linux-kernel - Re: [PATCH 3/3] x86/apic: Use smaller array for __apicid_to

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Date:	Tue, 13 Oct 2015 14:55:58 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Jiang Liu <jiang.liu@...ux.intel.com>
cc:	Daniel J Blueman <daniel@...ascale.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Ingo Molnar <mingo@...nel.org>,
	Len Brown <len.brown@...el.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] x86/apic: Use smaller array for __apicid_to_node[]
 mapping

On Tue, 13 Oct 2015, Jiang Liu wrote:
> On 2015/10/12 18:25, Thomas Gleixner wrote:
> > On Mon, 12 Oct 2015, Daniel J Blueman wrote:
> >> Another approach which may be suitable without changing SRAT parsing to be
> >> after the memory allocator is up, is to exploit the associativity of the
> >> bottom APIC ID bits.
> > 
> > What's the problem with moving (SRAT/ACPI/whatever) APIC parsing after
> > the memory allocator is up and available?
> Hi Thomas,
> 	The work flow is as below at boot:
> 1) figure out memory NUMA topology info by walking ACPI table or probing
>    AMD northbirdge.
> 2) initialize memory allocation based on memory NUMA topology.
> 
> And to make code simple, it also scan CPU NUMA topology in step 1, so
> we could avoid walking ACPI tables twice. On the other hand, there are
> several subsystems having code pattern as follow before booting APs.
> up:
> for_each_possible_cpu(cpu)
> 	alloc_page_node(size, cpu_to_node(cpu))
> So it's a little hard to find a suitable hook point to delay CPU NUMA
> topology scanning after memory allocator is ready.

Not really. The memory allocator is available very early and long
before smp_prepare_cpus(). So we should try hard to move it after that
point, even if that means that we need to walk the tables twice.

Alternatively, use a bootmem allocation and convert it to a radix tree
when the allocator is up.

Thanks,

	tglx


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