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Date: Fri, 19 Jan 2024 10:42:04 +0200
From: Mike Rapoport <rppt@kernel.org>
To: Shijie Huang <shijie@amperemail.onmicrosoft.com>
Cc: Yury Norov <yury.norov@gmail.com>,
	Huang Shijie <shijie@os.amperecomputing.com>,
	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: <Zao13I4Bb0tur0fZ@kernel.org>
References: <20240119033227.14113-1-shijie@os.amperecomputing.com>
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On Fri, Jan 19, 2024 at 02:46:16PM +0800, Shijie Huang wrote:
> 
> 在 2024/1/19 12:42, Yury Norov 写道:
> > 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?
> #./scripts/bloat-o-meter vmlinux vmlinux.new
> add/remove: 6/1 grow/shrink: 61/51 up/down: 1168/-588 (580)
> Function                                     old     new   delta
> numa_update_cpu                              148     244     +96
> 
>  ...................................................................................................................................(to many to skip)
> 
> Total: Before=32990130, After=32990710, chg +0.00%
 
It's not only about text size, the indirect call also hurts performance
 
> > 
> > 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.
> 
> It is easy to change to early_cpu_to_node() for sched_init(),
> init_sched_fair_class()
> 
> and workqueue_init_early(). These three places call the cpu_to_node() in the
> __init function.
> 
> 
> But it is a little hard to change the early_trace_init(), since it calls
> cpu_to_node in the deep
> 
> function stack:
> 
>   early_trace_init() --> ring_buffer_alloc() -->rb_allocate_cpu_buffer()
> 
> 
> For early_trace_init(), we need to change more code.
> 
> 
> Anyway, If we think it is not a good idea to change the common code, I am
> oaky too.
 
Is there a fundamental reason to have early_cpu_to_node() at all?
It seems that all the mappings are known by the end of setup_arch() and the
initialization of numa_node can be moved earlier. 
 
> > 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.
> 
> Even we set the numa_node with NUMA_NO_NODE, it does not always produce
> error.
> 
> Please see the alloc_pages_node().
> 
> 
> Thanks
> 
> Huang Shijie
> 

-- 
Sincerely yours,
Mike.