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

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Date:   Thu, 20 Apr 2017 18:21:40 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Boris Ostrovsky <boris.ostrovsky@...cle.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Prarit Bhargava <prarit@...hat.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, xen-devel@...ts.xenproject.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...e.de>, Juergen Gross <jgross@...e.com>
Subject: Re: [Xen-devel] [PATCH RFC] x86/smpboot: Set safer __max_logical_packages limit

Boris Ostrovsky <boris.ostrovsky@...cle.com> writes:

> On 04/20/2017 11:40 AM, Vitaly Kuznetsov wrote:
>> 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.
>
> Do you observe this failure on PV or HVM guest?
>
> We've had a number of issues with topology discovery for PV guests but
> AFAIK they have been addressed (so far). I wonder though whether it
> would make sense to have some sort of a callback (or an smp_ops.op) to
> override native topology info, if needed.
>

This is HVM.

I guess that CPUID handling for AMD processors in the hypervisor doesn't
adjust the core information and passes it from hardware as-is while it
should be tweaked.

-- 
  Vitaly