Re: [RFC PATCH 8/8] powerpc/papr_scm: Use FORM2 associativity details

[Daniel Henrique Barboza <danielhb413@gmail.com>]

On 6/17/21 4:46 AM, David Gibson wrote:
> On Tue, Jun 15, 2021 at 12:35:17PM +0530, Aneesh Kumar K.V wrote:
>> David Gibson <david@gibson.dropbear.id.au> writes:
>>
>>> On Tue, Jun 15, 2021 at 11:27:50AM +0530, Aneesh Kumar K.V wrote:
>>>> David Gibson <david@gibson.dropbear.id.au> writes:
>>>>
>>>>> On Mon, Jun 14, 2021 at 10:10:03PM +0530, Aneesh Kumar K.V wrote:
>>>>>> FORM2 introduce a concept of secondary domain which is identical to the
>>>>>> conceept of FORM1 primary domain. Use secondary domain as the numa node
>>>>>> when using persistent memory device. For DAX kmem use the logical domain
>>>>>> id introduced in FORM2. This new numa node
>>>>>>
>>>>>> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
>>>>>> ---
>>>>>>   arch/powerpc/mm/numa.c                    | 28 +++++++++++++++++++++++
>>>>>>   arch/powerpc/platforms/pseries/papr_scm.c | 26 +++++++++++++--------
>>>>>>   arch/powerpc/platforms/pseries/pseries.h  |  1 +
>>>>>>   3 files changed, 45 insertions(+), 10 deletions(-)
>>>>>>
>>>>>> diff --git a/arch/powerpc/mm/numa.c b/arch/powerpc/mm/numa.c
>>>>>> index 86cd2af014f7..b9ac6d02e944 100644
>>>>>> --- a/arch/powerpc/mm/numa.c
>>>>>> +++ b/arch/powerpc/mm/numa.c
>>>>>> @@ -265,6 +265,34 @@ static int associativity_to_nid(const __be32 *associativity)
>>>>>>   	return nid;
>>>>>>   }
>>>>>>   
>>>>>> +int get_primary_and_secondary_domain(struct device_node *node, int *primary, int *secondary)
>>>>>> +{
>>>>>> +	int secondary_index;
>>>>>> +	const __be32 *associativity;
>>>>>> +
>>>>>> +	if (!numa_enabled) {
>>>>>> +		*primary = NUMA_NO_NODE;
>>>>>> +		*secondary = NUMA_NO_NODE;
>>>>>> +		return 0;
>>>>>> +	}
>>>>>> +
>>>>>> +	associativity = of_get_associativity(node);
>>>>>> +	if (!associativity)
>>>>>> +		return -ENODEV;
>>>>>> +
>>>>>> +	if (of_read_number(associativity, 1) >= primary_domain_index) {
>>>>>> +		*primary = of_read_number(&associativity[primary_domain_index], 1);
>>>>>> +		secondary_index = of_read_number(&distance_ref_points[1], 1);
>>>>>
>>>>> Secondary ID is always the second reference point, but primary depends
>>>>> on the length of resources?  That seems very weird.
>>>>
>>>> primary_domain_index is distance_ref_point[0]. With Form2 we would find
>>>> both primary and secondary domain ID same for all resources other than
>>>> persistent memory device. The usage w.r.t. persistent memory is
>>>> explained in patch 7.
>>>
>>> Right, I misunderstood
>>>
>>>>
>>>> With Form2 the primary domainID and secondary domainID are used to identify the NUMA nodes
>>>> the kernel should use when using persistent memory devices.
>>>
>>> This seems kind of bogus.  With Form1, the primary/secondary ID are a
>>> sort of heirarchy of distance (things with same primary ID are very
>>> close, things with same secondary are kinda-close, etc.).  With Form2,
>>> it's referring to their effective node for different purposes.
>>>
>>> Using the same terms for different meanings seems unnecessarily
>>> confusing.
>>
>> They are essentially domainIDs. The interpretation of them are different
>> between Form1 and Form2. Hence I kept referring to them as primary and
>> secondary domainID. Any suggestion on what to name them with Form2?
> 
> My point is that reusing associativity-reference-points for something
> with completely unrelated semantics seems like a very poor choice.


I agree that this reuse can be confusing. I could argue that there is
precedent for that in PAPR - FORM0 puts a different spin on the same
property as well - but there is no need to keep following existing PAPR
practices in new spec (and some might argue it's best not to).

As far as QEMU goes, renaming this property to "numa-associativity-mode"
(just an example) is a quick change to do since we separated FORM1 and FORM2
code over there.

Doing such a rename can also help with the issue of having to describe new
FORM2 semantics using "least significant boundary" or "primary domain" or
any FORM0|FORM1 related terminology.


Thanks,


Daniel



> 
>>>> Persistent memory devices
>>>> can also be used as regular memory using DAX KMEM driver and primary domainID indicates
>>>> the numa node number OS should use when using these devices as regular memory. Secondary
>>>> domainID is the numa node number that should be used when using this device as
>>>> persistent memory.
>>>
>>> It's weird to me that you'd want to consider them in different nodes
>>> for those different purposes.
>>
>>
>>     --------------------------------------
>>    |                            NUMA node0 |
>>    |    ProcA -----> MEMA                  |
>>    |     |                                 |
>>    |	|                                 |
>>    |	-------------------> PMEMB        |
>>    |                                       |
>>     ---------------------------------------
>>
>>     ---------------------------------------
>>    |                            NUMA node1 |
>>    |                                       |
>>    |    ProcB -------> MEMC                |
>>    |	|                                 |
>>    |	-------------------> PMEMD        |
>>    |                                       |
>>    |                                       |
>>     ---------------------------------------
>>   
>>
>> For a topology like the above application running of ProcA wants to find out
>> persistent memory mount local to its NUMA node. Hence when using it as
>> pmem fsdax mount or devdax device we want PMEMB to have associativity
>> of NUMA node0 and PMEMD to have associativity of NUMA node 1. But when
>> we want to use it as memory using dax kmem driver, we want both PMEMB
>> and PMEMD to appear as memory only NUMA node at a distance that is
>> derived based on the latency of the media.
> 
> I'm still not understanding why the latency we care about is different
> in the two cases.  Can you give an example of when this would result
> in different actual node assignments for the two different cases?
>