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

[Yicong Yang <yangyicong@huawei.com>]

On 2025/2/28 21:54, Sudeep Holla wrote:
> On Tue, Feb 18, 2025 at 10:10:16PM +0800, 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
>>
>> 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@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 */
>> +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");
>> +
> 
> May be we need to make it more conditional as we may have to support
> systems with few cores that are single threaded ? I think Dietmar's
> comment is about that.
> 

it thought of ignoring the cores with single thread in one previous discussion
as replied in Dietmar's thread.

>> +	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.
>> +	 */
>> +	if (!max_smt_thread_num || ret)
>> +		max_smt_thread_num = 1;
>> +
> 
> For the failed parsing of topology, reset_cpu_topology() gets called.
> I suggest resetting max_smt_thread_num to 1 belongs there.

this is only used by ARM64 || RISCV for using arch_topology to parse
the CPU topology, but the reset_cpu_topology() is also shared by arm/parisc.
Should we move it there and add some ARM64 || RISCV protection macro?

> 
> And if you start with max_smt_thread_num, we don't need to update it to
> 1 explicitly here. So I would like to get rid of above check completely.
> 
> --
> Regards,
> Sudeep
> 
> .
>