linux-kernel - Re: [PATCH v4 2/2] x86/amd: Fixup cpu_core_id for family17h downcore configuration

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Date:   Tue, 25 Jul 2017 12:51:53 +0700
From:   Suravee Suthikulpanit <Suravee.Suthikulpanit@....com>
To:     Borislav Petkov <bp@...e.de>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org, tglx@...utronix.de,
        mingo@...hat.com, hpa@...or.com, peterz@...radead.org,
        Yazen.Ghannam@....com, Andreas Herrmann <aherrmann@...e.com>
Subject: Re: [PATCH v4 2/2] x86/amd: Fixup cpu_core_id for family17h downcore
 configuration

Boris,

On 7/24/17 21:44, Borislav Petkov wrote:
> On Mon, Jul 24, 2017 at 09:14:18PM +0700, Suravee Suthikulpanit wrote:
>> Actually, this is not totally accurate. My apology. This patch is
>> mainly fix to incorrect core ID in /proc/cpuinfo.
>
> So you're "fixing" only some numbering thing. Because core_id doesn't
> have any influence on anything. Here's on an Intel box I have here:
>
> processor :  0		 physical id : 0	 core id : 0
> processor :  1		 physical id : 1	 core id : 0
> processor :  2		 physical id : 2	 core id : 0
> processor :  3		 physical id : 3	 core id : 0
> processor :  4		 physical id : 0	 core id : 8
> processor :  5		 physical id : 1	 core id : 8
> processor :  6		 physical id : 2	 core id : 8
> processor :  7		 physical id : 3	 core id : 8
> processor :  8		 physical id : 0	 core id : 2
> processor :  9		 physical id : 1	 core id : 2
> processor : 10		 physical id : 2	 core id : 2
> processor : 11		 physical id : 3	 core id : 2
> processor : 12		 physical id : 0	 core id : 10
> processor : 13		 physical id : 1	 core id : 10
> processor : 14		 physical id : 2	 core id : 10
> processor : 15		 physical id : 3	 core id : 10
>
> [....]
>
> So those core id numbers can be anything as long as the cpumasks used by
> the scheduler are correct.

Ok. Sure, it doesn't need be contiguous. But at least the cpu_core_id should 
represent an ID that make some sense since it is used in the 
arch/x86/kernel/smpboot.c: match_smt() and some other places. So, if it's 
invalid for the downcore configuration (i.e. duplicated where it should not be), 
we should at least clean this up.

>> This is due to the cpu_core_id fixup in amd_get_topology() below:
>>
>>         /* fixup multi-node processor information */
>>         if (nodes_per_socket > 1) {
>>                 u32 cus_per_node;
>>
>>                 set_cpu_cap(c, X86_FEATURE_AMD_DCM);
>>                 cus_per_node = c->x86_max_cores / nodes_per_socket;
>>
>>                 /* core id has to be in the [0 .. cores_per_node - 1] range */
>>                 c->cpu_core_id %= cus_per_node;
>>         }
>
> AFAICT, Andreas did this for MC at the time:
>
> 4a376ec3a259 ("x86: Fix CPU llc_shared_map information for AMD Magny-Cours")
>
> but I don't think we need to care about core_ids fitting into the node
> range anymore. For the above reason - topology doesn't use core ids.

Agree to the point that it does not need to be fitting into the node range.

> So you can just as well let ->cpu_core_id be derived from the
> ->initial_apicid as it is being done now in amd_detect_cmp().

Actually, for family17h, this is from the CPUID_Fn8000001E_EBX[CoreId]. But I 
get your point.

> In order not to cause any more confusion, you can limit the above fixup
> to anything below F17h so that we don't upset existing users and add a
> big fat comment as to why we're doing this. But if it is only a silly
> numbering thing, I don't see the need for doing that jumping through
> hoops.
>

I will update the patch to only limit the fixup to pre-family17h.

Thanks,
Suravee