Message-ID: <3947cb79-3199-4cd6-b784-51a245084581@arm.com>
Date: Thu, 29 Aug 2024 14:46:09 +0200
From: Pierre Gondois <pierre.gondois@....com>
To: Yicong Yang <yangyicong@...wei.com>
Cc: yangyicong@...ilicon.com, linuxppc-dev@...ts.ozlabs.org, bp@...en8.de,
 dave.hansen@...ux.intel.com, mingo@...hat.com,
 linux-arm-kernel@...ts.infradead.org, mpe@...erman.id.au,
 peterz@...radead.org, tglx@...utronix.de, sudeep.holla@....com,
 will@...nel.org, catalin.marinas@....com, x86@...nel.org,
 linux-kernel@...r.kernel.org, dietmar.eggemann@....com,
 gregkh@...uxfoundation.org, rafael@...nel.org, jonathan.cameron@...wei.com,
 prime.zeng@...ilicon.com, linuxarm@...wei.com, xuwei5@...wei.com,
 guohanjun@...wei.com
Subject: Re: [PATCH v5 3/4] arm64: topology: Support SMT control on ACPI based
 system

Hello Yicong,

On 8/29/24 09:40, Yicong Yang wrote:
> Hi Pierre,
> 
> On 2024/8/27 23:40, Pierre Gondois wrote:
>> Hello Yicong,
>> IIUC we are looking for the maximum number of threads a CPU can have
>> in the platform. But is is actually possible to have a platform with
>> CPUs not having the same number of threads ?
>>
> 
> I was thinking of the heterogenous platforms, for example part of the
> cores have SMT and others don't (I don't have any though, but there
> should be some such platforms for arm64).
> 
>> For instance kernel/cpu.c::cpu_smt_num_threads_valid() will check
>> that the number of threads is either 1 or cpu_smt_max_threads (as
>> CONFIG_SMT_NUM_THREADS_DYNAMIC is not enabled for arm64). However
>> a (hypothetical) platform with:
>> - CPU0: 2 threads
>> - CPU1: 4 threads
>> should not be able to set the number of threads to 2, as
>> 1 < 2 < cpu_smt_max_threads (cf. cpu_smt_num_threads_valid()).
>>
> 
> It's a bit more complex. If we enable/disable the SMT using on/off control
> then we won't have this problem. We'll online all the CPUs on enabling and
> offline CPUs which is not primary thread and don't care about the thread
> number of each core.
> 
> Control using thread number is introduced by CONFIG_SMT_NUM_THREADS_DYNAMIC
> and only enabled on powerpc. I think this interface is not enough to handle
> the hypothetical we assumed, since it assumes the homogenous platform that
> all the CPUs have the same thread number. But this should be fine for
> the platforms with SMT(with same thread number) and non-SMT cores, since it do
> indicates the real thread number of the SMT cores.
> 
>> So if there is an assumption that all the CPUs have the same number of
>> threads, it should be sufficient to count the number of threads for one
>> CPU right ?
>>
> 
> Naturally and conveniently I may think use of the threads number of CPU0 (boot
> cpu) in such a solution. But this will be wrong if CPU0 is non-SMT on a heterogenous
> platform :(
> 
>> In the other (unlikely) case (i.e. CPUs can have various number of threads),
>> I think we should either:
>> - use your method and check that all the CPUs have the same number of threads
>> - get the number of threads for one CPU and check that all the CPUs are
>>     identical using the ACPI_PPTT_ACPI_IDENTICAL tag
> 
> I think this won't be simpler since we still need to iterate all the CPUs to see
> if they have the same hetero_id (checked how we're using this ACPI tag in
> arm_acpi_register_pmu_device()). We could already know if they're identical by
> comparing the thread number and do the update if necessary.
> 
>> - have a per-cpu cpu_smt_max_threads value ?
> 
> This should be unnecessary in currently stage if there's no platforms
> with several kind cores have different thread number like in your assumption
> and enable CONFIG_SMT_NUM_THREADS_DYNAMIC on such platforms. Otherwise using
> a global cpu_smt_max_threads to enable the SMT control should be enough.
> Does it make sense?

Yes, I agree with all the things you said:
- the current smt/control interface cannot handle asymmetric SMT platforms
- I am not aware if such platform exist so far

I think it would still be good to check all the CPUs have the same number of
threads. I tried to enable the SMT_NUM_THREADS_DYNAMIC Kconfig with the
patch attached (and to check CPUs have the same number of threads). Feel free
to take what you like/ignore what is inside the attached patch, or let me know
if I should submit a part in a separate patchset,

----------------------------

     [RFC] arm64: topology: Enable CONFIG_SMT_NUM_THREADS_DYNAMIC
     
     - On arm64 ACPI systems, change the thread_id assignment to have
       increasing values starting from 0. This is already the case for DT
       based systems. Doing so allows to uniformly identify the n-th
       thread of a given CPU.
     - Check that all CPUs have the same number of threads (for DT/ACPI)
     - Enable CONFIG_SMT_NUM_THREADS_DYNAMIC
     
