Re: [PATCH 1/1] powerpc: Clear cpu_sibling_map in cpu_die

[Brian King <brking@linux.vnet.ibm.com>]

On 08/24/2010 12:24 AM, Benjamin Herrenschmidt wrote:
> On Wed, 2010-08-11 at 15:34 -0500, Brian King wrote:
>> While testing CPU DLPAR, the following problem was discovered.
>> We were DLPAR removing the first CPU, which in this case was
>> logical CPUs 0-3. CPUs 0-2 were already marked offline and
>> we were in the process of offlining CPU 3. After marking
>> the CPU inactive and offline in cpu_disable, but before the
>> cpu was completely idle (cpu_die), we ended up in __make_request
>> on CPU 3. There we looked at the topology map to see which CPU
>> to complete the I/O on and found no CPUs in the cpu_sibling_map.
>> This resulted in the block layer setting the completion cpu
>> to be NR_CPUS, which then caused an oops when we tried to
>> complete the I/O.
>>
>> Fix this by delaying clearing the sibling map of the cpu we
>> are offlining for the cpu we are offlining until cpu_die.
> 
> So I'm not getting a clear mental picture of the situation, sorry about
> that.
> 
> We are offlining CPU 3, and we have already marked it inactive and
> online, so how come we end up in __make_request() on it at this stage

I'm not sure about that. My thought was that until we get into cpu_die,
the cpu could still be executing code.

> and shouldn't it be the block layer that notices that it's targeting an
> offlined CPU ?

It could be easily fixed in blk_cpu_to_group as well. I'll look into
this.

> IE. I have doubts about leaving a CPU in the sibling map which isn't
> online... Wouldn't we end up "scheduling" things to it after it's
> supposed to have freed itself of everything (timers, workqueues,
> etc...) ?

I was assuming this wouldn't happen since the cpu is no longer online.


Thanks,

Brian

> 
> As I said, I'm probably missing a part of the puzzle ..
> 
> Ben.
> 
>> Signed-off-by: Brian King <brking@linux.vnet.ibm.com>
>> ---
>>
>>  arch/powerpc/kernel/smp.c |    9 +++++++--
>>  1 file changed, 7 insertions(+), 2 deletions(-)
>>
>> diff -puN arch/powerpc/kernel/smp.c~powerpc_sibling_map_offline arch/powerpc/kernel/smp.c
>> --- linux-2.6/arch/powerpc/kernel/smp.c~powerpc_sibling_map_offline	2010-08-09 16:49:47.000000000 -0500
>> +++ linux-2.6-bjking1/arch/powerpc/kernel/smp.c	2010-08-09 16:49:47.000000000 -0500
>> @@ -598,8 +598,11 @@ int __cpu_disable(void)
>>  	/* Update sibling maps */
>>  	base = cpu_first_thread_in_core(cpu);
>>  	for (i = 0; i < threads_per_core; i++) {
>> -		cpumask_clear_cpu(cpu, cpu_sibling_mask(base + i));
>> -		cpumask_clear_cpu(base + i, cpu_sibling_mask(cpu));
>> +		if ((base + i) != cpu) {
>> +			cpumask_clear_cpu(cpu, cpu_sibling_mask(base + i));
>> +			cpumask_clear_cpu(base + i, cpu_sibling_mask(cpu));
>> +		}
>> +
>>  		cpumask_clear_cpu(cpu, cpu_core_mask(base + i));
>>  		cpumask_clear_cpu(base + i, cpu_core_mask(cpu));
>>  	}
>> @@ -641,6 +644,8 @@ void cpu_hotplug_driver_unlock()
>>  
>>  void cpu_die(void)
>>  {
>> +	cpumask_clear_cpu(smp_processor_id(), cpu_sibling_mask(smp_processor_id()));
>> +
>>  	if (ppc_md.cpu_die)
>>  		ppc_md.cpu_die();
>>  }
>> _
> 
> 
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-- 
Brian King
Linux on Power Virtualization
IBM Linux Technology Center