Subject: Re: [PATCH] power, sched: stop updating inside arch_update_cpu_topology() when nothing to be update

From: "Srivatsa S. Bhat" <[email-masked]>
Date: Mon, 07 Apr 2014 15:21:06 +0530

Hi Michael,

On 04/04/2014 09:18 AM, Michael wang wrote:
> 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...
>

Ok, thanks!
 
>>
>> 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().
> 

Ah, ok, that makes it very clear. So, I agree that your patch is correct,
but I think the comment in your patch can be enhanced a bit. I'll suggest
something if I manage to come up with a better wording.

> 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.
> 

That looks like a bug in the hypervisor/firmware. But the Linux kernel should
be able to handle such NULL updates without crashing. So yes, your patch makes
sense to me.

Thank you!

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 <[email-masked]>
>>> CC: Paul Mackerras <[email-masked]>
>>> CC: Nathan Fontenot <[email-masked]>
>>> CC: Stephen Rothwell <[email-masked]>
>>> CC: Andrew Morton <[email-masked]>
>>> CC: Robert Jennings <[email-masked]>
>>> CC: Jesse Larrew <[email-masked]>
>>> CC: "Srivatsa S. Bhat" <[email-masked]>
>>> CC: Alistair Popple <[email-masked]>
>>> Signed-off-by: Michael Wang <[email-masked]>
>>> ---
>>>  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;
>>>  }
>>>
>>