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

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Date:   Sun, 12 Nov 2017 11:55:25 -0500
From:   Prarit Bhargava <prarit@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Andi Kleen <ak@...ux.intel.com>
Cc:     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 11/12/2017 08:36 AM, Thomas Gleixner wrote:
> On Fri, 10 Nov 2017, 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.
> 
> Sure, we can use a separate per cpu variable. The race for looking up phys
> -> logical will always be there if that handles stuff like the uncore PCI
> physid one. There is not much which can prevent that.
> 
> The other option is to figure out what clears cpu_data on online and just
> preserve the logical/physcial translation across that clear. 
> 
> One thing you need to be careful about (in both cases) is the value. The
> data is zeroed on boot, so we either need to fill that with UINT_MAX at
> boot time in one of the functions which does a for_each_possible_cpu() loop
> anyway or just leave 0 as the 'not initialized' value and make the first
> logical package be '1'. The readout functions (percpu, translation,
> etc.) just can subtract 1.

I'll look into this (code & test) and get back with a v6.

Thanks Thomas,

P.

> 
> Thanks,
> 
> 	tglx
>