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

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Message-ID: <Z8bPtsO7dEV0lq2M@bogus>
Date: Tue, 4 Mar 2025 10:02:30 +0000
From: Sudeep Holla <sudeep.holla@....com>
To: Pierre Gondois <pierre.gondois@....com>
Cc: Yicong Yang <yangyicong@...wei.com>, <yangyicong@...ilicon.com>,
	Sudeep Holla <sudeep.holla@....com>, <catalin.marinas@....com>,
	<will@...nel.org>, <tglx@...utronix.de>, <peterz@...radead.org>,
	<mpe@...erman.id.au>, <linux-arm-kernel@...ts.infradead.org>,
	<mingo@...hat.com>, <bp@...en8.de>, <dave.hansen@...ux.intel.com>,
	<dietmar.eggemann@....com>, <linuxppc-dev@...ts.ozlabs.org>,
	<x86@...nel.org>, <linux-kernel@...r.kernel.org>,
	<morten.rasmussen@....com>, <msuchanek@...e.de>,
	<gregkh@...uxfoundation.org>, <rafael@...nel.org>,
	<jonathan.cameron@...wei.com>, <prime.zeng@...ilicon.com>,
	<linuxarm@...wei.com>, <xuwei5@...wei.com>, <guohanjun@...wei.com>,
	<sshegde@...ux.ibm.com>
Subject: Re: [PATCH v11 3/4] arm64: topology: Support SMT control on ACPI
 based system

On Tue, Mar 04, 2025 at 09:25:02AM +0100, Pierre Gondois wrote:
>
>
> On 3/3/25 15:40, Yicong Yang wrote:
> > On 2025/3/3 19:16, Sudeep Holla wrote:
> > > 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 think it will be problematic if we parse:
> - first a CPU with 1 thread
> - then a CPU with 2 threads
>
> in that case we should detect the 'Heterogeneous SMT topology',
> but we cannot because we don't know whether max_smt_thread_num=1
> because 1 is the default value or we found a CPU with one thread.

Right, but as per Dietmar's and my previous response, it may be a valid
case. See latest response from Dietmar which is explicitly requesting
support for this. It may need some special handling if we decide to support
that.

--
Regards,
Sudeep