linux-kernel - Re: [PATCH] thermal: cpufreq_cooling: freq_qos_update

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Date:   Wed, 17 Feb 2021 10:29:13 +0000
From:   Lukasz Luba <lukasz.luba@....com>
To:     Viresh Kumar <viresh.kumar@...aro.org>,
        Amit Kucheria <amitk@...nel.org>
Cc:     Amit Daniel Kachhap <amit.kachhap@...il.com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Javi Merino <javi.merino@...nel.org>,
        Zhang Rui <rui.zhang@...el.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thara Gopinath <thara.gopinath@...aro.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        "v5 . 7+" <stable@...r.kernel.org>, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] thermal: cpufreq_cooling: freq_qos_update_request()
 returns < 0 on error

Hi Viresh,

On 2/17/21 5:48 AM, Viresh Kumar wrote:
> freq_qos_update_request() returns 1 if the effective constraint value
> has changed, 0 if the effective constraint value has not changed, or a
> negative error code on failures.
> 
> The frequency constraints for CPUs can be set by different parts of the
> kernel. If the maximum frequency constraint set by other parts of the
> kernel are set at a lower value than the one corresponding to cooling
> state 0, then we will never be able to cool down the system as
> freq_qos_update_request() will keep on returning 0 and we will skip
> updating cpufreq_state and thermal pressure.

To be precised, thermal pressure signal is not so important in this
mechanism and the 'cpufreq_state' has changed recently:

236761f19a4f373354  thermal/drivers/cpufreq_cooling: Update 
cpufreq_state only if state has changed

> 
> Fix that by doing the updates even in the case where
> freq_qos_update_request() returns 0, as we have effectively set the
> constraint to a new value even if the consolidated value of the
> actual constraint is unchanged because of external factors.
> 
> Cc: v5.7+ <stable@...r.kernel.org> # v5.7+
> Reported-by: Thara Gopinath <thara.gopinath@...aro.org>
> Fixes: f12e4f66ab6a ("thermal/cpu-cooling: Update thermal pressure in case of a maximum frequency capping")

I'm not sure if that f12e4f is the root cause.

> Signed-off-by: Viresh Kumar <viresh.kumar@...aro.org>
> ---
> Hi Guys,
> 
> This needs to go in 5.12-rc.
> 
> Thara, please give this a try and give your tested-by :).
> 
>   drivers/thermal/cpufreq_cooling.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)


Anyway, the fix LGTM. I will have to make sure that I'm CC'ed for these
topic, so I can have a look (I missed somehow 236761f19)

Reviewed-by: Lukasz Luba <lukasz.luba@....com>
Tested-by: Lukasz Luba <lukasz.luba@....com>

Regards,
Lukasz