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

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Date: Tue, 20 Feb 2024 12:33:51 +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 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. 

-aneesh