linux-kernel - Re: [PATCH RFC] x86/smpboot: Set safer __max_logical

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Date:   Thu, 20 Apr 2017 17:06:15 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     x86@...nel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...e.de>,
        Prarit Bhargava <prarit@...hat.com>,
        linux-kernel@...r.kernel.org, xen-devel@...ts.xenproject.org
Subject: Re: [PATCH RFC] x86/smpboot: Set safer __max_logical_packages limit

On Thu, Apr 20, 2017 at 03:24:53PM +0200, Vitaly Kuznetsov wrote:
> In this patch I suggest we set __max_logical_packages based on the
> max_physical_pkg_id and total_cpus,

So my 4 socket 144 CPU system will then get max_physical_pkg_id=144,
instead of 4.

This wastes quite a bit of memory for the per-node arrays. Luckily most
are just pointer arrays, but still, wasting 140*8 bytes for each of
them.

> this should be safe and cover all
> possible cases. Alternatively, we may think about eliminating the concept
> of __max_logical_packages completely and relying on max_physical_pkg_id/
> total_cpus where we currently use topology_max_packages().
> 
> The issue could've been solved in Xen too I guess. CPUID returning
> x86_max_cores can be tweaked to be the lowerest(?) possible number of
> all logical packages of the guest.

This is getting ludicrous. Xen is plain broken, and instead of fixing
it, you propose to somehow deal with its obviously crack induced
behaviour :-(