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Message-ID: <CADcy93XwiJU8P42Zxb+zPihv5wKyoMnv0tPYFgOH7gDk_-P2Qw@mail.gmail.com>
Date: Wed, 1 Apr 2015 10:41:33 +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 v4 2/3] sched/rt: Fix wrong SMP scheduler behavior
for equal prio cases
On 27 March 2015 at 23:28, Steven Rostedt <rostedt@...dmis.org> wrote:
> On Mon, 9 Mar 2015 15:32:27 +0800
> Xunlei Pang <xlpang@....com> wrote:
>
>> From: Xunlei Pang <pang.xunlei@...aro.org>
>>
>> Currently, SMP RT scheduler has some trouble in dealing with
>> equal prio cases.
>>
>> For example, in check_preempt_equal_prio():
>> When RT1(current task) gets preempted by RT2, if there is a
>> migratable RT3 with same prio, RT3 will be pushed away instead
>> of RT1 afterwards, because RT1 will be enqueued to the tail of
>> the pushable list when going through succeeding put_prev_task_rt()
>> triggered by resched. This broke FIFO.
>>
>> Furthermore, this is also problematic for normal preempted cases
>> if there're some rt tasks queued with the same prio as current.
>> Because current will be put behind these tasks in the pushable
>> queue.
>>
>> So, if a task is running and gets preempted by a higher priority
>> task (or even with same priority for migrating), this patch ensures
>> that it is put ahead of any existing task with the same priority in
>> the pushable queue.
>>
>> Signed-off-by: Xunlei Pang <pang.xunlei@...aro.org>
>> ---
>> kernel/sched/rt.c | 23 ++++++++++++++++-------
>> 1 file changed, 16 insertions(+), 7 deletions(-)
>>
>> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
>> index f4d4b07..86cd79f 100644
>> --- a/kernel/sched/rt.c
>> +++ b/kernel/sched/rt.c
>> @@ -347,11 +347,15 @@ static inline void set_post_schedule(struct rq *rq)
>> rq->post_schedule = has_pushable_tasks(rq);
>> }
>>
>> -static void enqueue_pushable_task(struct rq *rq, struct task_struct *p)
>> +static void enqueue_pushable_task(struct rq *rq,
>> + struct task_struct *p, bool head)
>
> Nit.
>
> static void
> enqueue_pushable_task(struct rq *rq, struct task_struct *p, bool head)
>
> Is a better breaking of the line.
I'll adjust it to be one line.
>
>> {
>> plist_del(&p->pushable_tasks, &rq->rt.pushable_tasks);
>> plist_node_init(&p->pushable_tasks, p->prio);
>> - plist_add(&p->pushable_tasks, &rq->rt.pushable_tasks);
>> + if (head)
>> + plist_add_head(&p->pushable_tasks, &rq->rt.pushable_tasks);
>> + else
>> + plist_add_tail(&p->pushable_tasks, &rq->rt.pushable_tasks);
>>
>> /* Update the highest prio pushable task */
>> if (p->prio < rq->rt.highest_prio.next)
>> @@ -373,7 +377,8 @@ static void dequeue_pushable_task(struct rq *rq, struct task_struct *p)
>>
>> #else
>>
>> -static inline void enqueue_pushable_task(struct rq *rq, struct task_struct *p)
>> +static inline void enqueue_pushable_task(struct rq *rq,
>> + struct task_struct *p, bool head)
>
> Same here.
>
>> {
>> }
>>
>> @@ -1248,7 +1253,7 @@ enqueue_task_rt(struct rq *rq, struct task_struct *p, int flags)
>> enqueue_rt_entity(rt_se, flags & ENQUEUE_HEAD);
>>
>> if (!task_current(rq, p) && p->nr_cpus_allowed > 1)
>> - enqueue_pushable_task(rq, p);
>> + enqueue_pushable_task(rq, p, false);
>> }
>>
>> static void dequeue_task_rt(struct rq *rq, struct task_struct *p, int flags)
>> @@ -1494,8 +1499,12 @@ static void put_prev_task_rt(struct rq *rq, struct task_struct *p)
>> * The previous task needs to be made eligible for pushing
>> * if it is still active
>> */
>> - if (on_rt_rq(&p->rt) && p->nr_cpus_allowed > 1)
>> - enqueue_pushable_task(rq, p);
>> + if (on_rt_rq(&p->rt) && p->nr_cpus_allowed > 1) {
>> + if (task_running(rq, p) && (preempt_count() & PREEMPT_ACTIVE))
>
> put_prev_task_rt() is called by put_prev_task() which is called by
> several functions: rt_mutex_setprio(), __sched_setscheduler(),
> sched_setnuma(), migrate_tasks(), and sched_move_task(). It's not part
> of being preempted.
>
> Now it is also called by pick_next_task_rt() which I'm assuming is what
> you want it to affect.
>
> The above definitely needs a comment about what it is doing. Also, I'm
> not so sure we care about testing task_running(). I'm thinking the
> check for PREEMPT_ACTIVE is good enough, as that would only be set from
> being called within preempt_schedule().
Indeed.
>
> Also, we could get rid of the if statement and do:
>
> enqueue_pushable_task(rq, p, !!(preempt_count() & PREEMPT_ACTIVE));
Agree, will do. Thanks.
>
>
> -- Steve
>
>> + enqueue_pushable_task(rq, p, true);
>> + else
>> + enqueue_pushable_task(rq, p, false);
>> + }
>> }
>>
>> #ifdef CONFIG_SMP
>> @@ -1914,7 +1923,7 @@ static void set_cpus_allowed_rt(struct task_struct *p,
>> rq->rt.rt_nr_migratory--;
>> } else {
>> if (!task_current(rq, p))
>> - enqueue_pushable_task(rq, p);
>> + enqueue_pushable_task(rq, p, false);
>> rq->rt.rt_nr_migratory++;
>> }
>>
>
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