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Message-ID: <3181577e-d6dd-fd91-f51f-ae6ca0413687@huawei.com>
Date: Thu, 24 Oct 2024 23:09:59 +0800
From: Yicong Yang <yangyicong@...wei.com>
To: Pierre Gondois <pierre.gondois@....com>
CC: <catalin.marinas@....com>, <will@...nel.org>, <sudeep.holla@....com>,
<tglx@...utronix.de>, <peterz@...radead.org>, <mpe@...erman.id.au>,
<linux-arm-kernel@...ts.infradead.org>, <mingo@...hat.com>, <bp@...en8.de>,
<dave.hansen@...ux.intel.com>, <dietmar.eggemann@....com>,
<yangyicong@...ilicon.com>, <linuxppc-dev@...ts.ozlabs.org>,
<x86@...nel.org>, <linux-kernel@...r.kernel.org>, <morten.rasmussen@....com>,
<msuchanek@...e.de>, <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 v6 3/4] arm64: topology: Support SMT control on ACPI based
system
On 2024/10/24 16:44, Pierre Gondois wrote:
> Hello Yicong,
>
> On 10/15/24 04:18, 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 heterogeneous information (from ACPI_PPTT_ACPI_IDENTICAL)
>> and SMT information of the first core in its heterogeneous CPU cluster
>> when building the topology. Then we can know the largest SMT number
>> in the system.
>
> Is it possible to also state that the logic only work for PPTT versions
> prior to 6.2. Otherwise find_acpi_cpu_topology_hetero_id() will return
> the id of the root node of the PPTT, and all the CPUs of the platform
> will be assumed to be identical.
>
sure. of course.
>>
>> The core's SMT control provides two interface to the users [1]:
>> 1) enable/disable SMT by writing on/off
>> 2) enable/disable SMT by writing thread number 1/max_thread_number
>>
>> If a system have more than one SMT thread number the 2) may
>> not handle it well, since there're multiple thread numbers in the
>> system and 2) only accept 1/max_thread_number. So issue a warning
>> to notify the users if such system detected.
>>
>> [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/ABI/testing/sysfs-devices-system-cpu#n542
>> Signed-off-by: Yicong Yang <yangyicong@...ilicon.com>
>> ---
>> arch/arm64/kernel/topology.c | 61 ++++++++++++++++++++++++++++++++++++
>> 1 file changed, 61 insertions(+)
>>
>> diff --git a/arch/arm64/kernel/topology.c b/arch/arm64/kernel/topology.c
>> index 1a2c72f3e7f8..2fa584b932ee 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>
>> @@ -37,17 +39,29 @@ static bool __init acpi_cpu_is_threaded(int cpu)
>> return !!is_threaded;
>> }
>> +struct cpu_smt_info {
>> + int thread_num;
>> + int core_id;
>> + int cpu;
>
> It doesn't seem that the 'cpu' is used. Maybe this should be removed,
> or used in the error message (I think this could be removed).
>
sure. will remove..
>> +};
>> +
>> /*
>> * Propagate the topology information of the processor_topology_node tree to the
>> * cpu_topology array.
>> */
>> int __init parse_acpi_topology(void)
>> {
>> + int max_smt_thread_num = 0;
>> + struct cpu_smt_info *entry;
>> + struct xarray hetero_cpu;
>> + unsigned long hetero_id;
>> int cpu, topology_id;
>> if (acpi_disabled)
>> return 0;
>> + xa_init(&hetero_cpu);
>> +
>> for_each_possible_cpu(cpu) {
>> topology_id = find_acpi_cpu_topology(cpu, 0);
>> if (topology_id < 0)
>> @@ -57,6 +71,30 @@ 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;
>> +
>> + /*
>> + * Build up the XArray using the heterogeneous ID of
>> + * the CPU cluster. Store the CPU and SMT information
>> + * of the first appeared CPU in the CPU cluster of this
>> + * heterogeneous ID since the SMT information should be
>> + * the same in this CPU cluster. Then we can know the
>> + * SMT information of each heterogeneous CPUs in the
>> + * system.
>> + */
>
> A suggestion just in case:
> /*
> * In the PPTT, CPUs below a node with the 'identical
> * implementation' flag have the same number of threads.
> * Count the number of threads for only one CPU (i.e.
> * one core_id) among those with the same hetero_id.
> *
> * One entry is created for each node having:
> * - the 'identical implementation' flag
> * - its parent not having the flag
> */
>
ok maybe we can also refer to the comments of find_acpi_cpu_topology_hetero_id() [1] to lead
reader there if they want more details.
[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/acpi/pptt.c#n794
>
>> + hetero_id = find_acpi_cpu_topology_hetero_id(cpu);
>> + entry = (struct cpu_smt_info *)xa_load(&hetero_cpu, hetero_id);
>> + if (!entry) {
>> + entry = kzalloc(sizeof(*entry), GFP_KERNEL);
>> + WARN_ON(!entry);
>> +
>> + entry->cpu = cpu;
>> + entry->core_id = topology_id;
>> + entry->thread_num = 1;
>> + xa_store(&hetero_cpu, hetero_id,
>> + entry, GFP_KERNEL);
>> + } else if (entry->core_id == topology_id) {
>> + entry->thread_num++;
>> + }
>> } else {
>> cpu_topology[cpu].thread_id = -1;
>> cpu_topology[cpu].core_id = topology_id;
>> @@ -67,6 +105,29 @@ int __init parse_acpi_topology(void)
>> cpu_topology[cpu].package_id = topology_id;
>> }
>> + /*
>> + * This should be a short loop depending on the number of heterogeneous
>> + * CPU clusters. Typically on a homogeneous system there's only one
>> + * entry in the XArray.
>> + */
>
> I think it should be ok to remove the above/below comments
>
I'd leave the above comment since this won't make people who are awared of the efficiency feel
nervous about the loop here...
will drop the below comment.
>> + xa_for_each(&hetero_cpu, hetero_id, entry) {
>> + /*
>> + * If max_smt_thread_num has been initialized and doesn't match
>> + * the thread number of this entry, then the system has
>> + * heterogeneous SMT topology.
>> + */
>> + if (entry->thread_num != max_smt_thread_num && max_smt_thread_num)
>> + pr_warn_once("Heterogeneous SMT topology is partly supported by SMT control\n");
>> +
>> + if (entry->thread_num > max_smt_thread_num)
>> + max_smt_thread_num = entry->thread_num;
>> +
>> + xa_erase(&hetero_cpu, hetero_id);
>> + kfree(entry);
>> + }
>> +
>> + cpu_smt_set_num_threads(max_smt_thread_num, max_smt_thread_num);
>> + xa_destroy(&hetero_cpu);
>> return 0;
>> }
>> #endif
>
> Otherwise I tested the SMT detection and it worked well
>
appreciated for the tests!
Thanks.
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