linux-kernel - Re: [PATCH RFC] x86: Reduce MAX_LOCAL_APIC and MAX_IO

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Message-ID: <560C15EB.1000200@linux.intel.com>
Date:	Thu, 1 Oct 2015 01:03:39 +0800
From:	Jiang Liu <jiang.liu@...ux.intel.com>
To:	Denys Vlasenko <dvlasenk@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>
Cc:	Ingo Molnar <mingo@...nel.org>, Len Brown <len.brown@...el.com>,
	x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC] x86: Reduce MAX_LOCAL_APIC and MAX_IO_APICS

On 2015/9/30 23:49, Denys Vlasenko wrote:
> On 09/30/2015 05:11 PM, Thomas Gleixner wrote:
>> On Fri, 25 Sep 2015, Denys Vlasenko wrote:
>>>  
>>> +config MAX_LAPIC_ID
>>> +	int "Maximum APIC ID"
>>> +	range 8 32768
>>> +	default "8"
>>> +	---help---
>>> +	  Use this option to set maximum allowed Local APIC ID higher than
>>> +	  maximum number of CPUs. This may be necessary for machines
>>> +	  with large number of processor sockets and non-contiguous
>>> +	  LAPIC numbering.
>>> +	  This setting will be automatically rounded up, if necessary.
>>
>> This is wrong. If you would limit the APIC IDs then you really break
>> stuff. You can only limit the number of APICs.
> 
> This CONFIG setting allows to _increase_ max APIC ID.
> 
> Check out this part of the patch:
> 
> +/*
> + * Allow non-contiguous APIC IDs for small machines:
> + * APIC ids 0..15 are valid in any config.
> + * Typical SMP machines have contiguous APIC IDs: 0..NR_CPUS-1.
> + * CONFIG_MAX_LAPIC_ID can override.
> + */
> +#define MAX_LOCAL_APIC (NR_CPUS < 16 ? 16 : NR_CPUS)
> +#if MAX_LOCAL_APIC < CONFIG_MAX_LAPIC_ID
> +# undef  MAX_LOCAL_APIC
> +# define MAX_LOCAL_APIC CONFIG_MAX_LAPIC_ID
>  #endif
> 
> 
> For example, if you'd build with NR_CPUS=128
> (for example, Fedora kernels do that),
> max accepted APIC id will be NR_CPUS-1 = 127
> even if CONFIX_MAX_LAPIC_ID is 8.
> 
> If Fedora would want to support APIC ids up to
> 255, it will need to set CONFIG_MAX_LAPIC_ID=256.
> 
> Otherwise, if it's happy with "only" supporting up to 128,
> it does not need to change CONFIG_MAX_LAPIC_ID from default.
> 
> With current kernels, max APIC id for any kernel is 32768,
> which is in most cases way bigger than necessary.
> 
> 
> Perhaps I need to update the text.
> Something like:
> 
> - This setting will be automatically rounded up, if necessary
> + This setting will be increased to NR_CPUS, if necessary
> 
> 
>> ACPI: LAPIC (acpi_id[0x00] lapic_id[0x00] enabled)
>> ACPI: LAPIC (acpi_id[0x01] lapic_id[0x02] enabled)
> 
> Does it mean that on a 2-CPU machine, CPU #1 has APIC_ID=2?
Yes, APIC IDs are assigned by BIOS and may not be continuous.
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