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;
}
-aneesh
