Re: [PATCH v3 8/9] powerpc: Add HOTPLUG_SMT support

[Laurent Dufour <ldufour@linux.ibm.com>]

Hi Michael,

Le 29/06/2023 à 16:31, Laurent Dufour a écrit :
> From: Michael Ellerman <mpe@ellerman.id.au>
> 
> 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.
> 
> 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@ellerman.id.au>
> [ldufour: pass current SMT level to cpu_smt_set_num_threads]
> [ldufour: remove topology_smt_supported]
> [ldufour: remove topology_smt_threads_supported]
> [ldufour: select CONFIG_SMT_NUM_THREADS_DYNAMIC]
> [ldufour: update kernel-parameters.txt]
> Signed-off-by: Laurent Dufour <ldufour@linux.ibm.com>
> ---
>   Documentation/admin-guide/kernel-parameters.txt |  4 ++--
>   arch/powerpc/Kconfig                            |  2 ++
>   arch/powerpc/include/asm/topology.h             | 15 +++++++++++++++
>   arch/powerpc/kernel/smp.c                       |  8 +++++++-
>   4 files changed, 26 insertions(+), 3 deletions(-)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 9e5bab29685f..5efb6c73a928 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -3838,10 +3838,10 @@
>   	nosmp		[SMP] Tells an SMP kernel to act as a UP kernel,
>   			and disable the IO APIC.  legacy for "maxcpus=0".
>   
> -	nosmt		[KNL,S390] Disable symmetric multithreading (SMT).
> +	nosmt		[KNL,S390,PPC] Disable symmetric multithreading (SMT).
>   			Equivalent to smt=1.
>   
> -			[KNL,X86] Disable symmetric multithreading (SMT).
> +			[KNL,X86,PPC] Disable symmetric multithreading (SMT).
>   			nosmt=force: Force disable SMT, cannot be undone
>   				     via the sysfs control file.
>   
> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
> index 8b955bc7b59f..bacabc3d7f0c 100644
> --- a/arch/powerpc/Kconfig
> +++ b/arch/powerpc/Kconfig
> @@ -273,6 +273,8 @@ config PPC
>   	select HAVE_SYSCALL_TRACEPOINTS
>   	select HAVE_VIRT_CPU_ACCOUNTING
>   	select HAVE_VIRT_CPU_ACCOUNTING_GEN
> +	select HOTPLUG_SMT			if HOTPLUG_CPU
> +	select SMT_NUM_THREADS_DYNAMIC

I missed that this list should be kept sorted alphabetically.
Could you fix that when applying the series, or should I send a new 
version ?

Thanks,
Laurent.

>   	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..f4e6f2dd04b7 100644
> --- a/arch/powerpc/include/asm/topology.h
> +++ b/arch/powerpc/include/asm/topology.h
> @@ -143,5 +143,20 @@ 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_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 406e6d0ffae3..eb539325dff8 100644
> --- a/arch/powerpc/kernel/smp.c
> +++ b/arch/powerpc/kernel/smp.c
> @@ -1087,7 +1087,7 @@ static int __init init_big_cores(void)
>   
>   void __init smp_prepare_cpus(unsigned int max_cpus)
>   {
> -	unsigned int cpu;
> +	unsigned int cpu, num_threads;
>   
>   	DBG("smp_prepare_cpus\n");
>   
> @@ -1154,6 +1154,12 @@ void __init smp_prepare_cpus(unsigned int max_cpus)
>   
>   	if (smp_ops && smp_ops->probe)
>   		smp_ops->probe();
> +
> +	// Initalise the generic SMT topology support
> +	num_threads = 1;
> +	if (smt_enabled_at_boot)
> +		num_threads = smt_enabled_at_boot;
> +	cpu_smt_set_num_threads(num_threads, threads_per_core);
>   }
>   
>   void smp_prepare_boot_cpu(void)