linux-kernel - Re: [PATCH 1/3] mm/mempolicy: Use the already fetched local variable

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Date: Tue, 20 Feb 2024 12:02:55 +0530
From: "Aneesh Kumar K.V" <aneesh.kumar@...nel.org>
To: "Huang, Ying" <ying.huang@...el.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
 Donet Tom <donettom@...ux.ibm.com>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, Dave Hansen <dave.hansen@...ux.intel.com>,
 Mel Gorman <mgorman@...e.de>, Ben Widawsky <ben.widawsky@...el.com>,
 Feng Tang <feng.tang@...el.com>, Michal Hocko <mhocko@...nel.org>,
 Andrea Arcangeli <aarcange@...hat.com>, Peter Zijlstra
 <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>,
 Rik van Riel <riel@...riel.com>, Johannes Weiner <hannes@...xchg.org>,
 Matthew Wilcox <willy@...radead.org>, Mike Kravetz
 <mike.kravetz@...cle.com>, Vlastimil Babka <vbabka@...e.cz>,
 Dan Williams <dan.j.williams@...el.com>, Hugh Dickins <hughd@...gle.com>,
 Kefeng Wang <wangkefeng.wang@...wei.com>,
 Suren Baghdasaryan <surenb@...gle.com>
Subject: Re: [PATCH 1/3] mm/mempolicy: Use the already fetched local variable

On 2/20/24 11:55 AM, Huang, Ying wrote:
> "Aneesh Kumar K.V" <aneesh.kumar@...nel.org> writes:
> 
>> On 2/20/24 6:51 AM, Andrew Morton wrote:
>>> On Mon, 19 Feb 2024 14:04:23 +0530 Donet Tom <donettom@...ux.ibm.com> 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