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

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Date: Tue, 20 Feb 2024 15:22:07 +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 12:02 PM, Aneesh Kumar K.V wrote:
>> 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;
>>  	}
>>  
>> 
>
> 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. 

This isn't an issue now, because mpol_misplaced() are always called with
PTL held.  And, we can still keep thiscpu local variable.

--
Best Regards,
Huang, Ying