linux-kernel - Re: [PATCH v1 1/2] x86/tsc: use logical_package as a better estimation of socket numbers

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Date:   Sun, 23 Oct 2022 00:12:38 +0800
From:   Zhang Rui <rui.zhang@...el.com>
To:     Dave Hansen <dave.hansen@...el.com>,
        Feng Tang <feng.tang@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Cc:     tim.c.chen@...el.com, Xiongfeng Wang <wangxiongfeng2@...wei.com>,
        liaoyu15@...wei.com
Subject: Re: [PATCH v1 1/2] x86/tsc: use logical_package as a better
 estimation of socket numbers

On Fri, 2022-10-21 at 09:21 -0700, Dave Hansen wrote:
> On 10/21/22 08:00, Zhang Rui wrote:
> > I checked the history of '__max_logical_packages', and realized
> > that
> > 
> > 1. for topology_max_packages()/'__max_logical_packages', the
> > divisor
> >    'ncpus' uses cpu_data(0).booted_cores, which is based on the
> >    *online* CPUs. So when using kernel cmdlines like
> > maxcpus=/nr_cpus=,
> >    '__max_logical_packages' can get over-estimated.
> > 
> > 
> > 2. for 'logical_packages', it equals the number of different
> > physical
> >    Package IDs for all *online* CPUs. So with kernel cmdlines like
> >    nr_cpus=/maxcpus=, it can gets under-estimated.
> > 
> > BTW, I also checked CPUID.B/1F, which can tell a fixed number of
> > CPUs
> > within a package. But we don't have a fixed number of total CPUs
> > from
> > hardware.
> > On my Dell laptop, BIOS allows me to disable/enable one or several
> > cores. When this happens, the 'total_cpus' changes, but CPUID.B/1F
> > does
> > not change. So I don't think CPUID.B/1F can be used to optimize the
> > '__
> > max_logical_packages' calculation.
> > 
> > I'm not sure if we have a perfect solution here.
> 
> Are the implementations fixable?

currently, I don't have any idea.

>   Or, at least tolerable?
> 
> For instance, I can live with the implementation being a bit goofy
> when
> kernel commandlines are in play.  We can pr_info() about those cases.

My understanding is that the cpus in the last package may still have
small cpu id value. This means that the 'logical_packages' is hard to
break unless we boot with very small CPU count and happened to disable
all cpus in one/more packages. Feng is experiencing with this and may
have some update later.

If this is the case, is this a valid case that we need to take care of?

thanks,
rui