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Date: Thu, 18 Feb 2016 09:13:20 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Andi Kleen <andi@firstfloor.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
        Stephane Eranian <eranian@google.com>,
        LKML <linux-kernel@vger.kernel.org>, Ingo Molnar <mingo@kernel.org>,
        Borislav Petkov <bp@alien8.de>,
        Harish Chegondi <harish.chegondi@intel.com>,
        Kan Liang <kan.liang@intel.com>, Andi Kleen <andi.kleen@intel.com>
Subject: Re: [patch 07/11] x86/perf/uncore: Track packages not per cpu data
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On Wed, Feb 17, 2016 at 11:16:40PM +0100, Andi Kleen wrote:
> > Do you have any data to back that up or is that just "believe" ?
> 
> I've seen systems with discontiguous apic ids before.
> 
> It is obvious if you consider setups with node hotplug.

Those systems will also have a num_possible_cpus() that's inflated to
account for the possible hotplug of new sockets, right?

So while the phys_pkg_id of the present sockets might be 2 and 3
(leaving 0 and 1 available for hotplug), the num_possible_cpus() should
be big enough to actually allow for that hotplug to happen.

Because if num_possible_cpus() is too small, we could not accommodate
the hotplug operation.

And if num_possible_cpus() is of the right size, then the computed
max_packages() should be of the right size too.

Now clearly, BIOS can completely wreck things and indeed report too
small an apic_id range or whatever, and in this case we're up a creek
without a paddle.

But I think you can check for that at boot and report errors/warns
whatever, because if you trigger this, your system is not really
'correct' anyway.