Re: [PATCH v11 2/4] arch_topology: Support SMT control for OF based system

Yicong Yang
2025-03-03T14:21:29Z

On 2025/2/28 19:11, Dietmar Eggemann wrote:
> On 18/02/2025 15:10, Yicong Yang wrote:
>> From: Yicong Yang <yangyicong@hisilicon.com>
>>
>> On building the topology from the devicetree, we've already
>> gotten the SMT thread number of each core. Update the largest
>> SMT thread number and enable the SMT control by the end of
>> topology parsing.
>>
>> 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
>
> 1/max_thread_number stands for '1 or max_thread_number', right ?
>
> Aren't the two interfaces:
>
> (a) /sys/devices/system/cpu/smt/active
> (b) /sys/devices/system/cpu/smt/control
>
> and you write 1) or 2) (or 'forceoff') into (b)?

yes you're correct. "active" is a RO file for status only so not for this interface.
Let me explicitly mention the /sys/devices/system/cpu/smt/control here in the commit.

>
>> If a system have more than one SMT thread number the 2) may
>
> s/have/has
>
>> not handle it well, since there're multiple thread numbers in the
>
> multiple thread numbers other than 1, right?

according to the pr_warn_once() we implemented below it also includes the case
where the system have one type of SMT cores and non-SMT cores (the thread number is 1):
- 1 thread
- X (!= 1) threads

Discussion made in [1] and I thought we have agreement (hope I understood correctly)
that all the asymmetric cases need to notify. Do you and Sudeep think we should not
warn in such case?

[1] https://lore.kernel.org/linux-arm-kernel/10082e64-b00a-a30b-b9c5-1401a54f6717@huawei.com/

>
>> system and 2) only accept 1/max_thread_number. So issue a warning
>> to notify the users if such system detected.
>
> This paragraph seems to be about heterogeneous systems. Maybe mention this?
>
> Heterogeneous system with SMT only on a subset of cores (like Intel
> Hybrid): This one works (N threads per core with N=1 and N=2) just fine.
>
> But on Arm64 (default) we would still see:
>
> [0.075782] Heterogeneous SMT topology is partly supported by SMT control
>

more clearer, will add it. Thanks.

>> [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@hisilicon.com>
>> ---
>> drivers/base/arch_topology.c | 27 +++++++++++++++++++++++++++
>> 1 file changed, 27 insertions(+)
>>
>> diff --git a/drivers/base/arch_topology.c b/drivers/base/arch_topology.c
>> index 3ebe77566788..23f425a9d77a 100644
>> --- a/drivers/base/arch_topology.c
>> +++ b/drivers/base/arch_topology.c
>> @@ -11,6 +11,7 @@
>> #include <linux/cleanup.h>
>> #include <linux/cpu.h>
>> #include <linux/cpufreq.h>
>> +#include <linux/cpu_smt.h>
>> #include <linux/device.h>
>> #include <linux/of.h>
>> #include <linux/slab.h>
>> @@ -506,6 +507,10 @@ core_initcall(free_raw_capacity);
>> #endif
>>
>> #if defined(CONFIG_ARM64) || defined(CONFIG_RISCV)
>> +
>> +/* Maximum SMT thread number detected used to enable the SMT control */
>
> maybe shorter ?
>
> /* used to enable SMT control */
>

sure.

>> +static unsigned int max_smt_thread_num;
>> +
>> /*
>> * This function returns the logic cpu number of the node.
>> * There are basically three kinds of return values:
>> @@ -565,6 +570,16 @@ static int __init parse_core(struct device_node *core, int package_id,
>> i++;
>> } while (1);
>>
>> + /*
>> + * 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 (max_smt_thread_num && max_smt_thread_num != i)
>> + pr_warn_once("Heterogeneous SMT topology is partly supported by SMT control\n");
>> +
>> + max_smt_thread_num = max_t(unsigned int, max_smt_thread_num, i);
>> +
>> cpu = get_cpu_for_node(core);
>> if (cpu >= 0) {
>> if (!leaf) {
>> @@ -677,6 +692,18 @@ static int __init parse_socket(struct device_node *socket)
>> if (!has_socket)
>> ret = parse_cluster(socket, 0, -1, 0);
>>
>> + /*
>> + * Notify the CPU framework of the SMT support. Initialize the
>> + * max_smt_thread_num to 1 if no SMT support detected or failed
>> + * to parse the topology. A thread number of 1 can be handled by
>> + * the framework so we don't need to check max_smt_thread_num to
>> + * see we support SMT or not.
>
> Not sure whether the last sentence is needed here?
>

We always need to call cpu_smt_set_num_threads() to notify the framework
of the thread number even if SMT is not supported. In which case the
thread number is 1 but the framework can handle this well. I worry readers
may get confused for notifying a thread number of 1 so add this comment this.

Will get rid of this if thought redundant.

Thanks.