linux-kernel - Re: [RFCv6 PATCH 03/10] sched: scheduler-driven cpu frequency selection

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Date:	Sat, 19 Dec 2015 13:54:32 +0800
From:	Leo Yan <leo.yan@...aro.org>
To:	Steve Muckle <steve.muckle@...aro.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	linux-pm@...r.kernel.org,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Juri Lelli <Juri.Lelli@....com>,
	Patrick Bellasi <patrick.bellasi@....com>,
	Michael Turquette <mturquette@...libre.com>,
	Ricky Liang <jcliang@...omium.org>
Subject: Re: [RFCv6 PATCH 03/10] sched: scheduler-driven cpu frequency
 selection

Hi Steve,

On Fri, Dec 18, 2015 at 11:15:01AM -0800, Steve Muckle wrote:
> On 12/16/2015 11:17 PM, Leo Yan wrote:
> > Could you check if below corner case will introduce logic error?
> > The task still will be removed from rq if timer tick is triggered
> > between two time's set_current_state().
> > 
> > set_current_state(TASK_INTERRUPTIBLE);
> >            `-------> timer_tick and
> >                      schedule();
> > do_something...
> > set_current_state(TASK_RUNNING);
> > 
> > It will be safe for combination for set_current_state()/schedule()
> > with waken_up_process():
> > 
> > Thread_A:                                       Thread_B:
> > 
> > set_current_state(TASK_INTERRUPTIBLE);
> >              `-------> timer_tick and
> >                        schedule();
> > ....
> >                                                 wake_up_process(Thread_A);
> >                            <---------------------/
> > schedule();
> > 
> > The first time's schedule() will remove task from rq which is caused
> > by timer tick and call schedule(), and the second time schdule() will
> > be equal yeild().
> 
> I was initially concerned about preemption while task state =
> TASK_INTERRUPTIBLE as well, but a task with state TASK_INTERRUPTIBLE is
> not dequeued if it is preempted. See core.c:__schedule():
> 
>         if (!preempt && prev->state) {
>                 if (unlikely(signal_pending_state(prev->state, prev))) {
>                         prev->state = TASK_RUNNING;
>                 } else {
>                         deactivate_task(rq, prev, DEQUEUE_SLEEP);
>                         prev->on_rq = 0;
> 
> I knew this had to be the case, because this design pattern is used in
> many other places in the kernel, so many things would be very broken if
> this were a problem.

You are right, I went through the code again and sched tick irq will
call preempt_schedule_irq() and __schedule(true); so finally set the
parameter "preempt" = true. Sorry for noise :p

---8<---

arch/arm64/kernel/entry.S:

#ifdef CONFIG_PREEMPT
el1_preempt:
        mov     x24, lr
1:      bl      preempt_schedule_irq            // irq en/disable is done inside
        ldr     x0, [tsk, #TI_FLAGS]            // get new tasks TI_FLAGS
        tbnz    x0, #TIF_NEED_RESCHED, 1b       // needs rescheduling?
        ret     x24
#endif

Thanks,
Leo Yan
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