[PATCH] arm64: topology: Fix handling of multi-level cluster MPIDR-based detection

[Ganapatrao Kulkarni]

Hi Mark,

On Tue, Aug 19, 2014 at 9:02 PM, Mark Brown <broonie at kernel.org> wrote:
> From: Mark Brown <broonie at linaro.org>
>
> The only requirement the scheduler has on cluster IDs is that they must
> be unique.  When enumerating the topology based on MPIDR information the
> kernel currently generates cluster IDs by using the first level of
> affinity above the core ID (either level one or two depending on if the
> core has multiple threads) however the ARMv8 architecture allows for up
> to three levels of affinity.  This means that an ARMv8 system may
> contain cores which have MPIDRs identical other than affinity level
> three which with current code will cause us to report multiple cores
> with the same identification to the scheduler in violation of its
> uniqueness requirement.
>
> Ensure that we do not violate the scheduler requirements on systems that
> uses all the affinity levels by incorporating both affinity levels two
> and three into the cluser ID when the cores are not threaded.
>
> While no currently known hardware uses multi-level clusters it is better
> to program defensively, this will help ease bringup of systems that have
> them and will ensure that things like distribution install media do not
> need to be respun to replace kernels in order to deploy such systems.
> In the worst case the system will work but perform suboptimally until a
> kernel modified to handle the new topology better is installed, in the
> best case this will be an adequate description of such topologies for
> the scheduler to perform well.
>
> Signed-off-by: Mark Brown <broonie at linaro.org>
> ---
>  arch/arm64/kernel/topology.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/arch/arm64/kernel/topology.c b/arch/arm64/kernel/topology.c
> index b6ee26b..5752c1b 100644
> --- a/arch/arm64/kernel/topology.c
> +++ b/arch/arm64/kernel/topology.c
> @@ -255,7 +255,8 @@ void store_cpu_topology(unsigned int cpuid)
>                 /* Multiprocessor system : Multi-threads per core */
>                 cpuid_topo->thread_id  = MPIDR_AFFINITY_LEVEL(mpidr, 0);
>                 cpuid_topo->core_id    = MPIDR_AFFINITY_LEVEL(mpidr, 1);
> -               cpuid_topo->cluster_id = MPIDR_AFFINITY_LEVEL(mpidr, 2);
> +               cpuid_topo->cluster_id = MPIDR_AFFINITY_LEVEL(mpidr, 2) |
> +                                        MPIDR_AFFINITY_LEVEL(mpidr, 3) << 8;
>         } else {
>                 /* Multiprocessor system : Single-thread per core */
>                 cpuid_topo->thread_id  = -1;
Can you please extend this for non-SMT case also?
> --
> 2.1.0.rc1
>

thanks
ganapat