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Subject: Re: [PATCH V2 6/7] thermal/drivers/cpu_cooling: Introduce the cpu
 idle cooling driver
To:     Leo Yan <leo.yan@linaro.org>
Cc:     edubezval@gmail.com, kevin.wangtao@linaro.org,
        vincent.guittot@linaro.org, amit.kachhap@gmail.com,
        linux-kernel@vger.kernel.org, javi.merino@kernel.org,
        rui.zhang@intel.com, daniel.thompson@linaro.org,
        linux-pm@vger.kernel.org, Viresh Kumar <viresh.kumar@linaro.org>
References: <1519226968-19821-1-git-send-email-daniel.lezcano@linaro.org>
 <1519226968-19821-7-git-send-email-daniel.lezcano@linaro.org>
 <20180327033554.GB21693@leoy-ThinkPad-X240s>
From:   Daniel Lezcano <daniel.lezcano@linaro.org>
Message-ID: <3d1ba2a7-5520-9e80-4e30-a1a413fb417f@linaro.org>
Date:   Tue, 27 Mar 2018 12:56:28 +0200
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On 27/03/2018 05:35, Leo Yan wrote:
> On Wed, Feb 21, 2018 at 04:29:27PM +0100, Daniel Lezcano wrote:
> 
> [...]
> 
>> +/**
>> + * cpuidle_cooling_injection_thread - Idle injection mainloop thread function
>> + * @arg: a void pointer containing the idle cooling device address
>> + *
>> + * This main function does basically two operations:
>> + *
>> + * - Goes idle for a specific amount of time
>> + *
>> + * - Sets a timer to wake up all the idle injection threads after a
>> + *   running period
>> + *
>> + * That happens only when the mitigation is enabled, otherwise the
>> + * task is scheduled out.
>> + *
>> + * In order to keep the tasks synchronized together, it is the last
>> + * task exiting the idle period which is in charge of setting the
>> + * timer.
>> + *
>> + * This function never returns.
>> + */
>> +static int cpuidle_cooling_injection_thread(void *arg)
>> +{
>> +	struct sched_param param = { .sched_priority = MAX_USER_RT_PRIO/2 };
> 
> I am just wandering if should set priority to (MAX_RT_PRIO - 1)?
> Otherwise I am concern it might be cannot enter deep idle state when
> any CPU idle injection thread is preempted by other higher priority RT
> threads so all CPUs have no alignment for idle state entering/exiting.

I do believe we should consider other RT tasks more important than the
idle injection threads.

>> +	struct cpuidle_cooling_device *idle_cdev = arg;
>> +	struct cpuidle_cooling_tsk *cct = per_cpu_ptr(&cpuidle_cooling_tsk,
>> +						      smp_processor_id());
>> +	DEFINE_WAIT(wait);
>> +
>> +	set_freezable();
>> +
>> +	sched_setscheduler(current, SCHED_FIFO, &param);
>> +
>> +	while (1) {
>> +		s64 next_wakeup;
>> +
>> +		prepare_to_wait(&cct->waitq, &wait, TASK_INTERRUPTIBLE);
>> +
>> +		schedule();
>> +
>> +		atomic_inc(&idle_cdev->count);
>> +
>> +		play_idle(idle_cdev->idle_cycle / USEC_PER_MSEC);
>> +
>> +		/*
>> +		 * The last CPU waking up is in charge of setting the
>> +		 * timer. If the CPU is hotplugged, the timer will
>> +		 * move to another CPU (which may not belong to the
>> +		 * same cluster) but that is not a problem as the
>> +		 * timer will be set again by another CPU belonging to
>> +		 * the cluster, so this mechanism is self adaptive and
>> +		 * does not require any hotplugging dance.
>> +		 */
>> +		if (!atomic_dec_and_test(&idle_cdev->count))
>> +			continue;
>> +
>> +		if (!idle_cdev->state)
>> +			continue;
>> +
>> +		next_wakeup = cpuidle_cooling_runtime(idle_cdev);
>> +
>> +		hrtimer_start(&idle_cdev->timer, ns_to_ktime(next_wakeup),
>> +			      HRTIMER_MODE_REL_PINNED);
> 
> If SoC temperature descreases under tipping point, will the timer be
> disabled for this case?  Or will here set next timer event with big
> value from next_wakeup?

Another timer (the polling one) will update the 'state' variable to zero
in the set_cur_state. In the worst case, we check the idle_cdev->state
right before it is updated and we end up with an extra idle injection
cycle which is perfectly fine.


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