     On a Tx2, with 256 CPUs, threads siblings being 0,32,64,96
     for socket0 and 128 + (0,32,64,96) for socket1:
     $ cd /sys/devices/system/cpu/smt/
     $ cat ../online
     0-255
     $ echo 2 > control
     $ cat ../online
     0-63,128-191
     $ echo 3 > control
     $ cat ../online
     0-95,128-223
     $ echo on > control
     $ cat ../online
     0-255

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index bd3bc2f5e0ec..1d8521483065 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -239,6 +239,7 @@ config ARM64
         select HAVE_GENERIC_VDSO
         select HOTPLUG_CORE_SYNC_DEAD if HOTPLUG_CPU
         select HOTPLUG_SMT if (SMP && HOTPLUG_CPU)
+       select SMT_NUM_THREADS_DYNAMIC if HOTPLUG_SMT
         select IRQ_DOMAIN
         select IRQ_FORCED_THREADING
         select KASAN_VMALLOC if KASAN
diff --git a/arch/arm64/include/asm/topology.h b/arch/arm64/include/asm/topology.h
index 0f6ef432fb84..7dd211f81687 100644
--- a/arch/arm64/include/asm/topology.h
+++ b/arch/arm64/include/asm/topology.h
@@ -39,6 +39,14 @@ void update_freq_counters_refs(void);
  #define arch_scale_hw_pressure topology_get_hw_pressure
  #define arch_update_hw_pressure        topology_update_hw_pressure
  
+#ifdef CONFIG_SMT_NUM_THREADS_DYNAMIC
+#include <linux/cpu_smt.h>
+static inline bool topology_smt_thread_allowed(unsigned int cpu)
+{
+       return topology_thread_id(cpu) < cpu_smt_num_threads;
+}
+#endif
+
  #include <asm-generic/topology.h>
  
  #endif /* _ASM_ARM_TOPOLOGY_H */
diff --git a/arch/arm64/kernel/topology.c b/arch/arm64/kernel/topology.c
index f72e1e55b05e..a83babe19972 100644
--- a/arch/arm64/kernel/topology.c
+++ b/arch/arm64/kernel/topology.c
@@ -47,7 +47,9 @@ int __init parse_acpi_topology(void)
  {
         int thread_num, max_smt_thread_num = 1;
         struct xarray core_threads;
+       bool have_thread = false;
         int cpu, topology_id;
+       unsigned long i;
         void *entry;
  
         if (acpi_disabled)
@@ -61,6 +63,8 @@ int __init parse_acpi_topology(void)
                         return topology_id;
  
                 if (acpi_cpu_is_threaded(cpu)) {
+                       have_thread = true;
+
                         cpu_topology[cpu].thread_id = topology_id;
                         topology_id = find_acpi_cpu_topology(cpu, 1);
                         cpu_topology[cpu].core_id   = topology_id;
@@ -69,9 +73,10 @@ int __init parse_acpi_topology(void)
                         if (!entry) {
                                 xa_store(&core_threads, topology_id,
                                          xa_mk_value(1), GFP_KERNEL);
+                               cpu_topology[cpu].thread_id = 0;
                         } else {
                                 thread_num = xa_to_value(entry);
-                               thread_num++;
+                               cpu_topology[cpu].thread_id = thread_num++;
                                 xa_store(&core_threads, topology_id,
                                          xa_mk_value(thread_num), GFP_KERNEL);
  
@@ -86,8 +91,17 @@ int __init parse_acpi_topology(void)
                 cpu_topology[cpu].cluster_id = topology_id;
                 topology_id = find_acpi_cpu_topology_package(cpu);
                 cpu_topology[cpu].package_id = topology_id;
+
+               pr_debug("CPU%u: package=0x%x cluster=0x%x core=0x%x thread=0x%x\n",
+                       cpu, cpu_topology[cpu].package_id, cpu_topology[cpu].cluster_id,
+                       cpu_topology[cpu].core_id, cpu_topology[cpu].thread_id);
         }
  
+       if (have_thread)
+               xa_for_each(&core_threads, i, entry)
+                       if (xa_to_value(entry) != max_smt_thread_num)
+                               pr_warn("Heterogeneous SMT topology not handled");
+
         cpu_smt_set_num_threads(max_smt_thread_num, max_smt_thread_num);
  
         xa_destroy(&core_threads);
diff --git a/drivers/base/arch_topology.c b/drivers/base/arch_topology.c
index 95513abd664f..20d7f5b72ddd 100644
--- a/drivers/base/arch_topology.c
+++ b/drivers/base/arch_topology.c
@@ -532,13 +532,15 @@ static int __init get_cpu_for_node(struct device_node *node)
         return cpu;
  }
  
