linux-kernel - Re: [PATCH v5 2/3] x86/topology: Avoid wasting 128k for package id array

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Date:   Fri, 10 Nov 2017 13:35:41 -0500
From:   Prarit Bhargava <prarit@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Andi Kleen <ak@...ux.intel.com>, linux-kernel@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Piotr Luc <piotr.luc@...el.com>,
        Kan Liang <kan.liang@...el.com>, Borislav Petkov <bp@...e.de>,
        Stephane Eranian <eranian@...gle.com>,
        Arvind Yadav <arvind.yadav.cs@...il.com>,
        Andy Lutomirski <luto@...nel.org>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        He Chen <he.chen@...ux.intel.com>,
        Mathias Krause <minipli@...glemail.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>
Subject: Re: [PATCH v5 2/3] x86/topology: Avoid wasting 128k for package id
 array

On 11/10/2017 10:01 AM, Andi Kleen wrote:
>>> All of that works. There is no way to make sure that a lookup is fully
>>> serialized against a concurrent update. Even if the lookup holds
>>> cpu_read_lock() the new package might arrive right after the unlock.
>>>
>>
>> Thanks Thomas.
>>
>> Andi, do you want to take a look at this?
> 
> I was originally worried about races, that is why i tried to put 
> everything into cpu_data. But that didn't work out because something
> clears it. Perhaps the right solution would be some extra per_cpu
> data variables, and search for the first match. I suspect that would
> be simpler. But if that doesn't work I guess something like Thomas'
> example will work.
> 
> I assume you will handle it, Prarit?

I can, but let's discuss your paragraph above.  I'd like to get Thomas' thoughts
on the problem and see if he thinks your original idea is valid, or if he has a
better suggestion.

Thomas, what Andi was originally attempting to do (way back in v1) is store the
logical_proc_id in cpu_data and drop the array completely.

The problem is the procedure on a cpu up, smp_store_cpu_info() overwrites the
cpu's cpu_data with boot_cpu_data (arch/x86/kernel/smpboot.c:402).  AFAICT this
is done because *new* cpus (ie, physical hotplug) should be initialized with
some basic data from boot_cpu_data.  ie) for an already existing cpu it looks
like this is unnecessary as the data has already been initialized.

Andi's idea would work and the code would become much cleaner, if
smp_store_cpu_info() only overwrote cpu_data for new physically hotplugged cpus.

Admittedly, I might be missing some path to that code so I'm looking for
verification.  What do you think?

P.

> 
> -Andi
>