> when the class of current is a higher level. So if there is a DL
>> task running when doing this for RT, check_preempt_equal_prio()
>> will definitely miss, which may result in some response latency
>> 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.
>>
>> Signed-off-by: Xunlei Pang <pang.xunlei@...aro.org>
>> ---
>>  kernel/sched/rt.c | 15 +++++++++++++++
>>  1 file changed, 15 insertions(+)
>>
>> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
>> index 4dacb6e..b2385ee 100644
>> --- a/kernel/sched/rt.c
>> +++ b/kernel/sched/rt.c
>> @@ -1477,6 +1477,21 @@ pick_next_task_rt(struct rq *rq, struct task_struct *prev)
>>
>>       put_prev_task(rq, prev);
>>
>> +#ifdef CONFIG_SMP
>> +             /*
>> +              * If there's a running deadline task, check_preempt_curr()
>> +              * doesn't invoke check_preempt_curr_rt() for rt tasks, so
>> +              * we can do it here.
>> +              */
>
> Why the strange indentation?
>

Thanks for catching this, I'll fix it.

>> +             if (prev->sched_class == &dl_sched_class &&
>> +                 rq->rt.rt_nr_total > 1) {
>> +                     p = _pick_next_task_rt(rq, 1); /* peek only */
>
> I hate the "peek only". Just split the function into two, where you
> have something like check_next_task(rq) which does your "peek only"
> and the __pick_next_task_rt() calls check_next_task() first and then
> runs the rest of the code.
>

This sounds good, I'll make a new peek_next_task_rt() as the base one.

Thanks,
Xunlei
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