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Subject[PATCH] vmalloc: use rcu list iterator to reduce vmap_area_lock contention
Date
Richard Yao reported a month ago that his system have a trouble
with vmap_area_lock contention during performance analysis
by /proc/meminfo. Andrew asked why his analysis checks /proc/meminfo
stressfully, but he didn't answer it.

https://lkml.org/lkml/2014/4/10/416

Although I'm not sure that this is right usage or not, there is a solution
reducing vmap_area_lock contention with no side-effect. That is just
to use rcu list iterator in get_vmalloc_info(). This function only needs
values on vmap_area structure, so we don't need to grab a spinlock.

Reported-by: Richard Yao <ryao@gentoo.org>
Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index f64632b..fdbb116 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -2690,14 +2690,14 @@ void get_vmalloc_info(struct vmalloc_info *vmi)

prev_end = VMALLOC_START;

- spin_lock(&vmap_area_lock);
+ rcu_read_lock();

if (list_empty(&vmap_area_list)) {
vmi->largest_chunk = VMALLOC_TOTAL;
goto out;
}

- list_for_each_entry(va, &vmap_area_list, list) {
+ list_for_each_entry_rcu(va, &vmap_area_list, list) {
unsigned long addr = va->va_start;

/*
@@ -2724,7 +2724,7 @@ void get_vmalloc_info(struct vmalloc_info *vmi)
vmi->largest_chunk = VMALLOC_END - prev_end;

out:
- spin_unlock(&vmap_area_lock);
+ rcu_read_unlock();
}
#endif

--
1.7.9.5


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