linux-kernel - Re: x86_64: MAX_LOCAL

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Date:	Fri, 25 Sep 2015 21:44:41 +0200
From:	Denys Vlasenko <dvlasenk@...hat.com>
To:	Jiang Liu <jiang.liu@...ux.intel.com>,
	Ingo Molnar <mingo@...nel.org>
CC:	Thomas Gleixner <tglx@...utronix.de>,
	Len Brown <len.brown@...el.com>, X86 ML <x86@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: x86_64: MAX_LOCAL_APIC way too big?

On 09/25/2015 06:56 PM, Jiang Liu wrote:
> On 2015/9/26 0:16, Denys Vlasenko wrote:
>> 32 thousand APICs? That's a lot. Especially
>> considering that event with CONFIG_MAXSMP=y,
>> NR_CPUS is "only" 8096.
>>
>> After a quick glance through code, it looks like
>> such a big value causes several data arrays to be
>> quite oversized:
>>
>> phys_cpu_present_map is 4 kbytes (one bit per apicid)
>> __apicid_to_node is 64 kbytes
>> apic_version is 128 kbytes (!!!)
>>
...
>> Maybe we can reduce MAX_LOCAL_APIC?
>> Why it has to be this big in the first place?
>>
>> IIRC: APIC id at first was just a 8-bit quantity,
>> then x2apic mode it was extended to 32 bits.
>>
>> On "usual" systems, apic ids simply go from zero
>> to maximum logical CPU number, mirroring CPU ids.


> Hi Denys,
> 	The above assumption is risky with modern
> x86 platforms. APIC ids are assigned by firmware,
> and may be discrete.


I guess it means a new CONFIG option will be needed then,
for such machines. I'll send an RFC patch now.
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