linux-kernel - Re: [patch v2 21/30] x86/cpu/topology: Use topology bitmaps for sizing

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Date: Fri, 26 Jan 2024 21:22:47 +0100
From: Thomas Gleixner <tglx@...utronix.de>
To: "Zhang, Rui" <rui.zhang@...el.com>, "linux-kernel@...r.kernel.org"
 <linux-kernel@...r.kernel.org>
Cc: "Raj, Ashok" <ashok.raj@...el.com>, "mhklinux@...look.com"
 <mhklinux@...look.com>, "arjan@...ux.intel.com" <arjan@...ux.intel.com>,
 "ray.huang@....com" <ray.huang@....com>, "thomas.lendacky@....com"
 <thomas.lendacky@....com>, "andrew.cooper3@...rix.com"
 <andrew.cooper3@...rix.com>, "Sivanich, Dimitri"
 <dimitri.sivanich@....com>, "Tang, Feng" <feng.tang@...el.com>,
 "kan.liang@...ux.intel.com" <kan.liang@...ux.intel.com>, "Mehta, Sohil"
 <sohil.mehta@...el.com>, "peterz@...radead.org" <peterz@...radead.org>,
 "paulmck@...nel.org" <paulmck@...nel.org>, "kprateek.nayak@....com"
 <kprateek.nayak@....com>, "jgross@...e.com" <jgross@...e.com>,
 "andy@...radead.org" <andy@...radead.org>, "x86@...nel.org"
 <x86@...nel.org>
Subject: Re: [patch v2 21/30] x86/cpu/topology: Use topology bitmaps for sizing

On Fri, Jan 26 2024 at 07:07, Zhang, Rui wrote:
>> >  
>> > +       cnta = domain_weight(TOPO_PKG_DOMAIN);
>> > +       cntb = domain_weight(TOPO_DIE_DOMAIN);
>> > +       __max_logical_packages = cnta;
>> > +       __max_dies_per_package = 1U << (get_count_order(cntb) - >
>> > get_count_order(cnta));
>> > +
>> > +       pr_info("Max. logical packages: %3u\n", cnta);
>> > +       pr_info("Max. logical dies:     %3u\n", cntb);
>> > +       pr_info("Max. dies per package: %3u\n", >
>> > __max_dies_per_package);
>> > +
>> > +       cnta = domain_weight(TOPO_CORE_DOMAIN);
>> > +       cntb = domain_weight(TOPO_SMT_DOMAIN);
>> > +       smp_num_siblings = 1U << (get_count_order(cntb) - >
>> > get_count_order(cnta));
>> > +       pr_info("Max. threads per core: %3u\n", smp_num_siblings);
>> > +
>
> I missed this but Ashok catches it.
>
> Say, on my Adlerlake platform, which has 4 Pcores with HT + 8 Ecores,
> cnta is 12, cntb is 16, and smp_num_siblings is set to 1 erroneously.
>
> I think we should use
> 	smp_num_siblings = DIV_ROUND_UP(cntb, cnta);
> here.

Indeed. That's more than obvious.

> Or even check each core to get the maximum value (in case there are
> more than 2 siblings in a core some day).

We want to get rid of HT not make it worse.