linux-kernel - Re: [PATCH 1/2] x86/cpu hotplug: make apicid <--> cpuid mapping persistent

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Date:	Fri, 27 Mar 2015 01:31:23 +0900
From:	Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Gu Zheng <guz.fnst@...fujitsu.com>
CC:	<linux-kernel@...r.kernel.org>, <tj@...nel.org>,
	<laijs@...fujitsu.com>, <isimatu.yasuaki@...fujitsu.com>,
	<tangchen@...fujitsu.com>, <izumi.taku@...fujitsu.com>
Subject: Re: [PATCH 1/2] x86/cpu hotplug: make apicid <--> cpuid mapping persistent

On 2015/03/26 13:55, Gu Zheng wrote:
> Hi Kame-san,
> On 03/26/2015 11:19 AM, Kamezawa Hiroyuki wrote:
> 
>> On 2015/03/26 11:17, Gu Zheng wrote:
>>> Previously, we build the apicid <--> cpuid mapping when the cpu is present, but
>>> the relationship will be changed if the cpu/node hotplug happenned, because we
>>> always choose the first free cpuid for the hot added cpu (whether it is new-add
>>> or re-add), so this the cpuid <--> node mapping changed if node hot plug
>>> occurred, and it causes the wq sub-system allocation failture:
>>>     ==
>>>        SLUB: Unable to allocate memory on node 2 (gfp=0x80d0)
>>>         cache: kmalloc-192, object size: 192, buffer size: 192, default
>>> order:
>>>       1, min order: 0
>>>         node 0: slabs: 6172, objs: 259224, free: 245741
>>>         node 1: slabs: 3261, objs: 136962, free: 127656
>>>     ==
>>> So here we build the persistent [lapic id] <--> cpuid mapping when the cpu first
>>> present, and never change it.
>>>
>>> Suggested-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
>>> Signed-off-by: Gu Zheng <guz.fnst@...fujitsu.com>
>>> ---
>>>    arch/x86/kernel/apic/apic.c |   31 ++++++++++++++++++++++++++++++-
>>>    1 files changed, 30 insertions(+), 1 deletions(-)
>>>
>>> diff --git a/arch/x86/kernel/apic/apic.c b/arch/x86/kernel/apic/apic.c
>>> index ad3639a..d539ebc 100644
>>> --- a/arch/x86/kernel/apic/apic.c
>>> +++ b/arch/x86/kernel/apic/apic.c
>>> @@ -2038,6 +2038,30 @@ void disconnect_bsp_APIC(int virt_wire_setup)
>>>    	apic_write(APIC_LVT1, value);
>>>    }
>>>    
>>> +/*
>>> + * Logic cpu number(cpuid) to local APIC id persistent mappings.
>>> + * Do not clear the mapping even if cpu hot removed.
>>> + * */
>>> +static int apicid_to_x86_cpu[MAX_LOCAL_APIC] = {
>>> +	[0 ... MAX_LOCAL_APIC - 1] = -1,
>>> +};
>>
>>
>> This patch cannot handle x2apic, which is 32bit.
> 
> IMO, if the apicid is too big (larger than MAX_LOCAL_APIC), we will skip
> generating a logic cpu number for it, so it seems no problem here.
> 
you mean MAX_LOCAL_APIC=32768 ? ....isn't it too wasting ?

Anyway, APIC IDs are sparse values. Please use proper structure.

Thanks,
-Kame

>>
>> As far as I understand, it depends on CPU's spec and the newest cpu has 9bit apicid, at least.
>>
>> But you can't create inifinit array.
>>
>> If you can't allocate the array dynamically, How about adding
>>
>>   static int cpuid_to_apicid[MAX_CPU] = {}
>>
>> or using idr library ? (please see lib/idr.c)
>>
>> I guess you can update this map after boot(after mm initialization)
>> and make use of idr library.
>>
>> About this patch, Nack.
>>
>> -Kame
>>
>>
>>
>>> +
>>> +/*
>>> + * Internal cpu id bits, set the bit once cpu present, and never clear it.
>>> + * */
>>> +static cpumask_t cpuid_mask = CPU_MASK_NONE;
>>> +
>>> +static int get_cpuid(int apicid)
>>> +{
>>> +	int cpuid;
>>> +
>>> +	cpuid = apicid_to_x86_cpu[apicid];
>>> +	if (cpuid == -1)
>>> +		cpuid = cpumask_next_zero(-1, &cpuid_mask);
>>> +
>>> +	return cpuid;
>>> +}
>>> +
>>>    int generic_processor_info(int apicid, int version)
>>>    {
>>>    	int cpu, max = nr_cpu_ids;
>>> @@ -2115,7 +2139,10 @@ int generic_processor_info(int apicid, int version)
>>>    		 */
>>>    		cpu = 0;
>>>    	} else
>>> -		cpu = cpumask_next_zero(-1, cpu_present_mask);
>>> +		cpu = get_cpuid(apicid);
>>> +
>>> +	/* Store the mapping */
>>> +	apicid_to_x86_cpu[apicid] = cpu;
>>>    
>>>    	/*
>>>    	 * Validate version
>>> @@ -2144,6 +2171,8 @@ int generic_processor_info(int apicid, int version)
>>>    	early_per_cpu(x86_cpu_to_logical_apicid, cpu) =
>>>    		apic->x86_32_early_logical_apicid(cpu);
>>>    #endif
>>> +	/* Mark this cpu id as uesed (already mapping a local apic id) */
>>> +	cpumask_set_cpu(cpu, &cpuid_mask);
>>>    	set_cpu_possible(cpu, true);
>>>    	set_cpu_present(cpu, true);
>>>    
>>>
>>
>>
>> .
>>
> 
> 


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