Re: [RFC PATCH] mm/mempolicy: add MPOL_PREFERRED_STRICT memory policy

["Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>]

On 10/13/21 18:20, Michal Hocko wrote:
> On Wed 13-10-21 18:05:49, Aneesh Kumar K.V wrote:
>> On 10/13/21 16:18, Michal Hocko wrote:
>>> On Wed 13-10-21 12:42:34, Michal Hocko wrote:
>>>> [Cc linux-api]
>>>>
>>>> On Wed 13-10-21 15:15:39, Aneesh Kumar K.V wrote:
>>>>> This mempolicy mode can be used with either the set_mempolicy(2)
>>>>> or mbind(2) interfaces.  Like the MPOL_PREFERRED interface, it
>>>>> allows an application to set a preference node from which the kernel
>>>>> will fulfill memory allocation requests. Unlike the MPOL_PREFERRED mode,
>>>>> it takes a set of nodes. The nodes in the nodemask are used as fallback
>>>>> allocation nodes if memory is not available on the preferred node.
>>>>> Unlike MPOL_PREFERRED_MANY, it will not fall back memory allocations
>>>>> to all nodes in the system. Like the MPOL_BIND interface, it works over a
>>>>> set of nodes and will cause a SIGSEGV or invoke the OOM killer if
>>>>> memory is not available on those preferred nodes.
>>>>>
>>>>> This patch helps applications to hint a memory allocation preference node
>>>>> and fallback to _only_ a set of nodes if the memory is not available
>>>>> on the preferred node.  Fallback allocation is attempted from the node which is
>>>>> nearest to the preferred node.
>>>>>
>>>>> This new memory policy helps applications to have explicit control on slow
>>>>> memory allocation and avoids default fallback to slow memory NUMA nodes.
>>>>> The difference with MPOL_BIND is the ability to specify a preferred node
>>>>> which is the first node in the nodemask argument passed.
>>>
>>> I am sorry but I do not understand the semantic diffrence from
>>> MPOL_BIND. Could you be more specific please?
>>>
>>
>>
>>
>> MPOL_BIND
>> 	This mode specifies that memory must come from the set of
>> 	nodes specified by the policy.  Memory will be allocated from
>> 	the node in the set with sufficient free memory that is
>> 	closest to the node where the allocation takes place.
>>
>>
>> MPOL_PREFERRED_STRICT
>> 	This mode specifies that the allocation should be attempted
>> 	from the first node specified in the nodemask of the policy.
>> 	If that allocation fails, the kernel will search other nodes
>> 	in the nodemask, in order of increasing distance from the
>> 	preferred node based on information provided by the platform   firmware.
>>
>> The difference is the ability to specify the preferred node as the first
>> node in the nodemask and all fallback allocations are based on the distance
>> from the preferred node. With MPOL_BIND they base based on the node where
>> the allocation takes place.
> 
> OK, this makes it more clear. Thanks!
> 
> I am still not sure the semantic makes sense though. Why should
> the lowest node in the nodemask have any special meaning? What if it is
> a node with a higher number that somebody preferes to start with?
> 

That is true. I haven't been able to find an easy way to specify the 
preferred node other than expressing it as first node in the node mask. 
Yes, it limits the usage of the policy. Any alternate suggestion?

We could do
set_mempolicy(MPOLD_PREFERRED, nodemask(nodeX)))
set_mempolicy(MPOLD_PREFFERED_EXTEND, nodemask(fallback nodemask for 
above PREFERRED policy))

But that really complicates the interface?

-aneesh