Re: [PATCH v5 3/4] arm64: topology: Support SMT control on ACPI based system

[Dietmar Eggemann <dietmar.eggemann@arm.com>]

On 19/08/2024 09:03, Yicong Yang wrote:
> On 2024/8/16 23:55, Dietmar Eggemann wrote:
>> On 06/08/2024 10:53, Yicong Yang wrote:
>>> From: Yicong Yang <yangyicong@hisilicon.com>

[...]

>> So the xarray contains one element for each core_id with the information
>> how often the core_id occurs? I assume you have to iterate over all
>> possible CPUs since you don't know which logical CPUs belong to the same
>> core_id.
>>
> 
> Each xarray element counts the thread number of a certain core id. so the logic is like below:
> 1. if the "core id" entry doesn't exists, then we're accessing this core for the 1st time. create
>    one and make the thread number to 1
> 2. otherwise increment the thread number of "core id" this cpu belongs (PPTT already
>    told us which core this CPU belongs to). Update the max_smt_thread_num if necessary.
> 
> Then we can know max_smt_thread_num by meanwhile iterating the PPTT table and
> build the topology for all the possible CPUs.
> 
> Otherwise we need to do a second scan for the max thread number after built the
> topology. This way is implemented in v1 and it's complained about the overhead on large
> scale systems since we need to loop the CPUs twice.

OK.

[...]

>> Tested on ThunderX2:
>>
>> $ cat /proc/schedstat | head -6 | tail -4 | awk '{ print $1, $2 }'
>> cpu0 0
>> domain0 00000000,00000000,00000000,00000000,00000001,00000001,00000001,00000001
>>                                                    ^        ^        ^        ^
>> domain1 00000000,00000000,00000000,00000000,ffffffff,ffffffff,ffffffff,ffffffff
>> domain2 ffffffff,ffffffff,ffffffff,ffffffff,ffffffff,ffffffff,ffffffff,ffffffff
>>
>> detecting 'max_smt_thread_num = 4' correctly.
>>
> 
> Thanks for the testing. ok for a tag?

Yes, please go ahead.

[...]