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

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Date:   Mon, 16 Oct 2023 11:50:34 -0700
From:   srinivas pandruvada <srinivas.pandruvada@...ux.intel.com>
To:     "Wysocki, Rafael J" <rafael.j.wysocki@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Meng, Li (Jassmine)" <Li.Meng@....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 Mon, 2023-10-16 at 19:27 +0200, Wysocki, Rafael J wrote:
> 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.
> 
If we make
	static u32 max_highest_perf = 0, min_highest_perf = U32_MAX;
to
	static int max_highest_perf = INT_MIN, min_highest_perf =
INT_MAX;

Then in intel_pstate we will compare signed vs unsigned comparison as
cppc_perf.highest_perf is u32.


In reality this will be fine to change to "int" as we will never reach
u32 max as performance on any Intel platform.

> 
> > 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.
Should be fine as over clocked parts will set to max 0xff.

> 
> 
> > Also, same question as to the amd folks; what serializes those
> > static
> > variables?
> 
> That's a good one.

This function which is checking static variables is called from cpufreq
->init callback. Which in turn is called from a function which is
passed as startup() function pointer to
cpuhp_setup_state_nocalls_cpuslocked().

I see that startup() callbacks are called under a mutex
cpuhp_state_mutex for each present CPUs. So if some tear down happen,
that is also protected by the same mutex. The assumption is here is
that cpuhp_invoke_callback() in hotplug state machine is not called in
parallel on two CPUs by the hotplug state machine. But I see activity
on parallel bringup, so this is questionable now.

Thanks,
Srinivas

> 
>