Re: [PATCH 2/3] x86/topology: Fix AMD core count

[Huang Rui]

On Fri, Mar 18, 2016 at 11:03:47PM +0800, Peter Zijlstra wrote:
> It turns out AMD gets x86_max_cores wrong when there are compute
> units.
> 
> The issue is that Linux assumes:
> 
> 	nr_logical_cpus = nr_cores * nr_siblings
> 
> But AMD reports its CU unit as 2 cores, but then sets num_smp_siblings
> to 2 as well.
> 
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Borislav Petkov <bp@xxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Andreas Herrmann <aherrmann@xxxxxxxx>
> Reported-by: Xiong Zhou <jencce.kernel@xxxxxxxxx>
> Fixes: 1f12e32f4cd5 ("x86/topology: Create logical package id")
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>

I tested this patch both on fam15h and fam16h platforms, x86_max_cores
can report correct compute unit numbers. And
/sys/devices/system/cpu/cpuX/topology/thread_siblings_list
/sys/devices/system/cpu/cpuX/topology/core_siblings_list
are also right. Thanks.

Tested-by: Huang Rui <ray.huang@xxxxxxx>

> Link: http://lkml.kernel.org/r/20160317095220.GO6344@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> ---
>  arch/x86/kernel/cpu/amd.c |    8 ++++----
>  arch/x86/kernel/smpboot.c |   11 ++++++-----
>  2 files changed, 10 insertions(+), 9 deletions(-)
> 
> --- a/arch/x86/kernel/cpu/amd.c
> +++ b/arch/x86/kernel/cpu/amd.c
> @@ -313,9 +313,9 @@ static void amd_get_topology(struct cpui
>  		node_id = ecx & 7;
>  
>  		/* get compute unit information */
> -		smp_num_siblings = ((ebx >> 8) & 3) + 1;
> +		cores_per_cu = smp_num_siblings = ((ebx >> 8) & 3) + 1;
> +		c->x86_max_cores /= smp_num_siblings;
>  		c->compute_unit_id = ebx & 0xff;
> -		cores_per_cu += ((ebx >> 8) & 3);
>  	} else if (cpu_has(c, X86_FEATURE_NODEID_MSR)) {
>  		u64 value;
>  
> @@ -331,8 +331,8 @@ static void amd_get_topology(struct cpui
>  		u32 cus_per_node;
>  
>  		set_cpu_cap(c, X86_FEATURE_AMD_DCM);
> -		cores_per_node = c->x86_max_cores / nodes_per_socket;
> -		cus_per_node = cores_per_node / cores_per_cu;
> +		cus_per_node = c->x86_max_cores / nodes_per_socket;
> +		cores_per_node = cus_per_node * cores_per_cu;
>  
>  		/* store NodeID, use llc_shared_map to store sibling info */
>  		per_cpu(cpu_llc_id, cpu) = node_id;
> 
>