linux-kernel - Re: [PATCH v3 6/7] thermal/drivers/cpu_cooling: Introduce the cpu idle cooling driver

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Message-ID: <2d7f1caa-351a-8fdc-bb6b-151f7313a042@linaro.org>
Date:   Mon, 5 Aug 2019 09:37:43 +0200
From:   Daniel Lezcano <daniel.lezcano@...aro.org>
To:     Martin Kepplinger <martin.kepplinger@...i.sm>,
        viresh.kumar@...aro.org, kevin.wangtao@...aro.org,
        leo.yan@...aro.org, edubezval@...il.com,
        vincent.guittot@...aro.org, javi.merino@...nel.org,
        rui.zhang@...el.com, daniel.thompson@...aro.org
Cc:     linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 6/7] thermal/drivers/cpu_cooling: Introduce the cpu
 idle cooling driver

On 05/08/2019 07:11, Martin Kepplinger wrote:
> ---

[ ... ]

>> +static s64 cpuidle_cooling_runtime(struct cpuidle_cooling_device *idle_cdev)
>> +{
>> +	s64 next_wakeup;
>> +	unsigned long state = idle_cdev->state;
>> +
>> +	/*
>> +	 * The function should not be called when there is no
>> +	 * mitigation because:
>> +	 * - that does not make sense
>> +	 * - we end up with a division by zero
>> +	 */
>> +	if (!state)
>> +		return 0;
>> +
>> +	next_wakeup = (s64)((idle_cdev->idle_cycle * 100) / state) -
>> +		idle_cdev->idle_cycle;
>> +
>> +	return next_wakeup * NSEC_PER_USEC;
>> +}
>> +
> 
> There is a bug in your calculation formula here when "state" becomes 100.
> You return 0 for the injection rate, which is the same as "rate" being 0,
> which is dangerous. You stop cooling when it's most necessary :)

Right, thanks for spotting this.

> I'm not sure how much sense really being 100% idle makes, so I, when testing
> this, just say if (state == 100) { state = 99 }. Anyways, just don't return 0.
> 
> Daniel, thanks a lot for these additions! Could you send an update of this?

Yes, I'm working on a new version.

> btw, that's what I'm referring to:
> https://lore.kernel.org/linux-pm/1522945005-7165-1-git-send-email-daniel.lezcano@linaro.org/
> I know it's a little old already, but it seems like there hasn't been any
> equivalent solution in the meantime, has it?
> 
> Using cpuidle for cooling is way more effective than cpufreq (which often
> hardly is).

On which platform that happens?


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