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Date: Wed, 24 Jan 2024 09:41:18 -0800
From: Yury Norov <yury.norov@gmail.com>
To: "Lameter, Christopher" <cl@os.amperecomputing.com>
Cc: Huang Shijie <shijie@os.amperecomputing.com>,
	gregkh@linuxfoundation.org, patches@amperecomputing.com,
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Subject: Re: [PATCH v2] NUMA: Early use of cpu_to_node() returns 0 instead of
 the correct node id
Message-ID: <ZbFLvnMQ3wsQ0pIF@yury-ThinkPad>
References: <20240123045843.75969-1-shijie@os.amperecomputing.com>
 <4a13353c-cf4b-a388-5776-389c61c63ec0@os.amperecomputing.com>
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In-Reply-To: <4a13353c-cf4b-a388-5776-389c61c63ec0@os.amperecomputing.com>

On Wed, Jan 24, 2024 at 09:19:00AM -0800, Lameter, Christopher wrote:
> On Tue, 23 Jan 2024, Huang Shijie wrote:
> 
> > During the kernel booting, the generic cpu_to_node() is called too early in
> > arm64, powerpc and riscv when CONFIG_NUMA is enabled.
> > 
> > For arm64/powerpc/riscv, 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().
> 
> Would  you please fix this cleanly without a function pointer?
> 
> What I think needs to be done is a patch series.
> 
> 1. Instrument cpu_to_node so that some warning is issued if it is used too
> early. Preloading the array with NUMA_NO_NODE would allow us to do that.

By preloading do you mean compile-time initialization?
 
> 2. Implement early_cpu_to_node on platforms that currently do not have it.
> 
> 3. A series of patches that fix each place where cpu_to_node is used too
> early.

Agree. This is the right way to go. And pretty well all of it was discussed
in v1, isn't?

Thanks,
Yury