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

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Date: Tue, 20 Feb 2024 14:25:03 +0800
From: "Huang, Ying" <ying.huang@...el.com>
To: "Aneesh Kumar K.V" <aneesh.kumar@...nel.org>
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

"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.

--
Best Regards,
Huang, Ying