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

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Date:	Sat, 03 Oct 2015 22:26:32 +0200
From:	Denys Vlasenko <dvlasenk@...hat.com>
To:	Ingo Molnar <mingo@...nel.org>
CC:	Daniel J Blueman <daniel@...ascale.com>,
	Jiang Liu <jiang.liu@...ux.intel.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	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 10/03/2015 09:44 AM, Ingo Molnar wrote:
> 
> * Denys Vlasenko <dvlasenk@...hat.com> wrote:
> 
>> @@ -56,16 +56,34 @@ early_param("numa", numa_setup);
>>  /*
>>   * apicid, cpu, node mappings
>>   */
>> -s16 __apicid_to_node[MAX_LOCAL_APICID] = {
>> -	[0 ... MAX_LOCAL_APICID-1] = NUMA_NO_NODE
>> +
>> +struct apicid_to_node __apicid_to_node[NR_CPUS] = {
>> +	[0 ... NR_CPUS-1] = {-1, NUMA_NO_NODE}
>>  };
>>  
>> +void set_apicid_to_node(int apicid, s16 node)
>> +{
>> +	static int ent;
> 
> having such statics inside functions is really obscure and makes review harder.
> I had to look twice to see it. Please move it outside and also name it 
> appropriately.

Just to confirm: you want it to be a static data, but not inside a function?


>> +	/* Protect against small kernel on large system */
>> +	if (ent >= NR_CPUS)
>> +		return;
>> +
>> +	__apicid_to_node[ent].apicid = apicid;
>> +	__apicid_to_node[ent].node = node;
>> +	ent++;
>> +}
> 
> So what happens if we run a small kernel and run out of entries? We just silently 
> seem to return, no warning, no nothing - the system will likely fail to boot in 
> myserious ways, right?

Good question.

I tested this.

I built a NR_CPUS=8 kernel and booted it on 144 cpu and 240 cpu machines.
Both booted fine:

[    0.000000] ACPI: Local APIC address 0xfee00000
[    0.000000] ACPI: NR_CPUS/possible_cpus limit of 8 reached.  Processor 8/0x16 ignored.
...
[    0.000000] ACPI: NR_CPUS/possible_cpus limit of 8 reached.  Processor 143/0xf7 ignored.


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