linux-kernel - Re: [RFC PATCH] perf/x86/intel/rapl: avoid access unallocate memory

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Date:   Wed, 2 Nov 2016 18:47:49 -0400
From:   "Charles (Chas) Williams" <ciwillia@...cade.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>, <x86@...nel.org>
CC:     <linux-kernel@...r.kernel.org>,
        "M. Vefa Bicakci" <m.v.b@...box.com>
Subject: Re: [RFC PATCH] perf/x86/intel/rapl: avoid access unallocate memory

On 11/02/2016 08:25 AM, Sebastian Andrzej Siewior wrote:
> I am not sure if this a race with the new hotplug code or something that was
> always there. Both (M. Vefa Bicakc and Charles) say that the box boots
> sometimes fine (without the patch).  smp_store_boot_cpu_info() should have run
> before the notofoert and thus should have set the info properly. However I got
> the following bootlog from Charles with this patch:

I don't this this is a race.  Here is some debugging from the two CPU VM
(2 sockets, 1 core per socket).  In identify_cpu() we have:

         /* The boot/hotplug time assigment got cleared, restore it */
         c->logical_proc_id = topology_phys_to_logical_pkg(c->phys_proc_id);

The values just after this:

	[    0.228306] identify_cpu: c ffff88023fd0a040  logical_proc_id 65535  c->phys_proc_id 2

So what's interesting here, is the phys_proc_id of 2 for CPU1:

         int topology_phys_to_logical_pkg(unsigned int phys_pkg)
         {
                 if (phys_pkg >= max_physical_pkg_id)
                         return -1;
                 return physical_to_logical_pkg[phys_pkg];
         }

And we happen to know the max_physical_pkg_id is 2 in this case.
So apparently, topology_phys_to_logical_pkg() returns -1 and it gets
assigned to the logical_proc_id.

I don't know why the CPU's phys_proc_id is 2.