Re: [PATCH v11 3/4] arm64: topology: Support SMT control on ACPI based system

Sudeep Holla
2025-03-03T12:09:26Z

On Mon, Mar 03, 2025 at 10:56:12AM +0100, Pierre Gondois wrote:
> On 2/28/25 20:06, Sudeep Holla wrote:
> > > >
> > > > Ditto as previous patch, can get rid if it is default 1.
> > > >
> > >
> > > On non-SMT platforms, not calling cpu_smt_set_num_threads() leaves
> > > cpu_smt_num_threads uninitialized to UINT_MAX:
> > >
> > > smt/active:0
> > > smt/control:-1
> > >
> > > If cpu_smt_set_num_threads() is called:
> > > active:0
> > > control:notsupported
> > >
> > > So it might be slightly better to still initialize max_smt_thread_num.
> > >
> >
> > Sure, what I meant is to have max_smt_thread_num set to 1 by default is
> > that is what needed anyways and the above code does that now.
> >
> > Why not start with initialised to 1 instead ?
> > Of course some current logic needs to change around testing it for zero.
> >
>
> I think there would still be a way to check against the default value.
> If we have:
> unsigned int max_smt_thread_num = 1;
>
> then on a platform with 2 threads, the detection condition would trigger:
> xa_for_each(&hetero_cpu, hetero_id, entry) {
> if (entry->thread_num != max_smt_thread_num && max_smt_thread_num) <---- (entry->thread_num=2) and (max_smt_thread_num=1)
> pr_warn_once("Heterogeneous SMT topology is partly
> supported by SMT control\n");
>
> so we would need an additional variable:
> bool is_initialized = false;

Sure, we could do that or skip the check if max_smt_thread_num == 1 ?

I mean
if (entry->thread_num != max_smt_thread_num && max_smt_thread_num != 1)

I assume entry->thread_num must be set to 1 on single threaded cores
Won't that work ? Am I missing something still ?

--
Regards,
Sudeep