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Date: Fri, 4 Nov 2016 08:20:37 -0400 From: "Charles (Chas) Williams" <ciwillia@...cade.com> To: Sebastian Andrzej Siewior <bigeasy@...utronix.de> CC: <x86@...nel.org>, <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 11/03/2016 01:47 PM, Sebastian Andrzej Siewior wrote: > On 2016-11-02 18:47:49 [-0400], Charles (Chas) Williams wrote: >> 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. > > This is the physical ID. You have two logical IDs (on your two sockets > machine). What is max_physical_pkg_id? In order to get that -1 you would > have to max_physical_pkg_id of 1 but code does > max_physical_pkg_id = DIV_ROUND_UP(MAX_LOCAL_APIC, ncpus); > > and I would be a little surprised if this is 1. > > Sebastian The initial CPU boots and is identified: [ 0.009018] identify_boot_cpu [ 0.009174] generic_identify: phys_proc_id is now 0 ... [ 0.009427] identify_cpu: before c ffffffff81ae2680 logical_proc_id 0 c->phys_proc_id 0 [ 0.009506] identify_cpu: after c ffffffff81ae2680 logical_proc_id 65535 c->phys_proc_id 0 So, this is fine because the APIC hasn't been scanned yet. APIC now gets scanned: [ 0.015789] smpboot: APIC(0) Converting physical 0 to logical package 0, cpu 0 (ffff88023fc0a040) [ 0.015794] smpboot: APIC(1) Converting physical 1 to logical package 1, cpu 1 (ffff88023fd0a040) [ 0.015797] smpboot: Max logical packages: 2 So, at this point, I think everything is correct. But now the secondary CPU's "boot": [ 0.236569] identify_secondary_cpu [ 0.236620] generic_identify: phys_proc_id is now 2 [ 0.236745] identify_cpu: before c ffff88023fd0a040 logical_proc_id 65535 c->phys_proc_id 2 [ 0.236747] identify_cpu: after c ffff88023fd0a040 logical_proc_id 65535 c->phys_proc_id 2 So, APIC discovered I have a cpu 0 and 1 but generic_identify() is called my second CPU, 2. This is >= max_physical_pkg_id, so it is going to get set to -1. The comment at the end of identfy_cpu() says: /* The boot/hotplug time assigment got cleared, restore it */ So, logical_proc_id being wrong here before restoration doesn't bother me since I assume something in booting the secondary CPU's clears any existing cpu data. I know detect_extended_topology() is likely being called for both CPU's and getting the right values (checking this now). I don't know why generic_identify() is resetting this value.
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