linux-kernel - Re: [PATCH v12 2/4] arch_topology: Support SMT control for OF based system

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Message-ID: <c88e5b78-37b7-3023-7842-56a93fe119d2@huawei.com>
Date: Mon, 17 Mar 2025 19:29:19 +0800
From: Yicong Yang <yangyicong@...wei.com>
To: Dietmar Eggemann <dietmar.eggemann@....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>, <pierre.gondois@....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>,
	<sshegde@...ux.ibm.com>
Subject: Re: [PATCH v12 2/4] arch_topology: Support SMT control for OF based
 system

On 2025/3/17 17:56, Dietmar Eggemann wrote:
> On 11/03/2025 08:51, Yicong Yang wrote:
>> From: Yicong Yang <yangyicong@...ilicon.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.
> 
> [...]
> 
> .
>