On 2025/3/17 17:56, Dietmar Eggemann wrote:
> On 11/03/2025 08:51, Yicong Yang wrote:
>> From: Yicong Yang <yangyic...@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 framework's SMT control provides two interface to the users [1]
>> through /sys/devices/system/cpu/smt/control:
>> 1) enable SMT by writing "on" and disable by "off"
>> 2) enable SMT by writing max_thread_number or disable by writing 1
>>
>> Both method support to completely disable/enable the SMT cores so both
>> work correctly for symmetric SMT platform and asymmetric platform with
>> non-SMT and one type SMT cores like:
>> core A: 1 thread
>> core B: X (X!=1) threads
>>
>> Note that for a theoretically possible multiple SMT-X (X>1) core
>> platform the SMT control is also supported as expected but only
>> by writing the "on/off" method.
>
> Here we still have a little misunderstanding. IMHO, even on such a
> system 2) would work too.
>
yes but only by writing the max_thread_number. e.g. a system with SMT number
of 1 (no SMT core), X, Y (Y > X), 1 works same as "off" and Y works same as
"on", as you shown below. write X will be blocked by the CPU framework:
estuary:/sys/devices/system/cpu/smt$ cat control
off
# emulated CPU 0-7,16-22 as SMT-2, CPU 8-11,24-27 as SMT-4
estuary:/sys/devices/system/cpu/smt$ cat ../online
0,2,4,6,8,12-16,18,20,22,24,28-31
estuary:/sys/devices/system/cpu/smt$ echo 2 > control
bash: echo: write error: Invalid argument
estuary:/sys/devices/system/cpu/smt$ echo 4 > control
estuary:/sys/devices/system/cpu/smt$ cat ../online
0-31
so method 1) will always match the expectation to fully enable/disable the
SMT on all cores, that's I mean here. But 2) won't work if user expected
to write 2 to enable SMT-2 (I think it'll will work if we support
CONFIG_SMT_NUM_THREADS_DYNAMIC on arm64 later).
Thanks.
> My qemu example with SMT-1, SMT-2 and SMT-4 in one system from your v11:
>
> # cat /proc/schedstat | grep -v "^v\|^t" | awk '{print $1" "$2" "$3}'
> cpu0 0 0
> domain0 MC ff
> cpu1 0 0
> domain0 MC ff
> cpu2 0 0
> domain0 SMT 0c
> domain1 MC ff
> cpu3 0 0
> domain0 SMT 0c
> domain1 MC ff
> cpu4 0 0
> domain0 SMT f0
> domain1 MC ff
> cpu5 0 0
> domain0 SMT f0
> domain1 MC ff
> cpu6 0 0
> domain0 SMT f0
> domain1 MC ff
> cpu7 0 0
> domain0 SMT f0
> domain1 MC ff
>
> # cat /proc/cpuinfo | grep ^processor
> processor : 0
> processor : 1
> processor : 2
> processor : 3
> processor : 4
> processor : 5
> processor : 6
> processor : 7
>
> # echo 1 > /sys/devices/system/cpu/smt/control
>
> # cat /proc/cpuinfo | grep ^processor
> processor : 0
> processor : 1
> processor : 2
> processor : 4
>
> # echo 4 > /sys/devices/system/cpu/smt/control
>
> # cat /proc/cpuinfo | grep ^processor
> processor : 0
> processor : 1
> processor : 2
> processor : 3
> processor : 4
> processor : 5
> processor : 6
> processor : 7
>
> Whats doesn't work is to echoing a '2' but that's not
> 'max_thread_number' of the system.
>
> [...]
>
> .
>
