Subject: Re: [PATCH v10 2/4] arch_topology: Support SMT control for OF based system

From: Jonathan Cameron <[email-masked]>
Date: Mon, 23 Dec 2024 16:33:14 +0000

On Fri, 20 Dec 2024 15:53:11 +0800
Yicong Yang <[email-masked]> wrote:

> From: Yicong Yang <[email-masked]>
> 
> 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
> 
> If a system have more than one SMT thread number the 2) may
> not handle it well, since there're multiple thread numbers in the
> system and 2) only accept 1/max_thread_number. So issue a warning
> to notify the users if such system detected.
> 
> [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 <[email-masked]>
Hi Yicong,

Apologies that I'm late to the game on this one.

A few comments inline.  Only important one is whether to bail out early
on error from parse_cluster()

Thanks,

Jonathan

> ---
>  drivers/base/arch_topology.c | 26 ++++++++++++++++++++++++++
>  1 file changed, 26 insertions(+)
> 
> diff --git a/drivers/base/arch_topology.c b/drivers/base/arch_topology.c
> index 3ebe77566788..9e81060144c7 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 */
> +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 +570,17 @@ 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");
> +
> +	if (max_smt_thread_num < i)
> +		max_smt_thread_num = i;

Maybe more self documenting if you use min()? I'm not sure...
	max_smt_thread_num = min(max_smt_thread_num, i);

> +
>  	cpu = get_cpu_for_node(core);
>  	if (cpu >= 0) {
>  		if (!leaf) {
> @@ -677,6 +693,16 @@ static int __init parse_socket(struct device_node *socket)
>  	if (!has_socket)
>  		ret = parse_cluster(socket, 0, -1, 0);

Is it appropriate to check ret before setting num threads?
	if (!has_socket) {
		ret = parse_cluster(socket, 0, -1, 0);
		if (ret)
			return ret;
	}
...

	return 0;

>  
> +	/*
> +	 * Notify the CPU framework of the SMT support. Initialize the
> +	 * max_smt_thread_num to 1 if no SMT support detected. 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.
> +	 */
> +	if (!max_smt_thread_num)
> +		max_smt_thread_num = 1;
> +
> +	cpu_smt_set_num_threads(max_smt_thread_num, max_smt_thread_num);

Trivial but I'd put a blank line here.

>  	return ret;
>  }