Re: [PATCH] arch_topology: Support SMT control on arm64

[Yicong Yang]

On 2023/9/21 1:08, Dietmar Eggemann wrote:
> On 19/09/2023 14:33, Yicong Yang wrote:
>> From: Yicong Yang <yangyicong@xxxxxxxxxxxxx>
>>
>> The core CPU control framework supports runtime SMT control which
>> is not yet supported on arm64. Besides the general vulnerabilities
>> concerns we want this runtime control on our arm64 server for:
>>
>> - better single CPU performance in some cases
>> - saving overall power consumption
>>
>> This patch implements it in the following aspects:
>>
>> - implement the callbacks of the core
> 
> I see only 1 function here: topology_is_primary_thread() ?

Yes. Before 6.6-rc1 there's also one function topology_smt_supported() required,
I forgot to update the comment after rebase on 6.6-rc1.

> 
>> - update the SMT status after the topology enumerated on arm64
> 
> That's the call init_cpu_topology()
>                  topology_smt_set_num_threads()
>                   cpu_smt_set_num_threads()
> 

Yes.

>> - select HOTPLUG_SMT for arm64
>>
>> For disabling SMT we'll offline all the secondary threads and
> 
> `disabling SMT` means here setting cpu_smt_control=CPU_SMT_DISABLED ?
> 

Yes. The SMT control provides user interface like
`/sys/devices/system/cpu/smt/control` or cmdline option `nosmt=[force]`,
which will update cpu_smt_control = CPU_SMT_DISABLED.

>> only leave the primary thread. Since we don't have restriction
>> for primary thread selection, the first thread is chosen as the
>> primary thread in this implementation.
>>
>> Tests has been done on our ACPI based arm64 server and on
>> ACPI/OF based QEMU VMs.
>>
>> Signed-off-by: Yicong Yang <yangyicong@xxxxxxxxxxxxx>
>> ---
>>  arch/arm64/Kconfig            |  1 +
>>  drivers/base/arch_topology.c  | 63 +++++++++++++++++++++++++++++++++++
>>  include/linux/arch_topology.h | 11 ++++++
>>  3 files changed, 75 insertions(+)
>>
>> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
>> index b10515c0200b..531a71c7f499 100644
>> --- a/arch/arm64/Kconfig
>> +++ b/arch/arm64/Kconfig
>> @@ -233,6 +233,7 @@ config ARM64
>>  	select HAVE_KRETPROBES
>>  	select HAVE_GENERIC_VDSO
>>  	select HOTPLUG_CORE_SYNC_DEAD if HOTPLUG_CPU
>> +	select HOTPLUG_SMT if SMP
>>  	select IRQ_DOMAIN
>>  	select IRQ_FORCED_THREADING
>>  	select KASAN_VMALLOC if KASAN
>> diff --git a/drivers/base/arch_topology.c b/drivers/base/arch_topology.c
>> index b741b5ba82bd..75a693834fff 100644
>> --- a/drivers/base/arch_topology.c
>> +++ b/drivers/base/arch_topology.c
>> @@ -729,6 +729,63 @@ const struct cpumask *cpu_clustergroup_mask(int cpu)
>>  	return &cpu_topology[cpu].cluster_sibling;
>>  }
>>  
>> +#ifdef CONFIG_HOTPLUG_SMT
>> +static int topology_smt_num_threads = 1;
>> +
>> +void __init topology_smt_set_num_threads(void)
>> +{
>> +	int cpu, sibling, threads;
>> +
>> +	/*
>> +	 * Walk all the CPUs to find the largest thread number, in case we're
>> +	 * on a heterogeneous platform with only part of the CPU cores support
>> +	 * SMT.
>> +	 *
>> +	 * Get the thread number by checking the CPUs with same core id
>> +	 * rather than checking the topology_sibling_cpumask(), since the
>> +	 * sibling mask will not cover all the CPUs if there's CPU offline.
>> +	 */
>> +	for_each_possible_cpu(cpu) {
>> +		threads = 1;
>> +
>> +		/* Invalid thread id, this CPU is not in a SMT core */
>> +		if (cpu_topology[cpu].thread_id == -1)
>> +			continue;
>> +
>> +		for_each_possible_cpu(sibling) {
>> +			if (sibling == cpu || cpu_topology[sibling].thread_id == -1)
>> +				continue;
>> +
>> +			if (cpu_topology[cpu].core_id == cpu_topology[sibling].core_id)
>> +				threads++;
>> +		}
>> +
>> +		if (threads > topology_smt_num_threads)
>> +			topology_smt_num_threads = threads;
>> +	}
>> +
>> +	/*
>> +	 * We don't support CONFIG_SMT_NUM_THREADS_DYNAMIC so make the
>> +	 * max_threads == num_threads.
>> +	 */
>> +	cpu_smt_set_num_threads(topology_smt_num_threads, topology_smt_num_threads);
>> +}
>> +
>> +/*
>> + * On SMT Hotplug the primary thread of the SMT won't be disabled. For x86 they
>> + * seem to have a primary thread for special purpose. For other arthitectures
>> + * like arm64 there's no such restriction for a primary thread, so make the
>> + * first thread in the SMT as the primary thread.
>> + */
>> +bool topology_is_primary_thread(unsigned int cpu)
>> +{
>> +	if (cpu == cpumask_first(topology_sibling_cpumask(cpu)))
>> +		return true;
>> +
>> +	return false;
>> +}
>> +#endif
>> +
>>  void update_siblings_masks(unsigned int cpuid)
>>  {
>>  	struct cpu_topology *cpu_topo, *cpuid_topo = &cpu_topology[cpuid];
>> @@ -841,6 +898,12 @@ void __init init_cpu_topology(void)
>>  		reset_cpu_topology();
>>  	}
>>  
>> +	/*
>> +	 * By this stage we get to know whether we support SMT or not, update
>> +	 * the information for the core.
>> +	 */
>> +	topology_smt_set_num_threads();
>> +
> 
> So this would be the diff between x86 and arm64:
> 
> start_kernel() [init/main.c]
> 
>  arch_cpu_finalize_init() [arch/x86/kernel/cpu/common.c] <- x86
> 
>   identify_boot_cpu() [arch/x86/kernel/cpu/common.c]
> 
>    detect_ht() [arch/x86/kernel/cpu/common.c]
> 
>     detect_ht_early() [arch/x86/kernel/cpu/common.c]
> 
>   cpu_smt_set_num_threads(smp_num_siblings, smp_num_siblings) <- (1)
>    
> 
>  arch_call_rest_init() [init/main.c] <- arm64
> 
>   rest_init() [init/main.c]
> 
>    kernel_init() [init/main.c]
> 
>     kernel_init_freeable() [init/main.c]
> 
>      smp_prepare_cpus() [arch/arm64/kernel/smp.c]
> 
>       init_cpu_topology() [drivers/base/arch_topology.c]
> 
>        topology_smt_set_num_threads()
> 
>         cpu_smt_set_num_threads(topology_smt_num_threads, topology_smt_num_threads) <- (1) 
> 
> [...]
> 

