Subject: Re: [RFCv6 PATCH 03/10] sched: scheduler-driven cpu frequency
 selection
To: Leo Yan <leo.yan@linaro.org>
References: <1449641971-20827-1-git-send-email-smuckle@linaro.org>
 <1449641971-20827-4-git-send-email-smuckle@linaro.org>
 <20151216034825.GA11303@leoy-linaro> <56720EE8.1090507@linaro.org>
 <20151217071733.GB6195@leoy-linaro>
Cc: Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@redhat.com>,
        linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Morten Rasmussen <morten.rasmussen@arm.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Juri Lelli <Juri.Lelli@arm.com>,
        Patrick Bellasi <patrick.bellasi@arm.com>,
        Michael Turquette <mturquette@baylibre.com>,
        Ricky Liang <jcliang@chromium.org>
From: Steve Muckle <steve.muckle@linaro.org>
Message-ID: <56745B35.5050704@linaro.org>
Date: Fri, 18 Dec 2015 11:15:01 -0800
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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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