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

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Date:	Fri, 18 Dec 2015 11:15:01 -0800
From:	Steve Muckle <steve.muckle@...aro.org>
To:	Leo Yan <leo.yan@...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 Leo,

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.

thanks,
Steve

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