linux-kernel - Re: [PATCH v6 2/4] arch_topology: Support SMT control for OF based system

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Message-ID: <eb7ec4c3-5995-4040-8992-bb95f4b9f923@arm.com>
Date: Wed, 23 Oct 2024 17:43:29 +0200
From: Pierre Gondois <pierre.gondois@....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, dietmar.eggemann@....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
Subject: Re: [PATCH v6 2/4] arch_topology: Support SMT control for OF based
 system

Hello Yicong,

On 10/15/24 04:18, 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 | 21 +++++++++++++++++++++
>   1 file changed, 21 insertions(+)
> 
> diff --git a/drivers/base/arch_topology.c b/drivers/base/arch_topology.c
> index 75fcb75d5515..5eed864df5e6 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>
> @@ -28,6 +29,7 @@
>   static DEFINE_PER_CPU(struct scale_freq_data __rcu *, sft_data);
>   static struct cpumask scale_freq_counters_mask;
>   static bool scale_freq_invariant;
> +static unsigned int max_smt_thread_num;
>   DEFINE_PER_CPU(unsigned long, capacity_freq_ref) = 1;
>   EXPORT_PER_CPU_SYMBOL_GPL(capacity_freq_ref);
>   
> @@ -561,6 +563,17 @@ static int __init parse_core(struct device_node *core, int package_id,
>   		i++;
>   	} while (1);
>   
> +	if (max_smt_thread_num < i)
> +		max_smt_thread_num = i;

Shouldn't the conditions above/below be inverted ?
I.e. (max_smt_thread_num != i) should never be true if there is
   max_smt_thread_num = i;
just before

> +
> +	/*
> +	 * 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");
> +
>   	cpu = get_cpu_for_node(core);
>   	if (cpu >= 0) {
>   		if (!leaf) {
> @@ -673,6 +686,14 @@ 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. 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)
> +		cpu_smt_set_num_threads(max_smt_thread_num, max_smt_thread_num);
> +
>   	return ret;
>   }
>