Re: [PATCH 1/1] x86/topology: fix erroneous smp_num_siblings on Intel Hybrid platform

[Dave Hansen]

On 2/17/23 08:37, Zhang Rui wrote:
> The SMT siblings value returned by CPUID.1F SMT level EBX differs
> among CPUs on Intel Hybrid platforms like AlderLake and MeteorLake.
> It returns 2 for Pcore CPUs which have SMT siblings and returns 1 for
> Ecore CPUs which do not have SMT siblings.
> 
> Today, the CPU boot code sets the global variable smp_num_siblings when
> every CPU thread is brought up. The last thread to boot will overwrite
> it with the number of siblings of *that* thread. That last thread to
> boot will "win". If the thread is a Pcore, smp_num_siblings == 2.  If it
> is an Ecore, smp_num_siblings == 1.
> 
> smp_num_siblings describes if the *system* supports SMT.  It should
> specify the maximum number of SMT threads among all cores.

I was with you until here, but I'm having a hard time parsing this:

> On AlderLake-P/S platforms, it does not cause any functional issues so
> far.
> But on MeteorLake-P platform, when probing an Ecore CPU,
> a). smp_num_siblings varies like AlderLake and it is set to 1 for Ecore.
> b). x86_max_cores is totally broken and it is set to 1 for the boot cpu.
> Altogether, these two issues make the system being treated as an UP
> system in set_cpu_sibling_map() when probing Ecore CPUs, and the Ecore
> CPUs are not updated in any cpu sibling maps erroneously.

Let's try and focus this changelog on the problem at hand which is a
broken smp_num_siblings on MeterorLake.  Right?

> Below shows part of the CPU topology information before and after the
> fix, for both Pcore and Ecore CPU (cpu0 is Pcore, cpu 12 is Ecore).
> ...
> -/sys/devices/system/cpu/cpu0/topology/package_cpus:000fff
> -/sys/devices/system/cpu/cpu0/topology/package_cpus_list:0-11
> +/sys/devices/system/cpu/cpu0/topology/package_cpus:3fffff
> +/sys/devices/system/cpu/cpu0/topology/package_cpus_list:0-21
> ...
> -/sys/devices/system/cpu/cpu12/topology/package_cpus:001000
> -/sys/devices/system/cpu/cpu12/topology/package_cpus_list:12
> +/sys/devices/system/cpu/cpu12/topology/package_cpus:3fffff
> +/sys/devices/system/cpu/cpu12/topology/package_cpus_list:0-21
> 
> And this also breaks userspace tools like lscpu
> -Core(s) per socket:  1
> -Socket(s):           11
> +Core(s) per socket:  16
> +Socket(s):           1

Heh, yeah, 11 sockets is a tiny bug.

> To fix the first issue, ensure that smp_num_siblings represents the
> system-wide maximum number of siblings by always increasing its value.
> Never allow it to decrease.
> 
> Note that this fix is sufficient to make set_cpu_sibling_map() work
> correctly. And how to fix the bogus cpuinfo_x86.x86_max_cores will be
> addressed separately.

Having this note here is probably OK.  But, I'm not sure even mentioning
x86_max_cores is worth it.  Doesn't it just add confusion?

> diff --git a/arch/x86/kernel/cpu/topology.c b/arch/x86/kernel/cpu/topology.c
> index 5e868b62a7c4..0270925fe013 100644
> --- a/arch/x86/kernel/cpu/topology.c
> +++ b/arch/x86/kernel/cpu/topology.c
> @@ -79,7 +79,7 @@ int detect_extended_topology_early(struct cpuinfo_x86 *c)
>  	 * initial apic id, which also represents 32-bit extended x2apic id.
>  	 */
>  	c->initial_apicid = edx;
> -	smp_num_siblings = LEVEL_MAX_SIBLINGS(ebx);
> +	smp_num_siblings = max_t(int, smp_num_siblings, LEVEL_MAX_SIBLINGS(ebx));
>  #endif
>  	return 0;
>  }
> @@ -109,7 +109,8 @@ int detect_extended_topology(struct cpuinfo_x86 *c)
>  	 */
>  	cpuid_count(leaf, SMT_LEVEL, &eax, &ebx, &ecx, &edx);
>  	c->initial_apicid = edx;
> -	core_level_siblings = smp_num_siblings = LEVEL_MAX_SIBLINGS(ebx);
> +	core_level_siblings = LEVEL_MAX_SIBLINGS(ebx);
> +	smp_num_siblings = max_t(int, smp_num_siblings, LEVEL_MAX_SIBLINGS(ebx));
>  	core_plus_mask_width = ht_mask_width = BITS_SHIFT_NEXT_LEVEL(eax);
>  	die_level_siblings = LEVEL_MAX_SIBLINGS(ebx);
>  	pkg_mask_width = die_plus_mask_width = BITS_SHIFT_NEXT_LEVEL(eax);

The fix seems simple enough.