linux-kernel - Re: [PATCH 1/2] pseries/smp: export the smt level in the SYS FS.

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Date:   Thu, 13 Apr 2023 17:38:51 +0200
From:   Laurent Dufour <ldufour@...ux.ibm.com>
To:     Michael Ellerman <mpe@...erman.id.au>, npiggin@...il.com,
        christophe.leroy@...roup.eu
Cc:     msuchanek@...e.de, nathanl@...ux.ibm.com,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Subject: Re: [PATCH 1/2] pseries/smp: export the smt level in the SYS FS.

On 13/04/2023 15:37:59, Michael Ellerman wrote:
> Hi Laurent,
> 
> Laurent Dufour <ldufour@...ux.ibm.com> writes:
>> There is no SMT level recorded in the kernel neither in user space.
>> Indeed there is no real constraint about that and mixed SMT levels are
>> allowed and system is working fine this way.
>>
>> However when new CPU are added, the kernel is onlining all the threads
>> which is leading to mixed SMT levels and confuse end user a bit.
>>
>> To prevent this exports a SMT level from the kernel so user space
>> application like the energy daemon, could read it to adjust their settings.
>> There is no action unless recording the value when a SMT value is written
>> into the new sysfs entry. User space applications like ppc64_cpu should
>> update the sysfs when changing the SMT level to keep the system consistent.
>>
>> Suggested-by: Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
>> Signed-off-by: Laurent Dufour <ldufour@...ux.ibm.com>
>> ---
>>  arch/powerpc/platforms/pseries/pseries.h |  3 ++
>>  arch/powerpc/platforms/pseries/smp.c     | 39 ++++++++++++++++++++++++
>>  2 files changed, 42 insertions(+)
> 
> There is a generic sysfs interface for smt in /sys/devices/system/cpu/smt
> 
> I think we should be enabling that on powerpc and then adapting it to
> our needs, rather than adding a pseries specific file.

Thanks Michael, I was not aware of this sysfs interface.

> Currently the generic code is only aware of SMT on/off, so it would need
> to be taught about SMT4 and 8 at least.

Do you think we should limit our support to SMT4 and SMT8 only?

> There are already hooks in the generic code to check the SMT level when
> bringing CPUs up, see cpu_smt_allowed(), they may work for the pseries
> hotplug case too, though maybe we need some additional logic.
> 
> Wiring up the basic support is pretty straight forward, something like
> the diff below.

I'll look into how to wire this up.
Thanks a lot!

> cheers
> 
> 
> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
> index 0f123f1f62a1..a48576f1c579 100644
> --- a/arch/powerpc/Kconfig
> +++ b/arch/powerpc/Kconfig
> @@ -260,6 +260,7 @@ 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..bd23ba716d23 100644
> --- a/arch/powerpc/include/asm/topology.h
> +++ b/arch/powerpc/include/asm/topology.h
> @@ -143,5 +143,8 @@ static inline int cpu_to_coregroup_id(int cpu)
>  #endif
>  #endif
> 
> +bool topology_is_primary_thread(unsigned int cpu);
> +bool topology_smt_supported(void);
> +
>  #endif /* __KERNEL__ */
>  #endif	/* _ASM_POWERPC_TOPOLOGY_H */
> diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
> index 265801a3e94c..8619609809d5 100644
> --- a/arch/powerpc/kernel/smp.c
> +++ b/arch/powerpc/kernel/smp.c
> @@ -1769,4 +1769,20 @@ void __noreturn arch_cpu_idle_dead(void)
>  	start_secondary_resume();
>  }
> 
> +/**
> + * topology_is_primary_thread - Check whether CPU is the primary SMT thread
> + * @cpu:	CPU to check
> + */
> +bool topology_is_primary_thread(unsigned int cpu)
> +{
> +	return cpu == cpu_first_thread_sibling(cpu);
> +}
> +
> +/**
> + * topology_smt_supported - Check whether SMT is supported by the CPUs
> + */
> +bool topology_smt_supported(void)
> +{
> +	return threads_per_core > 1;
> +}
>  #endif