Date:   Mon, 16 Oct 2023 06:20:53 +0000
From:   "Meng, Li (Jassmine)" <Li.Meng@....com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        "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.

[AMD Official Use Only - General]

Hi Peter:

> -----Original Message-----
> From: Peter Zijlstra <peterz@...radead.org>
> Sent: Saturday, October 14, 2023 12:01 AM
> To: Meng, Li (Jassmine) <Li.Meng@....com>
> Cc: Rafael J . Wysocki <rafael.j.wysocki@...el.com>; Huang, Ray
> <Ray.Huang@....com>; linux-pm@...r.kernel.org; linux-
> kernel@...r.kernel.org; x86@...nel.org; linux-acpi@...r.kernel.org; Shuah
> Khan <skhan@...uxfoundation.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.
>
> Caution: This message originated from an External Source. Use proper
> caution when opening attachments, clicking links, or responding.
>
>
> On Fri, Oct 13, 2023 at 11:31:14AM +0800, Meng Li wrote:
>
> > +#define AMD_PSTATE_PREFCORE_THRESHOLD        166
> > +#define AMD_PSTATE_MAX_CPPC_PERF     255
>
> > +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.

> > +
> > +     ret = amd_pstate_get_highest_perf(cpudata->cpu, &highest_perf);
> > +     if (ret)
> > +             return;
> > +
> > +     cpudata->hw_prefcore = true;
> > +     /* check if CPPC preferred core feature is enabled*/
> > +     if (highest_perf == AMD_PSTATE_MAX_CPPC_PERF) {
>
> Which effectively means <255 (also, seems to suggest MAX_CPPC_PERF
> might not be the best name, hmm?)
>
> Should you not write '>= 255' then? Just in case something 'funny'
> happens?
>
[Meng, Li (Jassmine)]
OK, I will modify these.

> > +             pr_debug("AMD CPPC preferred core is unsupported!\n");
> > +             cpudata->hw_prefcore = false;
> > +             return;
> > +     }
> > +
> > +     if (!amd_pstate_prefcore)
> > +             return;
> > +
> > +     /* The maximum value of highest perf is 255 */
> > +     prio = (int)(highest_perf & 0xff);
>
> If for some weird reason you get 0x1ff or whatever above (dodgy BIOS never
> happens, right) then this makes sense how?
>
> Perhaps stop sending patches at break-nek speed and think for a little while
> on how to write this and not be confused?
>
[Meng, Li (Jassmine)]
If I use '>= 255' to check, the issue mentioned will not exist.
Because it will be returned when highest_perff>0xff.
>
> > +     /*
> > +      * The priorities can be set regardless of whether or not
> > +      * sched_set_itmt_support(true) has been called and it is valid to
> > +      * update them at any time after it has been called.
> > +      */
> > +     sched_set_itmt_core_prio(prio, cpudata->cpu);
> > +
> > +     if (max_highest_perf <= min_highest_perf) {
> > +             if (highest_perf > max_highest_perf)
> > +                     max_highest_perf = highest_perf;
> > +
> > +             if (highest_perf < min_highest_perf)
> > +                     min_highest_perf = highest_perf;
> > +
> > +             if (max_highest_perf > min_highest_perf) {
> > +                     /*
> > +                      * This code can be run during CPU online under the
> > +                      * CPU hotplug locks, so sched_set_itmt_support()
> > +                      * cannot be called from here.  Queue up a work item
> > +                      * to invoke it.
> > +                      */
> > +                     schedule_work(&sched_prefcore_work);
> > +             }
> > +     }
>
> Not a word about what serializes these variables.
>
> > +}

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