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

[Wysocki, Rafael J]

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.