linux-kernel - Re: [RESEND PATCH V9 3/7] cpufreq: amd-pstate: Enable amd-pstate preferred core supporting.

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Message-ID: <e82fc689-5cc3-d799-6e5f-a9e4ac98e26f@intel.com>
Date:   Mon, 16 Oct 2023 19:27:03 +0200
From:   "Wysocki, Rafael J" <rafael.j.wysocki@...el.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        "Meng, Li (Jassmine)" <Li.Meng@....com>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>
CC:     "Huang, Ray" <Ray.Huang@....com>,
        "linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>,
        Shuah Khan <skhan@...uxfoundation.org>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
        "Fontenot, Nathan" <Nathan.Fontenot@....com>,
        "Sharma, Deepak" <Deepak.Sharma@....com>,
        "Deucher, Alexander" <Alexander.Deucher@....com>,
        "Limonciello, Mario" <Mario.Limonciello@....com>,
        "Huang, Shimmer" <Shimmer.Huang@....com>,
        "Yuan, Perry" <Perry.Yuan@....com>,
        "Du, Xiaojian" <Xiaojian.Du@....com>,
        "Viresh Kumar" <viresh.kumar@...aro.org>,
        Borislav Petkov <bp@...en8.de>,
        "Oleksandr Natalenko" <oleksandr@...alenko.name>,
        "Karny, Wyes" <Wyes.Karny@....com>
Subject: Re: [RESEND PATCH V9 3/7] cpufreq: amd-pstate: Enable amd-pstate
 preferred core supporting.

On 10/16/2023 12:58 PM, Peter Zijlstra wrote:
> On Mon, Oct 16, 2023 at 06:20:53AM +0000, Meng, Li (Jassmine) wrote:
>>>> +static void amd_pstate_init_prefcore(struct amd_cpudata *cpudata) {
>>>> +     int ret, prio;
>>>> +     u32 highest_perf;
>>>> +     static u32 max_highest_perf = 0, min_highest_perf = U32_MAX;
>>> What serializes these things?
>>>
>>> Also, *why* are you using u32 here, what's wrong with something like:
>>>
>>>          int max_hp = INT_MIN, min_hp = INT_MAX;
>>>
>> [Meng, Li (Jassmine)]
>> We use ITMT architecture to utilize preferred core features.
>> Therefore, we need to try to be consistent with Intel's implementation
>> as much as possible.  For details, please refer to the
>> intel_pstate_set_itmt_prio function in file intel_pstate.c. (Line 355)
>>
>> I think using the data type of u32 is consistent with the data
>> structures of cppc_perf_ctrls and amd_cpudata etc.
> Rafael, should we fix intel_pstate too?

Srinivas should be more familiar with this code than I am, so adding him.


> The point is, that sched_asym_prefer(), the final consumer of these
> values uses int and thus an explicitly signed compare.
>
> Using u32 and U32_MAX anywhere near the setting the priority makes
> absolutely no sense.
>
> If you were to have the high bit set, things do not behave as expected.

Right, but in practice these values are always between 0 and 255 
inclusive AFAICS.

It would have been better to use u8 I suppose.


> Also, same question as to the amd folks; what serializes those static
> variables?

That's a good one.