-static void __init update_smt_num_threads(unsigned int num_threads)
+static void __init update_smt_num_threads(int num_threads)
  {
-       static unsigned int max_smt_thread_num = 1;
+       static int max_smt_thread_num = -1;
  
-       if (num_threads > max_smt_thread_num) {
+       if (max_smt_thread_num < 0) {
                 max_smt_thread_num = num_threads;
                 cpu_smt_set_num_threads(max_smt_thread_num, max_smt_thread_num);
+       } else if (num_threads != max_smt_thread_num) {
+               pr_warn("Heterogeneous SMT topology not handled");
         }
  }
  
diff --git a/include/linux/arch_topology.h b/include/linux/arch_topology.h
index b721f360d759..afdfdc64a0a1 100644
--- a/include/linux/arch_topology.h
+++ b/include/linux/arch_topology.h
@@ -87,6 +87,7 @@ extern struct cpu_topology cpu_topology[NR_CPUS];
  #define topology_physical_package_id(cpu)      (cpu_topology[cpu].package_id)
  #define topology_cluster_id(cpu)       (cpu_topology[cpu].cluster_id)
  #define topology_core_id(cpu)          (cpu_topology[cpu].core_id)
+#define topology_thread_id(cpu)                (cpu_topology[cpu].thread_id)
  #define topology_core_cpumask(cpu)     (&cpu_topology[cpu].core_sibling)
  #define topology_sibling_cpumask(cpu)  (&cpu_topology[cpu].thread_sibling)
  #define topology_cluster_cpumask(cpu)  (&cpu_topology[cpu].cluster_sibling)

----------------------------


Regards,
Pierre

> 
> Thanks,
> Yicong
> 
>>
>> Same comment for the DT patch. If there is an assumption that all CPUs have
>> the same number of threads, then update_smt_num_threads() could only be called
>> once I suppose,
>>
>> Regards,
>> Pierre
>>
>>
>> On 8/6/24 10:53, Yicong Yang wrote:
>>> From: Yicong Yang <yangyicong@...ilicon.com>
>>>
>>> For ACPI we'll build the topology from PPTT and we cannot directly
>>> get the SMT number of each core. Instead using a temporary xarray
>>> to record the SMT number of each core when building the topology
>>> and we can know the largest SMT number in the system. Then we can
>>> enable the support of SMT control.
>>>
>>> Signed-off-by: Yicong Yang <yangyicong@...ilicon.com>
>>> ---
>>>    arch/arm64/kernel/topology.c | 24 ++++++++++++++++++++++++
>>>    1 file changed, 24 insertions(+)
>>>
>>> diff --git a/arch/arm64/kernel/topology.c b/arch/arm64/kernel/topology.c
>>> index 1a2c72f3e7f8..f72e1e55b05e 100644
>>> --- a/arch/arm64/kernel/topology.c
>>> +++ b/arch/arm64/kernel/topology.c
>>> @@ -15,8 +15,10 @@
>>>    #include <linux/arch_topology.h>
>>>    #include <linux/cacheinfo.h>
>>>    #include <linux/cpufreq.h>
>>> +#include <linux/cpu_smt.h>
>>>    #include <linux/init.h>
>>>    #include <linux/percpu.h>
>>> +#include <linux/xarray.h>
>>>      #include <asm/cpu.h>
>>>    #include <asm/cputype.h>
>>> @@ -43,11 +45,16 @@ static bool __init acpi_cpu_is_threaded(int cpu)
>>>     */
>>>    int __init parse_acpi_topology(void)
>>>    {
>>> +    int thread_num, max_smt_thread_num = 1;
>>> +    struct xarray core_threads;
>>>        int cpu, topology_id;
>>> +    void *entry;
>>>          if (acpi_disabled)
>>>            return 0;
>>>    +    xa_init(&core_threads);
>>> +
>>>        for_each_possible_cpu(cpu) {
>>>            topology_id = find_acpi_cpu_topology(cpu, 0);
>>>            if (topology_id < 0)
>>> @@ -57,6 +64,20 @@ int __init parse_acpi_topology(void)
>>>                cpu_topology[cpu].thread_id = topology_id;
>>>                topology_id = find_acpi_cpu_topology(cpu, 1);
>>>                cpu_topology[cpu].core_id   = 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;
>>> +            }
>>>            } else {
>>>                cpu_topology[cpu].thread_id  = -1;
>>>                cpu_topology[cpu].core_id    = topology_id;
>>> @@ -67,6 +88,9 @@ int __init parse_acpi_topology(void)
>>>            cpu_topology[cpu].package_id = topology_id;
>>>        }
>>>    +    cpu_smt_set_num_threads(max_smt_thread_num, max_smt_thread_num);
>>> +
>>> +    xa_destroy(&core_threads);
>>>        return 0;
>>>    }
>>>    #endif
>>
>> .
View attachment "0001-RFC-arm64-topology-Enable-CONFIG_SMT_NUM_THREADS_DYN.patch" of type "text/x-patch" (5575 bytes)

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