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Message-ID: <2c091ba8-4d5c-46d4-bf27-453ef96b1aa2@arm.com>
Date: Tue, 2 Jan 2024 09:42:02 +0000
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Subject: Re: [PATCH v5 03/23] PM: EM: Find first CPU active while updating OPP
 efficiency
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To: Qais Yousef <qyousef@layalina.io>
Cc: linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org,
 rafael@kernel.org, dietmar.eggemann@arm.com, rui.zhang@intel.com,
 amit.kucheria@verdurent.com, amit.kachhap@gmail.com,
 daniel.lezcano@linaro.org, viresh.kumar@linaro.org, len.brown@intel.com,
 pavel@ucw.cz, mhiramat@kernel.org, wvw@google.com
References: <20231129110853.94344-1-lukasz.luba@arm.com>
 <20231129110853.94344-4-lukasz.luba@arm.com>
 <20231217175829.a6hqz7mqlvrujsvs@airbuntu>
 <b31672e4-ab41-4724-86ef-038606318663@arm.com>
 <20231228171315.bmjdo5eztyix5o3r@airbuntu>
From: Lukasz Luba <lukasz.luba@arm.com>
In-Reply-To: <20231228171315.bmjdo5eztyix5o3r@airbuntu>
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On 12/28/23 17:13, Qais Yousef wrote:
> On 12/19/23 10:53, Lukasz Luba wrote:
>>
>>
>> On 12/17/23 17:58, Qais Yousef wrote:
>>> On 11/29/23 11:08, Lukasz Luba wrote:
>>>> The Energy Model might be updated at runtime and the energy efficiency
>>>> for each OPP may change. Thus, there is a need to update also the
>>>> cpufreq framework and make it aligned to the new values. In order to
>>>> do that, use a first active CPU from the Performance Domain. This is
>>>> needed since the first CPU in the cpumask might be offline when we
>>>> run this code path.
>>>
>>> I didn't understand the problem here. It seems you're fixing a race, but the
>>> description is not clear to me what the race is.
>>
>> I have explained that in v1, v4 comments for this patch.
>> When the EM is registered the fist CPU is always online. No problem
>> for the old code, but for new code with runtime modification at
>> later time, potentially from different subsystems - it it (e.g. thermal,
>> drivers, etc). The fist CPU might be offline, but still such EM
>> update for this domain shouldn'y fail. Although, when the CPU is offline
>> we cannot get the valid policy...
>>
>> We can get it for next cpu in the cpumask, that's what the code is
>> doing.
> 
> Okay, I can see now that cpufreq_cpu_get_raw() ignores offline CPUs
> intentionally.
> 
> A new variant seems better to me. But the experts know better. So LGTM.

Thanks. Yes, I will gently ask Viresh to have a look at those places
cpufreq related places.