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

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Message-ID: <2bd3aa0a-d700-46bf-81d1-a5fb0364d1e0@arm.com>
Date: Mon, 17 Mar 2025 10:56:43 +0100
From: Dietmar Eggemann <dietmar.eggemann@....com>
To: Yicong Yang <yangyicong@...wei.com>, 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
Cc: 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, yangyicong@...ilicon.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 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.

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.

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