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

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Message-ID: <Z8HAFftf7rAdc_MC@bogus>
Date: Fri, 28 Feb 2025 13:54:29 +0000
From: Sudeep Holla <sudeep.holla@....com>
To: Yicong Yang <yangyicong@...wei.com>
Cc: <catalin.marinas@....com>, <will@...nel.org>, <tglx@...utronix.de>,
	Sudeep Holla <sudeep.holla@....com>, <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>, <dietmar.eggemann@....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>,
	<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 Tue, Feb 18, 2025 at 10:10:16PM +0800, 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
> 
> 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 <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 */
> +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");
> +

May be we need to make it more conditional as we may have to support
systems with few cores that are single threaded ? I think Dietmar's
comment is about that.

> +	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.
> +	 */
> +	if (!max_smt_thread_num || ret)
> +		max_smt_thread_num = 1;
> +

For the failed parsing of topology, reset_cpu_topology() gets called.
I suggest resetting max_smt_thread_num to 1 belongs there.

And if you start with max_smt_thread_num, we don't need to update it to
1 explicitly here. So I would like to get rid of above check completely.

--
Regards,
Sudeep