linux-kernel - Re: [Need Help] Cpuhotplug operations on 32-bit mode of xeon-64bit processor crashes the system.

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Message-ID: <45C0206E.9030505@in.ibm.com>
Date:	Wed, 31 Jan 2007 10:21:58 +0530
From:	Srinivasa DS <srinivasa@...ibm.com>
To:	unlisted-recipients:; (no To-header on input)
CC:	"Siddha, Suresh B" <suresh.b.siddha@...el.com>,
	ashok.raj@...el.AU.IBM.COM, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...e.hu>, mingo@...hat.com
Subject: Re: [Need Help] Cpuhotplug operations on 32-bit mode of xeon-64bit
 processor crashes the system.

Siddha, Suresh B wrote:
> Sorry for my delayed response. I was away on vacation.
>
> What platform is this? what do you mean by crashing? Do you see a
> system freeze or oops?
>   
Its xeon-64 bit processor,running in 32-bit compatibility 
mode(i386-code). We have not seen this problem in x86_64 envioronment. 
It happens in 32-bit compatibility mode.

Problem is in calculation of apicid's and delivery of IPI's.

I saw a oops,when I do cpuhotplug operations on it.

If you want any further information,please free to ask.

Thanks
Srinivasa Ds

> thanks,
> suresh
>
> On Mon, Jan 22, 2007 at 01:42:48PM +0530, Srinivasa Ds wrote:
>   
>> I saw cpuhotplug operations on 32-bit mode of xeon-64bit processors 
>> crashing the system. This happens on latest 2.6.20-rc5 kernel also. Same 
>> (i386 cpuhotplug code) runs fine on xeon-32bit processors.
>> Steps to reproduce.
>> ====================
>> echo 0 > /sys/devices/system/cpu/cpu6/online
>> echo 1 > /sys/devices/system/cpu/cpu6/online
>> ================================
>> dmesg shows.
>> ==============
>> Breaking affinity for irq 4
>> cpu_mask_to_apicid: Not a valid mask!
>> CPU 6 is now offline
>> =======================
>>
>> On debugging the problem, I found that problem is not in cpuhotplug code 
>> but in apic part. Execution of  "stale" IPI's by onlined cpus(which we 
>> offlined earlier) is causing the crash. Now we need to debug,why IPI's 
>> are reaching the offlined cpu's too.
>>
>> 1)   During the calculation of apicid's, if cpu to which IPI has to 
>> deliver is not in
>> same apic cluster,it prints "Not a valid mask" error and returns "0xFF" 
>> which means broadcast the IPI's to all cpus(which are offlined too) and 
>> hence the problem.
>>
>> 2) I booted the system with maxcpus=2 boot parameter, and tried cpu 
>> hotplugging on it.
>> but still problem recreates(I think there is no concept of apic clusters 
>> if there are only 2 cpus). Hence it makes me to conclude that problem is 
>> in delivery of IPI's.
>>
>> So Iam completely stuck here. Iam not able to move forward in debugging. 
>> So could someone(may be intel folks) please throw some light on this.
>>
>> Thanks in advance
>>   Srinivasa DS
>>   LTC-IBM
>>     

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