Yes we have some differences. On arm64 the SMT information is retrieved from firmware
(ACPI/OF, parse_{acpi, dt}_topology()). So we need to update the SMT information
to the core after parsing the ACPI/OF in init_cpu_topology().

> Did some rough testing with your patch on an SMT4 Arm64 server with 256
> CPUs:
> 
> (1) CPU hp out all secondaries from the thread_siblings masks
> 
> for i in {32..255}; do echo 0 > /sys/devices/system/cpu/cpu$i/online; done
> 
> (2) Check thread_siblings cpumasks
> 
> cat /sys/devices/system/cpu/cpu*/topology/thread_siblings
> 00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000001
> 00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000002
> 00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000400
> 00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000800
> 00000000,00000000,00000000,00000000,00000000,00000000,00000000,00001000
> 00000000,00000000,00000000,00000000,00000000,00000000,00000000,00002000
> 00000000,00000000,00000000,00000000,00000000,00000000,00000000,00004000
> 00000000,00000000,00000000,00000000,00000000,00000000,00000000,00008000
> 00000000,00000000,00000000,00000000,00000000,00000000,00000000,00010000
> 00000000,00000000,00000000,00000000,00000000,00000000,00000000,00020000
> 00000000,00000000,00000000,00000000,00000000,00000000,00000000,00040000
> 00000000,00000000,00000000,00000000,00000000,00000000,00000000,00080000
> 00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000004
> 00000000,00000000,00000000,00000000,00000000,00000000,00000000,00100000
> 00000000,00000000,00000000,00000000,00000000,00000000,00000000,00200000
> 00000000,00000000,00000000,00000000,00000000,00000000,00000000,00400000
> 00000000,00000000,00000000,00000000,00000000,00000000,00000000,00800000
> 00000000,00000000,00000000,00000000,00000000,00000000,00000000,01000000
> 00000000,00000000,00000000,00000000,00000000,00000000,00000000,02000000
> 00000000,00000000,00000000,00000000,00000000,00000000,00000000,04000000
> 00000000,00000000,00000000,00000000,00000000,00000000,00000000,08000000
> 00000000,00000000,00000000,00000000,00000000,00000000,00000000,10000000
> 00000000,00000000,00000000,00000000,00000000,00000000,00000000,20000000
> 00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000008
> 00000000,00000000,00000000,00000000,00000000,00000000,00000000,40000000
> 00000000,00000000,00000000,00000000,00000000,00000000,00000000,80000000
> 00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000010
> 00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000020
> 00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000040
> 00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000080
> 00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000100
> 00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000200
> 
> (3) CPU hp out and out CPU31
> 
> echo 0 > /sys/devices/system/cpu/cpu31/online
> echo 1 > /sys/devices/system/cpu/cpu31/online
> 
> cpu_smt_control is still CPU_SMT_ENABLED in cpu_smt_allowed() so
> topology_is_primary_thread() isn't called? 
> 

If you manually disable SMT by offline each CPUs the cpu_smt_control will
not be updated. It'll updated when using the interface like
`/sys/devices/system/cpu/smt/control` or cmdline. By these means,
the framework will use topology_is_primary_thread() to decide which CPU
in the SMT will keep online:

// e.g. echo off > /sys/devices/system/cpu/smt/control
[ kernel/cpu.c ]
control_store()
  __store_smt_control()
    cpuhp_smt_disable()
      for_each_online_cpu(cpu)
        if (topology_is_primary_thread(cpu))
          continue;            <---------- will skip the primary thread
      [...]
      cpu_smt_control = CPU_SMT_DISABLED;

topology_is_primary_thread() checking only applies to the SMT control but
not to the CPU offline.

Thanks,
Yicong