Re: [PATCH 8/9] powerpc: Add HOTPLUG_SMT support

[Laurent Dufour]

On 24/05/2023 17:56:29, Michael Ellerman wrote:
> Add support for HOTPLUG_SMT, which enables the generic sysfs SMT support
> files in /sys/devices/system/cpu/smt, as well as the "nosmt" boot
> parameter.

Hi Michael,

It seems that there is now a conflict between with the PPC 'smt-enabled'
boot option.

Booting the patched kernel with 'smt-enabled=4', later, change to the SMT
level (for instance to 6) done through /sys/devices/system/cpu/smt/control
are not applied. Nothing happens.
Based on my early debug, I think the reasons is that cpu_smt_num_threads=8
when entering __store_smt_control(). But I need to dig further.

BTW, should the 'smt-enabled' PPC specific option remain?

Cheers,
Laurent.

> Implement the recently added hooks to allow partial SMT states, allow
> any number of threads per core.
> 
> Tie the config symbol to HOTPLUG_CPU, which enables it on the major
> platforms that support SMT. If there are other platforms that want the
> SMT support that can be tweaked in future.
> 
> Signed-off-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
> ---
>  arch/powerpc/Kconfig                |  1 +
>  arch/powerpc/include/asm/topology.h | 25 +++++++++++++++++++++++++
>  arch/powerpc/kernel/smp.c           |  3 +++
>  3 files changed, 29 insertions(+)
> 
> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
> index 539d1f03ff42..5cf87ca10a9c 100644
> --- a/arch/powerpc/Kconfig
> +++ b/arch/powerpc/Kconfig
> @@ -273,6 +273,7 @@ config PPC
>  	select HAVE_SYSCALL_TRACEPOINTS
>  	select HAVE_VIRT_CPU_ACCOUNTING
>  	select HAVE_VIRT_CPU_ACCOUNTING_GEN
> +	select HOTPLUG_SMT			if HOTPLUG_CPU
>  	select HUGETLB_PAGE_SIZE_VARIABLE	if PPC_BOOK3S_64 && HUGETLB_PAGE
>  	select IOMMU_HELPER			if PPC64
>  	select IRQ_DOMAIN
> diff --git a/arch/powerpc/include/asm/topology.h b/arch/powerpc/include/asm/topology.h
> index 8a4d4f4d9749..1e9117a22d14 100644
> --- a/arch/powerpc/include/asm/topology.h
> +++ b/arch/powerpc/include/asm/topology.h
> @@ -143,5 +143,30 @@ static inline int cpu_to_coregroup_id(int cpu)
>  #endif
>  #endif
>  
> +#ifdef CONFIG_HOTPLUG_SMT
> +#include <linux/cpu_smt.h>
> +#include <asm/cputhreads.h>
> +
> +static inline bool topology_smt_supported(void)
> +{
> +	return threads_per_core > 1;
> +}
> +
> +static inline bool topology_smt_threads_supported(unsigned int num_threads)
> +{
> +	return num_threads <= threads_per_core;
> +}
> +
> +static inline bool topology_is_primary_thread(unsigned int cpu)
> +{
> +	return cpu == cpu_first_thread_sibling(cpu);
> +}
> +
> +static inline bool topology_smt_thread_allowed(unsigned int cpu)
> +{
> +	return cpu_thread_in_core(cpu) < cpu_smt_num_threads;
> +}
> +#endif
> +
>  #endif /* __KERNEL__ */
>  #endif	/* _ASM_POWERPC_TOPOLOGY_H */
> diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
> index 265801a3e94c..eed20b9253b7 100644
> --- a/arch/powerpc/kernel/smp.c
> +++ b/arch/powerpc/kernel/smp.c
> @@ -1154,6 +1154,9 @@ void __init smp_prepare_cpus(unsigned int max_cpus)
>  
>  	if (smp_ops && smp_ops->probe)
>  		smp_ops->probe();
> +
> +	// Initalise the generic SMT topology support
> +	cpu_smt_check_topology(threads_per_core);
>  }
>  
>  void smp_prepare_boot_cpu(void)