Subject: Re: [PATCH v10 1/4] cpu/SMT: Provide a default topology_is_primary_thread()

From: Jonathan Cameron <[email-masked]>
Date: Mon, 23 Dec 2024 16:34:48 +0000

On Fri, 20 Dec 2024 15:59:27 +0800
Yicong Yang <[email-masked]> wrote:

> On 2024/12/20 15:53, Yicong Yang wrote:
> > From: Yicong Yang <[email-masked]>
> > 
> > Currently if architectures want to support HOTPLUG_SMT they need to
> > provide a topology_is_primary_thread() telling the framework which
> > thread in the SMT cannot offline. However arm64 doesn't have a
> > restriction on which thread in the SMT cannot offline, a simplest
> > choice is that just make 1st thread as the "primary" thread. So
> > just make this as the default implementation in the framework and
> > let architectures like x86 that have special primary thread to
> > override this function (which they've already done).
> > 
> > There's no need to provide a stub function if !CONFIG_SMP or
> > !CONFIG_HOTPLUG_SMP. In such case the testing CPU is already
> > the 1st CPU in the SMT so it's always the primary thread.
> > 
> > Signed-off-by: Yicong Yang <[email-masked]>
> > ---
> > As questioned in v9 [1] whether this works on architectures not using
> > CONFIG_GENERIC_ARCH_TOPOLOGY, hacked on LoongArch VM and this also works.
> > Architectures should use this on their own situation.
> > [1] https://lore.kernel.org/linux-arm-kernel/[email-masked]/
> > 
> > [root@localhost smt]# uname -m
> > loongarch64
> > [root@localhost smt]# pwd
> > /sys/devices/system/cpu/smt
> > [root@localhost smt]# cat ../possible 
> > 0-3
> > [root@localhost smt]# cat ../online 
> > 0-3
> > [root@localhost smt]# cat control 
> > on
> > [root@localhost smt]# echo off > control 
> > [root@localhost smt]# cat control 
> > off
> > [root@localhost smt]# cat ../online 
> > 0,2
> > [root@localhost smt]# echo on > control 
> > [root@localhost smt]# cat control 
> > on
> > [root@localhost smt]# cat ../online 
> > 0-3  
> 
> Tested with below code using the topology_is_primary_thread() introduced
> in this patch. Tested on an ACPI-based QEMU VM emulating SMT2.
Nice bit of testing.

Given it all seems fine. FWIW
Reviewed-by: Jonathan Cameron <[email-masked]>
(for original patch, not the longarch one!)

> 
> Subject: [PATCH] LoongArch: Support HOTPLUG_SMT on ACPI-based system
> 
> Support HOTPLUG_SMT on ACPI-based system using generic
> topology_is_primary_thread().
> 
> Signed-off-by: Yicong Yang <[email-masked]>
> ---
>  arch/loongarch/Kconfig       |  1 +
>  arch/loongarch/kernel/acpi.c | 26 ++++++++++++++++++++++++--
>  2 files changed, 25 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/loongarch/Kconfig b/arch/loongarch/Kconfig
> index dae3a9104ca6..bed1b0640b97 100644
> --- a/arch/loongarch/Kconfig
> +++ b/arch/loongarch/Kconfig
> @@ -172,6 +172,7 @@ config LOONGARCH
>  	select HAVE_SYSCALL_TRACEPOINTS
>  	select HAVE_TIF_NOHZ
>  	select HAVE_VIRT_CPU_ACCOUNTING_GEN if !SMP
> +	select HOTPLUG_SMT if HOTPLUG_CPU
>  	select IRQ_FORCED_THREADING
>  	select IRQ_LOONGARCH_CPU
>  	select LOCK_MM_AND_FIND_VMA
> diff --git a/arch/loongarch/kernel/acpi.c b/arch/loongarch/kernel/acpi.c
> index 382a09a7152c..e642b0de57e7 100644
> --- a/arch/loongarch/kernel/acpi.c
> +++ b/arch/loongarch/kernel/acpi.c
> @@ -15,9 +15,11 @@
>  #include <linux/memblock.h>
>  #include <linux/of_fdt.h>
>  #include <linux/serial_core.h>
> +#include <linux/xarray.h>
>  #include <asm/io.h>
>  #include <asm/numa.h>
>  #include <asm/loongson.h>
> +#include <linux/cpu_smt.h>
> 
>  int acpi_disabled;
>  EXPORT_SYMBOL(acpi_disabled);
> @@ -175,8 +177,12 @@ int pptt_enabled;
> 
>  int __init parse_acpi_topology(void)
>  {
> +	int thread_num, max_smt_thread_num = 1;
> +	struct xarray core_threads;
>  	int cpu, topology_id;
> +	void *entry;
> 
> +	xa_init(&core_threads);
>  	for_each_possible_cpu(cpu) {
>  		topology_id = find_acpi_cpu_topology(cpu, 0);
>  		if (topology_id < 0) {
> @@ -184,19 +190,35 @@ int __init parse_acpi_topology(void)
>  			return -ENOENT;
>  		}
> 
> -		if (acpi_pptt_cpu_is_thread(cpu) <= 0)
> +		if (acpi_pptt_cpu_is_thread(cpu) <= 0) {
>  			cpu_data[cpu].core = topology_id;
> -		else {
> +		} else {
>  			topology_id = find_acpi_cpu_topology(cpu, 1);
>  			if (topology_id < 0)
>  				return -ENOENT;
> 
>  			cpu_data[cpu].core = topology_id;
> +
> +			entry = xa_load(&core_threads, topology_id);
> +			if (!entry) {
> +				xa_store(&core_threads, topology_id,
> +					 xa_mk_value(1), GFP_KERNEL);
> +			} else {
> +				thread_num = xa_to_value(entry);
> +				thread_num++;
> +				xa_store(&core_threads, topology_id,
> +					 xa_mk_value(thread_num), GFP_KERNEL);
> +
> +				if (thread_num > max_smt_thread_num)
> +					max_smt_thread_num = thread_num;
> +			}
>  		}
>  	}
> 
>  	pptt_enabled = 1;
> 
> +	cpu_smt_set_num_threads(max_smt_thread_num, max_smt_thread_num);
> +	xa_destroy(&core_threads);
>  	return 0;
>  }
>