linux-kernel - Re: [RFC PATCH V2 0/1] x86: cpu topology fix and question on x86_max

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Date:   Mon, 20 Feb 2023 14:40:55 +0000
From:   "Zhang, Rui" <rui.zhang@...el.com>
To:     "peterz@...radead.org" <peterz@...radead.org>
CC:     "Brown, Len" <len.brown@...el.com>,
        "zhang.jia@...ux.alibaba.com" <zhang.jia@...ux.alibaba.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "hpa@...or.com" <hpa@...or.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH V2 0/1] x86: cpu topology fix and question on
 x86_max_cores

On Mon, 2023-02-20 at 11:36 +0100, Peter Zijlstra wrote:
> On Mon, Feb 20, 2023 at 11:28:55AM +0800, Zhang Rui wrote:
> 
> > Solution for fix smp_num_sibling
> > --------------------------------
> > 
> > Patch 1/1 ensures that smp_num_siblings represents the system-wide
> > maximum
> > number of siblings by always increasing its value. Never allow it
> > to
> > decrease.
> > 
> > It is sufficient to make the problem go away.
> > 
> > However, there is a pontenial problem left. That is, when boot CPU
> > is an
> > Ecore CPU, smp_num_sibling is set to 1 during BSP probe, kernel
> > disables
> > SMT support by setting cpu_smt_control to CPU_SMT_NOT_SUPPORTED in
> > start_kernel()->check_bugs()->cpu_smt_check_topology().
> > So far, we don't have such platforms.
> 
> This is the much recurring problem of 


> the boot CPU not having access to
> the system topology.
> 
Exactly!


> Instead of fixing that, Intel seems to work at making it worse. At
> some
> point, we're just going to have to give up and move to DT or
> something
> :/
> 
> Please communicate (again), that only knowing the topology/setup of
> the
> system once all the CPUs are online is crap. Once you start bringing
> up
> APs some things are fixed -- if we guessed wrong, we're hosed.
> 
> Specific examples of this that we've ran into in the past are:
> 
>  - does the machine have SMT
>  - is the machine Hybrid
>    (and if so, how many different core types will be have)
> 

It is "good" for me to know this is not the first time we run into such
issues. Less effort for me to sell the problem.
I will bring your suggestions back and see if we can improve this
instead of complicating software.

thanks,
rui