The patch titled
Subject: vmalloc: use rcu list iterator to reduce vmap_area_lock contention
has been removed from the -mm tree. Its filename was
vmalloc-use-rcu-list-iterator-to-reduce-vmap_area_lock-contention.patch
This patch was dropped because it was merged into mainline or a subsystem tree
------------------------------------------------------
From: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
Subject: vmalloc: use rcu list iterator to reduce vmap_area_lock contention
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().
rcu can be used in this function because all RCU protocol is already
respected by writers, since Nick Piggin commit db64fe02258f1 ("mm: rewrite
vmap layer") back in linux-2.6.28
Specifically :
insertions use list_add_rcu(),
deletions use list_del_rcu() and kfree_rcu().
Note the rb tree is not used from rcu reader (it would not be safe),
only the vmap_area_list has full RCU protection.
Note that __purge_vmap_area_lazy() already uses this rcu protection.
rcu_read_lock();
list_for_each_entry_rcu(va, &vmap_area_list, list) {
if (va->flags & VM_LAZY_FREE) {
if (va->va_start < *start)
*start = va->va_start;
if (va->va_end > *end)
*end = va->va_end;
nr += (va->va_end - va->va_start) >> PAGE_SHIFT;
list_add_tail(&va->purge_list, &valist);
va->flags |= VM_LAZY_FREEING;
va->flags &= ~VM_LAZY_FREE;
}
}
rcu_read_unlock();
Peter:
: While rcu list traversal over the vmap_area_list is safe, this may
: arrive at different results than the spinlocked version. The rcu list
: traversal version will not be a 'snapshot' of a single, valid instant
: of the entire vmap_area_list, but rather a potential amalgam of
: different list states.
Joonsoo:
: Yes, you are right, but I don't think that we should be strict here.
: Meminfo is already not a 'snapshot' at specific time. While we try to get
: certain stats, the other stats can change. And, although we may arrive at
: different results than the spinlocked version, the difference would not be
: large and would not make serious side-effect.
[edumazet@xxxxxxxxxx: add more commit description]
Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
Reported-by: Richard Yao <ryao@xxxxxxxxxx>
Acked-by: Eric Dumazet <edumazet@xxxxxxxxxx>
Cc: Peter Hurley <peter@xxxxxxxxxxxxxxxxxx>
Cc: Zhang Yanfei <zhangyanfei.yes@xxxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Andi Kleen <andi@xxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/vmalloc.c | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)
diff -puN mm/vmalloc.c~vmalloc-use-rcu-list-iterator-to-reduce-vmap_area_lock-contention mm/vmalloc.c
--- a/mm/vmalloc.c~vmalloc-use-rcu-list-iterator-to-reduce-vmap_area_lock-contention
+++ a/mm/vmalloc.c
@@ -2690,14 +2690,14 @@ void get_vmalloc_info(struct vmalloc_inf
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_inf
vmi->largest_chunk = VMALLOC_END - prev_end;
out:
- spin_unlock(&vmap_area_lock);
+ rcu_read_unlock();
}
#endif
_
Patches currently in -mm which might be from iamjoonsoo.kim@xxxxxxx are
origin.patch
mm-compactionc-isolate_freepages_block-small-tuneup.patch
page-owners-correct-page-order-when-to-free-page.patch
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