linux-kernel - Re:[PATCH]sched/rt: Do not try to push tasks if pinned task switches to RT

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Message-Id: <446051359450817@web10d.yandex.ru>
Date:	Tue, 29 Jan 2013 13:13:37 +0400
From:	Kirill Tkhai <tkhai@...dex.ru>
To:	Libo Chen <chenlibo.3@...il.com>
Cc:	linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-rt-users <linux-rt-users@...r.kernel.org>,
	lizefan@...wei.com, jovi.zhangwei@...wei.com
Subject: Re:[PATCH]sched/rt: Do not try to push tasks if pinned task switches to RT



> From: Libo Chen <libo.chen@...wei.com>
> 
> On 2013-1-29 4:23, Kirill Tkhai wrote:
> 
>> Just switched pinned task is not able to be pushed. If the rq had had
>> several RT tasks before they have already been considered as candidates
>> to be pushed (or pulled).
>>
>> Signed-off-by: Kirill V Tkhai <tkhai@...dex.ru>
>> CC: Steven Rostedt <rostedt@...dmis.org>
>> CC: Ingo Molnar <mingo@...nel.org>
>> CC: Peter Zijlstra <peterz@...radead.org>
>> CC: linux-rt-users <linux-rt-users@...r.kernel.org>
>> ---
>> kernel/sched/rt.c | 4 ++--
>> 1 file changed, 2 insertions(+), 2 deletions(-)
>> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
>> index 4e8f0f4..5f7d92b 100644
>> --- a/kernel/sched/rt.c
>> +++ b/kernel/sched/rt.c
>> @@ -1925,9 +1925,9 @@ static void switched_to_rt(struct rq *rq, struct task_struct *p)
>> */
>> if (p->on_rq && rq->curr != p) {
>> #ifdef CONFIG_SMP
>> - if (rq->rt.overloaded && push_rt_task(rq) &&
>> + if (rq->rt.overloaded && p->nr_cpus_allowed > 1 &&
>> /* Don't resched if we changed runqueues */
>> - rq != task_rq(p))
>> + push_rt_task(rq) && rq != task_rq(p))
> 
> I think you worry about it was excess to call push_rt_task, since the task of p->nr_cpus_allowed=1 can`t be pushed. is that right?

Sure.

> 
> The task of p->nr_cpus_allowed =1 would`t be added to pushable_tasks list （see the enqueue_task_rt()）and this push_rt_task() need to push other tasks when rt.overloaded.

If rq has already had 2 or more pushable tasks and we try to add a pinned task then call of push_rt_task will just waste a time.

> 
> So I don`t agree this patch.
> 
>> check_resched = 0;
>> #endif /* CONFIG_SMP */
>> if (check_resched && p->prio < rq->curr->prio)
>> --
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