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Date:	Fri, 04 Apr 2014 11:48:50 +0800
From:	Michael wang <wangyun@...ux.vnet.ibm.com>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
CC:	linuxppc-dev@...ts.ozlabs.org, LKML <linux-kernel@...r.kernel.org>,
	benh@...nel.crashing.org, paulus@...ba.org,
	nfont@...ux.vnet.ibm.com, sfr@...b.auug.org.au,
	Andrew Morton <akpm@...ux-foundation.org>,
	rcj@...ux.vnet.ibm.com, jlarrew@...ux.vnet.ibm.com,
	alistair@...ple.id.au,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Subject: Re: [PATCH] power, sched: stop updating inside arch_update_cpu_topology()
 when nothing to be update

Hi, Srivatsa

Thanks for your reply :)

On 04/03/2014 04:50 PM, Srivatsa S. Bhat wrote:
[snip]
> 
> Now, the interesting thing to note here is that, if CPU0's node was already
> set as node0, *nothing* should go wrong, since its just a redundant update.
> However, if CPU0's original node mapping was something different, or if
> node0 doesn't even exist in the machine, then the system can crash.

By printk I confirmed all cpus was belong to node 1 at very beginning,
and things become magically after the wrong updating...

> 
> Have you verified that CPU0's node mapping is different from node 0?
> That is, boot the kernel with "numa=debug" in the kernel command line and
> it will print out the cpu-to-node associativity during boot. That way you
> can figure out what was the original associativity that was set. This will
> confirm the theory that the hypervisor sent a redundant update, but because
> of the weird pre-allocation using kzalloc that we do inside
> arch_update_cpu_topology(), we wrongly updated CPU0's mapping as CPU0 <-> Node0.

Associativity should changes, otherwise we won't continue the updating,
and empty updates[] was confirmed to show up inside
arch_update_cpu_topology().

What I can't make sure is whether this is legal, notify changes but no
changes happen sounds weird...however, even if it's legal, a check in
here still make sense IMHO.

Regards,
Michael Wang

> 
> 
> Regards,
> Srivatsa S. Bhat
> 
>> Thus we should stop the updating in such cases, this patch will achieve
>> this and fix the issue.
>>
>> CC: Benjamin Herrenschmidt <benh@...nel.crashing.org>
>> CC: Paul Mackerras <paulus@...ba.org>
>> CC: Nathan Fontenot <nfont@...ux.vnet.ibm.com>
>> CC: Stephen Rothwell <sfr@...b.auug.org.au>
>> CC: Andrew Morton <akpm@...ux-foundation.org>
>> CC: Robert Jennings <rcj@...ux.vnet.ibm.com>
>> CC: Jesse Larrew <jlarrew@...ux.vnet.ibm.com>
>> CC: "Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
>> CC: Alistair Popple <alistair@...ple.id.au>
>> Signed-off-by: Michael Wang <wangyun@...ux.vnet.ibm.com>
>> ---
>>  arch/powerpc/mm/numa.c |    9 +++++++++
>>  1 file changed, 9 insertions(+)
>>
>> diff --git a/arch/powerpc/mm/numa.c b/arch/powerpc/mm/numa.c
>> index 30a42e2..6757690 100644
>> --- a/arch/powerpc/mm/numa.c
>> +++ b/arch/powerpc/mm/numa.c
>> @@ -1591,6 +1591,14 @@ int arch_update_cpu_topology(void)
>>  		cpu = cpu_last_thread_sibling(cpu);
>>  	}
>>
>> +	/*
>> +	 * The 'cpu_associativity_changes_mask' could be cleared if
>> +	 * all the cpus it indicates won't change their node, in
>> +	 * which case the 'updated_cpus' will be empty.
>> +	 */
>> +	if (!cpumask_weight(&updated_cpus))
>> +		goto out;
>> +
>>  	stop_machine(update_cpu_topology, &updates[0], &updated_cpus);
>>
>>  	/*
>> @@ -1612,6 +1620,7 @@ int arch_update_cpu_topology(void)
>>  		changed = 1;
>>  	}
>>
>> +out:
>>  	kfree(updates);
>>  	return changed;
>>  }
>>
> 
> --
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