linux-kernel - Re: [PATCH] cpufreq: qcom-cpufreq-hw: Revert adding cpufreq qos

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Message-ID: <20230330040030.bal3eqrq22xluxca@vireshk-i7>
Date:   Thu, 30 Mar 2023 09:30:30 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Bjorn Andersson <quic_bjorande@...cinc.com>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Xuewen Yan <xuewen.yan@...soc.com>,
        Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
        Konrad Dybcio <konrad.dybcio@...aro.org>,
        linux-arm-msm@...r.kernel.org, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH] cpufreq: qcom-cpufreq-hw: Revert adding cpufreq qos

On 23-03-23, 15:33, Bjorn Andersson wrote:
> The OSM/EPSS hardware controls the frequency of each CPU cluster based
> on requests from the OS and various throttling events in the system.
> While throttling is in effect the related dcvs interrupt will be kept
> high. The purpose of the code handling this interrupt is to
> continuously report the thermal pressure based on the throttled
> frequency.
> 
> The reasoning for adding QoS control to this mechanism is not entirely
> clear, but the introduction of commit 'c4c0efb06f17 ("cpufreq:
> qcom-cpufreq-hw: Add cpufreq qos for LMh")' causes the
> scaling_max_frequncy to be set to the throttled frequency. On the next
> iteration of polling, the throttled frequency is above or equal to the
> newly requested frequency, so the polling is stopped.
> 
> With cpufreq limiting the max frequency, the hardware no longer report a
> throttling state and no further updates to thermal pressure or qos
> state are made.
> 
> The result of this is that scaling_max_frequency can only go down, and
> the system becomes slower and slower every time a thermal throttling
> event is reported by the hardware.
> 
> Even if the logic could be improved, there is no reason for software to
> limit the max freqency in response to the hardware limiting the max
> frequency. At best software will follow the reported hardware state, but
> typically it will cause slower backoff of the throttling.
> 
> This reverts commit c4c0efb06f17fa4a37ad99e7752b18a5405c76dc.
> 
> Fixes: c4c0efb06f17 ("cpufreq: qcom-cpufreq-hw: Add cpufreq qos for LMh")
> Cc: stable@...r.kernel.org
> Reported-by: Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
> Signed-off-by: Bjorn Andersson <quic_bjorande@...cinc.com>
> ---
>  drivers/cpufreq/qcom-cpufreq-hw.c | 14 --------------
>  1 file changed, 14 deletions(-)

Applied. Thanks.

-- 
viresh