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

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Message-ID: <8a9aedef-08d7-445f-9b67-85e74ec6bd50@arm.com>
Date: Fri, 28 Feb 2025 12:11:16 +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 v11 2/4] arch_topology: Support SMT control for OF based
 system

On 18/02/2025 15:10, 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 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

1/max_thread_number stands for '1 or max_thread_number', right ?

Aren't the two interfaces:

(a) /sys/devices/system/cpu/smt/active
(b) /sys/devices/system/cpu/smt/control

and you write 1) or 2) (or 'forceoff') into (b)?

> If a system have more than one SMT thread number the 2) may

s/have/has

> not handle it well, since there're multiple thread numbers in the

multiple thread numbers other than 1, right?

> system and 2) only accept 1/max_thread_number. So issue a warning
> to notify the users if such system detected.

This paragraph seems to be about heterogeneous systems. Maybe mention this?

Heterogeneous system with SMT only on a subset of cores (like Intel
Hybrid): This one works (N threads per core with N=1 and N=2) just fine.

But on Arm64 (default) we would still see:

[0.075782] Heterogeneous SMT topology is partly supported by SMT control

> [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@...ilicon.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 */

maybe shorter ?

/* used to enable 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");
> +
> +	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.

Not sure whether the last sentence is needed here?

[...]