linux-kernel - Re: [PATCH] arm64: implement arm64 specific cpu_to

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Message-ID: <ZaUajJbKGywgC-AX@FVFF77S0Q05N>
Date: Mon, 15 Jan 2024 11:44:12 +0000
From: Mark Rutland <mark.rutland@....com>
To: Huang Shijie <shijie@...amperecomputing.com>
Cc: catalin.marinas@....com, patches@...erecomputing.com, will@...nel.org,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64: implement arm64 specific cpu_to_node

On Mon, Jan 15, 2024 at 05:59:31PM +0800, Huang Shijie wrote:
> After setting the right NUMA node for VMAP stack,
>   https://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git/commit/?id=75b5e0bf90bf
> 
> I found there are at least four places in the common code where
> the cpu_to_node() is called before it is initialized:
>    0.) early_trace_init()         in kernel/trace/trace.c
>    1.) sched_init()               in kernel/sched/core.c
>    2.) init_sched_fair_class()    in kernel/sched/fair.c
>    3.) workqueue_init_early()     in kernel/workqueue.c
> 
> We cannot use early_cpu_to_node() for them, since early_cpu_to_node()
> does not work for some ARCHs, such as x86, riscv, etc.

I spot that x86 seems to have an implementation of early_cpu_to_node(); what's
wrong with it?

> So we have to implement the arm64 specific cpu_to_node().

Surely those early uses of cpu_to_node() are equally broken on those other
architectures, so why should this be specific to arm64?

> This patch
>    0.) introduces the __cpu_to_node function pointer,
>        and exports it for kernel modules.
> 
>    1.) defines a macro cpu_to_node to override the
>        generic percpu implementation of cpu_to_node.
> 
>    2.) __cpu_to_node is initialized with early_cpu_to_node() before
>        numa_node is initialized.
> 
>    3.) __cpu_to_node is set to arm64_cpu_to_node() when numa_node is ready.
>       arm64_cpu_to_node() is a clone of the generic cpu_to_node().

I don't think this is the right approach. Regardlesss of anything else, we
shouldn't have a solution that only fixes arm64.

Why can't we mandate an early_cpu_to_node(), and have the other architectures
implement that?

Why can't we change cpu_to_node() to automatically do the right thing?

Mark.

> 
> Signed-off-by: Huang Shijie <shijie@...amperecomputing.com>
> ---
>  arch/arm64/include/asm/topology.h |  4 ++++
>  arch/arm64/kernel/smp.c           | 21 ++++++++++++++++++++-
>  2 files changed, 24 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/include/asm/topology.h b/arch/arm64/include/asm/topology.h
> index a323b109b9c4..3e99b40b5f9f 100644
> --- a/arch/arm64/include/asm/topology.h
> +++ b/arch/arm64/include/asm/topology.h
> @@ -12,6 +12,10 @@ int pcibus_to_node(struct pci_bus *bus);
>  				 cpu_all_mask :				\
>  				 cpumask_of_node(pcibus_to_node(bus)))
>  
> +/* override generic percpu implementation of cpu_to_node */
> +extern int (*__cpu_to_node)(int cpu);
> +#define cpu_to_node(cpu) __cpu_to_node(cpu)
> +
>  #endif /* CONFIG_NUMA */
>  
>  #include <linux/arch_topology.h>
> diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
> index 4ced34f62dab..8a3bc101eaed 100644
> --- a/arch/arm64/kernel/smp.c
> +++ b/arch/arm64/kernel/smp.c
> @@ -90,6 +90,21 @@ static struct irq_desc *ipi_desc[MAX_IPI] __ro_after_init;
>  
>  static void ipi_setup(int cpu);
>  
> +int (*__cpu_to_node)(int cpu);
> +EXPORT_SYMBOL(__cpu_to_node);
> +
> +#ifdef CONFIG_USE_PERCPU_NUMA_NODE_ID
> +static int arm64_cpu_to_node(int cpu)
> +{
> +	return per_cpu(numa_node, cpu);
> +}
> +#else
> +static int arm64_cpu_to_node(int cpu)
> +{
> +	return 0;
> +}
> +#endif
> +
>  #ifdef CONFIG_HOTPLUG_CPU
>  static void ipi_teardown(int cpu);
>  static int op_cpu_kill(unsigned int cpu);
> @@ -613,6 +628,7 @@ static void __init acpi_parse_and_init_cpus(void)
>  
>  	for (i = 0; i < nr_cpu_ids; i++)
>  		early_map_cpu_to_node(i, acpi_numa_get_nid(i));
> +	__cpu_to_node = early_cpu_to_node;
>  }
>  #else
>  #define acpi_parse_and_init_cpus(...)	do { } while (0)
> @@ -674,6 +690,7 @@ static void __init of_parse_and_init_cpus(void)
>  next:
>  		cpu_count++;
>  	}
> +	__cpu_to_node = early_cpu_to_node;
>  }
>  
>  /*
> @@ -733,7 +750,7 @@ void __init smp_prepare_cpus(unsigned int max_cpus)
>  	 * secondary CPUs present.
>  	 */
>  	if (max_cpus == 0)
> -		return;
> +		goto out;
>  
>  	/*
>  	 * Initialise the present map (which describes the set of CPUs
> @@ -758,6 +775,8 @@ void __init smp_prepare_cpus(unsigned int max_cpus)
>  		set_cpu_present(cpu, true);
>  		numa_store_cpu_info(cpu);
>  	}
> +out:
> +	__cpu_to_node = arm64_cpu_to_node;
>  }
>  
>  static const char *ipi_types[NR_IPI] __tracepoint_string = {
> -- 
> 2.40.1
>