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Date: Thu, 18 Jan 2024 20:42:25 -0800
From: Yury Norov <yury.norov@gmail.com>
To: Huang Shijie <shijie@os.amperecomputing.com>
Cc: gregkh@linuxfoundation.org, patches@amperecomputing.com,
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Subject: Re: [PATCH] NUMA: Early use of cpu_to_node() returns 0 instead of
 the correct node id
Message-ID: <Zan9sb0vtSvVvQeA@yury-ThinkPad>
References: <20240119033227.14113-1-shijie@os.amperecomputing.com>
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In-Reply-To: <20240119033227.14113-1-shijie@os.amperecomputing.com>

On Fri, Jan 19, 2024 at 11:32:27AM +0800, Huang Shijie wrote:
> hZ7bkEvc+Z19RHkS/HVG3KMg
> X-MS-Exchange-Transport-CrossTenantHeadersStamped: DM8PR01MB7144
> Status: O
> Content-Length: 3779
> Lines: 126
> 
> During the kernel booting, the generic cpu_to_node() is called too early in
> arm64, powerpc and riscv when CONFIG_NUMA is enabled.
> 
> There are at least four places in the common code where
> the generic cpu_to_node() is called before it is initialized:
> 	   1.) early_trace_init()         in kernel/trace/trace.c
> 	   2.) sched_init()               in kernel/sched/core.c
> 	   3.) init_sched_fair_class()    in kernel/sched/fair.c
> 	   4.) workqueue_init_early()     in kernel/workqueue.c
> 
> In order to fix the bug, the patch changes generic cpu_to_node to
> function pointer, and export it for kernel modules.
> Introduce smp_prepare_boot_cpu_start() to wrap the original
> smp_prepare_boot_cpu(), and set cpu_to_node with early_cpu_to_node.
> Introduce smp_prepare_cpus_done() to wrap the original smp_prepare_cpus(),
> and set the cpu_to_node to formal _cpu_to_node().

This adds another level of indirection, I think. Currently cpu_to_node
is a simple inliner. After the patch it would be a real function with
all the associate overhead. Can you share a bloat-o-meter output here?

Regardless, I don't think that the approach is correct. As per your
description, some initialization functions erroneously call
cpu_to_node() instead of early_cpu_to_node() which exists specifically
for that case.

If the above correct, it's clearly a caller problem, and the fix is to
simply switch all those callers to use early version.

I would also initialize the numa_node with NUMA_NO_NODE at declaration,
so that if someone calls cpu_to_node() before the variable is properly
initialized at runtime, he'll get NO_NODE, which is obviously an error.

Thanks,
Yury
 
> Signed-off-by: Huang Shijie <shijie@os.amperecomputing.com>
> ---
>  drivers/base/arch_numa.c | 11 +++++++++++
>  include/linux/topology.h |  6 ++----
>  init/main.c              | 29 +++++++++++++++++++++++++++--
>  3 files changed, 40 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/base/arch_numa.c b/drivers/base/arch_numa.c
> index 5b59d133b6af..867a477fa975 100644
> --- a/drivers/base/arch_numa.c
> +++ b/drivers/base/arch_numa.c
> @@ -61,6 +61,17 @@ EXPORT_SYMBOL(cpumask_of_node);
>  
>  #endif
>  
> +#ifdef CONFIG_USE_PERCPU_NUMA_NODE_ID
> +#ifndef cpu_to_node
> +int _cpu_to_node(int cpu)
> +{
> +	return per_cpu(numa_node, cpu);
> +}
> +int (*cpu_to_node)(int cpu);
> +EXPORT_SYMBOL(cpu_to_node);
> +#endif
> +#endif
> +
>  static void numa_update_cpu(unsigned int cpu, bool remove)
>  {
>  	int nid = cpu_to_node(cpu);
> diff --git a/include/linux/topology.h b/include/linux/topology.h
> index 52f5850730b3..e7ce2bae11dd 100644
> --- a/include/linux/topology.h
> +++ b/include/linux/topology.h
> @@ -91,10 +91,8 @@ static inline int numa_node_id(void)
>  #endif
>  
>  #ifndef cpu_to_node
> -static inline int cpu_to_node(int cpu)
> -{
> -	return per_cpu(numa_node, cpu);
> -}
> +extern int (*cpu_to_node)(int cpu);
> +extern int _cpu_to_node(int cpu);
>  #endif
>  
>  #ifndef set_numa_node
> diff --git a/init/main.c b/init/main.c
> index e24b0780fdff..b142e9c51161 100644
> --- a/init/main.c
> +++ b/init/main.c
> @@ -870,6 +870,18 @@ static void __init print_unknown_bootoptions(void)
>  	memblock_free(unknown_options, len);
>  }
>  
> +static void __init smp_prepare_boot_cpu_start(void)
> +{
> +	smp_prepare_boot_cpu();	/* arch-specific boot-cpu hooks */
> +
> +#ifdef CONFIG_USE_PERCPU_NUMA_NODE_ID
> +#ifndef cpu_to_node
> +	/* The early_cpu_to_node should be ready now. */
> +	cpu_to_node = early_cpu_to_node;
> +#endif
> +#endif
> +}
> +
>  asmlinkage __visible __init __no_sanitize_address __noreturn __no_stack_protector
>  void start_kernel(void)
>  {
> @@ -899,7 +911,7 @@ void start_kernel(void)
>  	setup_command_line(command_line);
>  	setup_nr_cpu_ids();
>  	setup_per_cpu_areas();
> -	smp_prepare_boot_cpu();	/* arch-specific boot-cpu hooks */
> +	smp_prepare_boot_cpu_start();
>  	boot_cpu_hotplug_init();
>  
>  	pr_notice("Kernel command line: %s\n", saved_command_line);
> @@ -1519,6 +1531,19 @@ void __init console_on_rootfs(void)
>  	fput(file);
>  }
>  
> +static void __init smp_prepare_cpus_done(unsigned int setup_max_cpus)
> +{
> +	/* Different ARCHs may override smp_prepare_cpus() */
> +	smp_prepare_cpus(setup_max_cpus);
> +
> +#ifdef CONFIG_USE_PERCPU_NUMA_NODE_ID
> +#ifndef cpu_to_node
> +	/* Change to the formal function. */
> +	cpu_to_node = _cpu_to_node;
> +#endif
> +#endif
> +}
> +
>  static noinline void __init kernel_init_freeable(void)
>  {
>  	/* Now the scheduler is fully set up and can do blocking allocations */
> @@ -1531,7 +1556,7 @@ static noinline void __init kernel_init_freeable(void)
>  
>  	cad_pid = get_pid(t