Re: [PATCH] mm/mmap.c: refine data locality of find_vma_prev

[Wei Yang]

On Tue, Aug 06, 2019 at 11:29:52AM +0200, Vlastimil Babka wrote:
>On 8/6/19 10:11 AM, Wei Yang wrote:
>> When addr is out of the range of the whole rb_tree, pprev will points to
>> the biggest node. find_vma_prev gets is by going through the right most
>
>s/biggest/last/ ? or right-most?
>
>> node of the tree.
>> 
>> Since only the last node is the one it is looking for, it is not
>> necessary to assign pprev to those middle stage nodes. By assigning
>> pprev to the last node directly, it tries to improve the function
>> locality a little.
>
>In the end, it will always write to the cacheline of pprev. The caller has most
>likely have it on stack, so it's already hot, and there's no other CPU stealing
>it. So I don't understand where the improved locality comes from. The compiler
>can also optimize the patched code so the assembly is identical to the previous
>code, or vice versa. Did you check for differences?

Vlastimil

Thanks for your comment.

I believe you get a point. I may not use the word locality. This patch tries
to reduce some unnecessary assignment of pprev.

Original code would assign the value on each node during iteration, this is
what I want to reduce.

The generated code looks different from my side. Would you mind sharing me how
you compare the generated code?

>
>The previous code is somewhat more obvious to me, so unless I'm missing
>something, readability and less churn suggests to not change.
>
>> Signed-off-by: Wei Yang <richardw.yang@xxxxxxxxxxxxxxx>
>> ---
>>  mm/mmap.c | 7 +++----
>>  1 file changed, 3 insertions(+), 4 deletions(-)
>> 
>> diff --git a/mm/mmap.c b/mm/mmap.c
>> index 7e8c3e8ae75f..284bc7e51f9c 100644
>> --- a/mm/mmap.c
>> +++ b/mm/mmap.c
>> @@ -2271,11 +2271,10 @@ find_vma_prev(struct mm_struct *mm, unsigned long addr,
>>  		*pprev = vma->vm_prev;
>>  	} else {
>>  		struct rb_node *rb_node = mm->mm_rb.rb_node;
>> -		*pprev = NULL;
>> -		while (rb_node) {
>> -			*pprev = rb_entry(rb_node, struct vm_area_struct, vm_rb);
>> +		while (rb_node && rb_node->rb_right)
>>  			rb_node = rb_node->rb_right;
>> -		}
>> +		*pprev = rb_node ? NULL
>> +			 : rb_entry(rb_node, struct vm_area_struct, vm_rb);
>>  	}
>>  	return vma;
>>  }
>> 

-- 
Wei Yang
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