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 wrote:
> On Wed, 4 Feb 2015 09:12:21 +0800
> Xunlei Pang wrote:
>
>> From: Xunlei Pang
>>
>> 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
>> ---
>> 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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Re: [PATCH RESEND 2/2] sched/rt: Add check_preempt_equal_prio() logic in pick_next_task_rt()
On Wed, 4 Feb 2015 09:12:21 +0800
Xunlei Pang wrote:
> From: Xunlei Pang
>
> 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
> ---
> 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?
> + 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.
-- Steve
> + if (p->nr_cpus_allowed != 1 &&
> + cpupri_find(&rq->rd->cpupri, p, NULL))
> + requeue_task_rt(rq, p, 0);
> + }
> +#endif
> +
> p = _pick_next_task_rt(rq, 0);
>
> /* The running task is never eligible for pushing */
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