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

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Message-ID: <87fuh3xf2i.fsf@vitty.brq.redhat.com>
Date:   Thu, 20 Apr 2017 17:40:37 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
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

Peter Zijlstra <peterz@...radead.org> writes:

> 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 :-(

Totally agree and I don't like the solution I propose (and that's why
this is RFC)... The problem is that there are such Xen setups in the
wild and with the recent changes some guests will BUG() :-(

Alternatively, we can just remove the BUG() and do something with CPUs
which have their pkg >= __max_logical_packages, e.g. assign them to the
last package. Far from ideal but will help to avoid the regression.

-- 
  Vitaly