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

[Wei Yang]

On Wed, Aug 07, 2019 at 09:51:01AM +0200, Michal Hocko wrote:
>On Wed 07-08-19 08:31:09, Wei Yang wrote:
>> 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.
>
>Is there any measurable difference (on micro benchmarks or regular
>workloads)?

I wrote a test case to compare these two methods, but not find visible
difference in run time.

While I found we may leverage rb_last to refine the code a little.

@@ -2270,12 +2270,9 @@ find_vma_prev(struct mm_struct *mm, unsigned long addr,
        if (vma) {
                *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);
-                       rb_node = rb_node->rb_right;
-               }
+               struct rb_node *rb_node = rb_last(&mm->mm_rb);
+               *pprev = !rb_node ? NULL :
+                        rb_entry(rb_node, struct vm_area_struct, vm_rb);
        }
        return vma;

Not sure this style would help a little in understanding the code?

>-- 
>Michal Hocko
>SUSE Labs

-- 
Wei Yang
Help you, Help me