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Subject[PATCH] mm/vmalloc: use rb_tree instead of list for vread() lookups
Date
vread() has been linearly searching vmap_area_list for looking up
vmalloc areas to read from. These same areas are also tracked by
a rb_tree (vmap_area_root) which offers logarithmic lookup.

This patch modifies vread() to use the rb_tree structure instead
of the list and the speedup for heavy /proc/kcore readers can
be pretty significant. Below are the wall clock measurements of
a Python application that leverages the drgn debugging library
to read and interpret data read from /proc/kcore.

Before the patch:
-----
$ time sudo sdb -e 'dbuf | head 2500 | wc'
(unsigned long)2500

real 0m21.128s
user 0m2.321s
sys 0m19.227s
-----

With the patch:
-----
$ time sudo sdb -e 'dbuf | head 2500 | wc'
(unsigned long)2500

real 0m1.870s
user 0m1.628s
sys 0m0.660s
-----

Signed-off-by: Serapheim Dimitropoulos <serapheim@delphix.com>
---
mm/vmalloc.c | 19 ++++++++++++-------
1 file changed, 12 insertions(+), 7 deletions(-)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 49ab9b6c001d..86343b879938 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -2851,6 +2851,7 @@ long vread(char *buf, char *addr, unsigned long count)
{
struct vmap_area *va;
struct vm_struct *vm;
+ struct rb_node *node;
char *vaddr, *buf_start = buf;
unsigned long buflen = count;
unsigned long n;
@@ -2860,17 +2861,15 @@ long vread(char *buf, char *addr, unsigned long count)
count = -(unsigned long) addr;

spin_lock(&vmap_area_lock);
- list_for_each_entry(va, &vmap_area_list, list) {
- if (!count)
- break;
-
+ va = __find_vmap_area((unsigned long)addr);
+ if (!va)
+ goto finished;
+ while (count) {
if (!va->vm)
- continue;
+ goto next_node;

vm = va->vm;
vaddr = (char *) vm->addr;
- if (addr >= vaddr + get_vm_area_size(vm))
- continue;
while (addr < vaddr) {
if (count == 0)
goto finished;
@@ -2889,6 +2888,12 @@ long vread(char *buf, char *addr, unsigned long count)
buf += n;
addr += n;
count -= n;
+
+next_node:
+ node = rb_next(&va->rb_node);
+ if (!node)
+ break;
+ va = rb_entry(node, struct vmap_area, rb_node);
}
finished:
spin_unlock(&vmap_area_lock);
--
2.17.1
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