[RFC] drm/amdkfd: Use logical cpu id for building vcrat

[Kuehling, Felix]

On x86 we use the apicid to associate caches with CPU cores. See the Thunk code in libhsakmt/src/topology.c (static void find_cpu_cache_siblings()). If we used a different way to identify CPU cores, I think that would break. This code in the Thunk is x86-specific as it uses the CPUID instruction. We don't have equivalent code for ARM. So for ARM it doesn't really matter much, how you count your CPU cores in the CRAT table.

I think eventually we want to get rid of that fragile CPUID code in the Thunk and get the cache information in kernel mode and report it to user mode through the KFD topology sysfs filesystem. Then we could also move away from using apicids as CPU IDs on x86.

It's not a high priority as I'm not aware of any applications that actually make use of the cache information.

Regards,
  Felix

On 2019-04-15 22:39, Hillf Danton wrote:
Hi folks

In commit d1c234e2cd, arm64 is granted to build kfd. Currently, it is physical
cpu id that is used for building the x86_64 vcrat, but logical cpu id is used
instead for arm64, though the function name requires apicid. Can we use the
physical id for both arches if it really has an up-hand over the logical one,
as the following tiny diff represents?

--- linux-5.1-rc4/drivers/gpu/drm/amd/amdkfd/kfd_topology.c    2019-04-16 07:55:56.611685400 +0800
+++ b/drivers/gpu/drm/amd/amdkfd/kfd_topology.c 2019-04-16 09:16:50.506126600 +0800
@@ -1405,11 +1405,7 @@ static int kfd_cpumask_to_apic_id(const
       first_cpu_of_numa_node = cpumask_first(cpumask);
       if (first_cpu_of_numa_node >= nr_cpu_ids)
             return -1;
-#ifdef CONFIG_X86_64
-     return cpu_data(first_cpu_of_numa_node).apicid;
-#else
-     return first_cpu_of_numa_node;
-#endif
+    return cpu_physical_id(first_cpu_of_numa_node);
}
/* kfd_numa_node_to_apic_id - Returns the APIC ID of the first logical processor
--


Or is logical cpu id enough to do the work, with some cosmetic applied to the
function names(not included in the following simple diff yet)?

thanks
Hillf


--- linux-5.1-rc4/drivers/gpu/drm/amd/amdkfd/kfd_topology.c    2019-04-16 07:55:56.611685400 +0800
+++ b/drivers/gpu/drm/amd/amdkfd/kfd_topology.c 2019-04-16 09:18:24.546578400 +0800
@@ -1405,11 +1405,7 @@ static int kfd_cpumask_to_apic_id(const
       first_cpu_of_numa_node = cpumask_first(cpumask);
       if (first_cpu_of_numa_node >= nr_cpu_ids)
             return -1;
-#ifdef CONFIG_X86_64
-     return cpu_data(first_cpu_of_numa_node).apicid;
-#else
       return first_cpu_of_numa_node;
-#endif
}
/* kfd_numa_node_to_apic_id - Returns the APIC ID of the first logical processor
--

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