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[PATCH v1] mm/vmalloc: add a node corresponding to cached_hole_size

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  [PATCH v1] mm/vmalloc: add a node corresponding to cached_hole_size Zhaoyang Huang <huangzhaoyang@gmail.com> - 2017-07-21 12:10 +0200
    Re: [PATCH v1] mm/vmalloc: add a node corresponding to  cached_hole_size Matthew Wilcox <willy@infradead.org> - 2017-07-21 13:50 +0200
      Re: [PATCH v1] mm/vmalloc: add a node corresponding to  cached_hole_size Michal Hocko <mhocko@kernel.org> - 2017-07-21 16:50 +0200

#1693556 — [PATCH v1] mm/vmalloc: add a node corresponding to cached_hole_size

we just record the cached_hole_size now, which will be used when
the criteria meet both of 'free_vmap_cache == NULL' and 'size <
cached_hole_size'. However, under above scenario, the search will
start from the rb_root and then find the node which just in front
of the cached hole.

free_vmap_cache miss:
      vmap_area_root
          /      \
       _next     U
        /  (T1)
 cached_hole_node
       /
     ...   (T2)
      /
    first

vmap_area_list->first->......->cached_hole_node->cached_hole_node.list.next
                  |-------(T3)-------| | <<< cached_hole_size >>> |

vmap_area_list->......->cached_hole_node->cached_hole_node.list.next
                               | <<< cached_hole_size >>> |

The time cost to search the node now is T = T1 + T2 + T3.
The commit add a cached_hole_node here to record the one just in front of
the cached_hole_size, which can help to avoid walking the rb tree and
the list and make the T = 0;

Signed-off-by: Zhaoyang Huang <zhaoyang.huang@spreadtrum.com>
---
 mm/vmalloc.c | 23 +++++++++++++++++++++--
 1 file changed, 21 insertions(+), 2 deletions(-)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 8698c1c..4e76e7f 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -336,6 +336,7 @@ unsigned long vmalloc_to_pfn(const void *vmalloc_addr)
 
 /* The vmap cache globals are protected by vmap_area_lock */
 static struct rb_node *free_vmap_cache;
+static struct vmap_area *cached_hole_node;
 static unsigned long cached_hole_size;
 static unsigned long cached_vstart;
 static unsigned long cached_align;
@@ -444,6 +445,12 @@ static struct vmap_area *alloc_vmap_area(unsigned long size,
 			size < cached_hole_size ||
 			vstart < cached_vstart ||
 			align < cached_align) {
+	/*if we have a cached node, just use it*/
+	if ((size < cached_hole_size) && cached_hole_node != NULL) {
+		addr = ALIGN(cached_hole_node->va_end, align);
+		cached_hole_node = NULL;
+		goto found;
+	}
 nocache:
 		cached_hole_size = 0;
 		free_vmap_cache = NULL;
@@ -487,8 +494,13 @@ static struct vmap_area *alloc_vmap_area(unsigned long size,
 
 	/* from the starting point, walk areas until a suitable hole is found */
 	while (addr + size > first->va_start && addr + size <= vend) {
-		if (addr + cached_hole_size < first->va_start)
+		if (addr + cached_hole_size < first->va_start) {
 			cached_hole_size = first->va_start - addr;
+			/*record the node corresponding to the hole*/
+			cached_hole_node = (first->list.prev ==
+					    &vmap_area_list) ?
+					    NULL : list_prev_entry(first, list);
+		}
 		addr = ALIGN(first->va_end, align);
 		if (addr + size < addr)
 			goto overflow;
@@ -571,10 +583,17 @@ static void __free_vmap_area(struct vmap_area *va)
 			}
 		}
 	}
+	if (va == cached_hole_node) {
+		/*cached node is freed, the hole get bigger*/
+		if (cached_hole_node->list.prev != &vmap_area_list)
+			cached_hole_node = list_prev_entry(cached_hole_node,
+							   list);
+		else
+			cached_hole_node = NULL;
+	}
 	rb_erase(&va->rb_node, &vmap_area_root);
 	RB_CLEAR_NODE(&va->rb_node);
 	list_del_rcu(&va->list);
-
 	/*
 	 * Track the highest possible candidate for pcpu area
 	 * allocation.  Areas outside of vmalloc area can be returned
-- 
1.9.1

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#1693641 — Re: [PATCH v1] mm/vmalloc: add a node corresponding to cached_hole_size

On Fri, Jul 21, 2017 at 06:01:41PM +0800, Zhaoyang Huang wrote:
> we just record the cached_hole_size now, which will be used when
> the criteria meet both of 'free_vmap_cache == NULL' and 'size <
> cached_hole_size'. However, under above scenario, the search will
> start from the rb_root and then find the node which just in front
> of the cached hole.
> 
> free_vmap_cache miss:
>       vmap_area_root
>           /      \
>        _next     U
>         /  (T1)
>  cached_hole_node
>        /
>      ...   (T2)
>       /
>     first
> 
> vmap_area_list->first->......->cached_hole_node->cached_hole_node.list.next
>                   |-------(T3)-------| | <<< cached_hole_size >>> |
> 
> vmap_area_list->......->cached_hole_node->cached_hole_node.list.next
>                                | <<< cached_hole_size >>> |
> 
> The time cost to search the node now is T = T1 + T2 + T3.
> The commit add a cached_hole_node here to record the one just in front of
> the cached_hole_size, which can help to avoid walking the rb tree and
> the list and make the T = 0;

Yes, but does this matter in practice?  Are there any workloads where
this makes a difference?  If so, how much?

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#1693759 — Re: [PATCH v1] mm/vmalloc: add a node corresponding to cached_hole_size

On Fri 21-07-17 04:39:48, Matthew Wilcox wrote:
> On Fri, Jul 21, 2017 at 06:01:41PM +0800, Zhaoyang Huang wrote:
> > we just record the cached_hole_size now, which will be used when
> > the criteria meet both of 'free_vmap_cache == NULL' and 'size <
> > cached_hole_size'. However, under above scenario, the search will
> > start from the rb_root and then find the node which just in front
> > of the cached hole.
> > 
> > free_vmap_cache miss:
> >       vmap_area_root
> >           /      \
> >        _next     U
> >         /  (T1)
> >  cached_hole_node
> >        /
> >      ...   (T2)
> >       /
> >     first
> > 
> > vmap_area_list->first->......->cached_hole_node->cached_hole_node.list.next
> >                   |-------(T3)-------| | <<< cached_hole_size >>> |
> > 
> > vmap_area_list->......->cached_hole_node->cached_hole_node.list.next
> >                                | <<< cached_hole_size >>> |
> > 
> > The time cost to search the node now is T = T1 + T2 + T3.
> > The commit add a cached_hole_node here to record the one just in front of
> > the cached_hole_size, which can help to avoid walking the rb tree and
> > the list and make the T = 0;
> 
> Yes, but does this matter in practice?  Are there any workloads where
> this makes a difference?  If so, how much?

I have already asked this and didn't get any response. There were other
versions of a similar patch without a good clarification...

Zhaoyang Huang, please try to formulate the problem you are fixing and
why. While it is clear that you add _an_ optimization it is not really
clear why we need it and whether it might adversely affect existing
workloads. I would rather not touch this code unless there is a strong
justification for it.
-- 
Michal Hocko
SUSE Labs

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