On 2/20/24 12:02 PM, Aneesh Kumar K.V wrote:
> On 2/20/24 11:55 AM, Huang, Ying wrote:
>> "Aneesh Kumar K.V" <aneesh.kumar@xxxxxxxxxx> writes:
>>
>>> On 2/20/24 6:51 AM, Andrew Morton wrote:
>>>> On Mon, 19 Feb 2024 14:04:23 +0530 Donet Tom <donettom@xxxxxxxxxxxxx> wrote:
>>>>
>>>>>>> --- a/mm/mempolicy.c
>>>>>>> +++ b/mm/mempolicy.c
>>>>>>> @@ -2526,7 +2526,7 @@ int mpol_misplaced(struct folio *folio, struct vm_area_struct *vma,
>>>>>>> if (node_isset(curnid, pol->nodes))
>>>>>>> goto out;
>>>>>>> z = first_zones_zonelist(
>>>>>>> - node_zonelist(numa_node_id(), GFP_HIGHUSER),
>>>>>>> + node_zonelist(thisnid, GFP_HIGHUSER),
>>>>>>> gfp_zone(GFP_HIGHUSER),
>>>>>>> &pol->nodes);
>>>>>>> polnid = zone_to_nid(z->zone);
>>>>>> int thisnid = cpu_to_node(thiscpu);
>>>>>>
>>>>>> Is there any dofference between numa_node_id() and
>>>>>> cpu_to_node(raw_smp_processor_id())? And it it explicable that we're
>>>>>> using one here and not the other?
>>>>>
>>>>> Hi Andrew
>>>>>
>>>>> Both numa_node_id() and cpu_to_node(raw_smp_processor_id()) return the current execution node id,
>>>>> Since the current execution node is already fetched at the beginning (thisnid) we can reuse it instead of getting it again.
>>>>
>>>> Sure, but mine was a broader thought: why do we have both? Is one
>>>> preferable and if so why?
>>>
>>> IIUC these are two helpers to fetch current numa node id. and either of them can be used based on need. The default implementation shows the details.
>>> (One small difference is numa_node_id() can use optimized per cpu reader because it is fetching the per cpu variable of the currently running cpu.)
>>>
>>> #ifndef numa_node_id
>>> /* Returns the number of the current Node. */
>>> static inline int numa_node_id(void)
>>> {
>>> return raw_cpu_read(numa_node);
>>> }
>>> #endif
>>>
>>> #ifndef cpu_to_node
>>> static inline int cpu_to_node(int cpu)
>>> {
>>> return per_cpu(numa_node, cpu);
>>> }
>>> #endif
>>>
>>> In mpol_misplaced function, we need the cpu details because we are using that in other place (should_numa_migreate_memory()). So it makes it easy
>>> to use cpu_to_node(thiscpu) instead of numa_node_id().
>>
>> IIUC, numa_node_id() is faster than cpu_to_node(thiscpu), even if we
>> have thiscpu already. cpu_to_node() is mainly used to get the node of
>> NOT current CPU. So, IMHO, we should only use numa_node_id() in this
>> function.
>>
>
> This change?
>
> modified mm/mempolicy.c
> @@ -2502,8 +2502,7 @@ int mpol_misplaced(struct folio *folio, struct vm_area_struct *vma,
> pgoff_t ilx;
> struct zoneref *z;
> int curnid = folio_nid(folio);
> - int thiscpu = raw_smp_processor_id();
> - int thisnid = cpu_to_node(thiscpu);
> + int thisnid = numa_node_id();
> int polnid = NUMA_NO_NODE;
> int ret = NUMA_NO_NODE;
>
> @@ -2573,7 +2572,7 @@ int mpol_misplaced(struct folio *folio, struct vm_area_struct *vma,
> polnid = thisnid;
>
> if (!should_numa_migrate_memory(current, folio, curnid,
> - thiscpu))
> + raw_smp_processor_id()))
> goto out;
> }
>
>
One of the problem with the above change will be the need to make sure smp processor id remaining stable, which
I am not sure we want in this function. With that we can end up with processor id not related to the numa node id
we are using.
-aneesh
