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

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Message-ID: <20171110150112.GI8522@tassilo.jf.intel.com>
Date:   Fri, 10 Nov 2017 07:01:12 -0800
From:   Andi Kleen <ak@...ux.intel.com>
To:     Prarit Bhargava <prarit@...hat.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>, 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

> > 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?

-Andi