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List:       linux-stable-commits
Subject:    Patch "vmalloc: use rcu list iterator to reduce vmap_area_lock contention" has been added to the 3.1
From:       <gregkh () linuxfoundation ! org>
Date:       2015-01-28 1:07:08
Message-ID: 142240722824338 () kroah ! com
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This is a note to let you know that I've just added the patch titled

    vmalloc: use rcu list iterator to reduce vmap_area_lock contention

to the 3.14-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     vmalloc-use-rcu-list-iterator-to-reduce-vmap_area_lock-contention.patch
and it can be found in the queue-3.14 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@vger.kernel.org> know about it.


From 474750aba88817c53f39424e5567b8e4acc4b39b Mon Sep 17 00:00:00 2001
From: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Date: Wed, 6 Aug 2014 16:05:06 -0700
Subject: vmalloc: use rcu list iterator to reduce vmap_area_lock contention

From: Joonsoo Kim <iamjoonsoo.kim@lge.com>

commit 474750aba88817c53f39424e5567b8e4acc4b39b upstream.

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@google.com: add more commit description]
Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Reported-by: Richard Yao <ryao@gentoo.org>
Acked-by: Eric Dumazet <edumazet@google.com>
Cc: Peter Hurley <peter@hurleysoftware.com>
Cc: Zhang Yanfei <zhangyanfei.yes@gmail.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Andi Kleen <andi@firstfloor.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Mel Gorman <mgorman@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 mm/vmalloc.c |    6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -2681,14 +2681,14 @@ 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;
 
 		/*
@@ -2715,7 +2715,7 @@ 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 stable-queue which might be from iamjoonsoo.kim@lge.com are

queue-3.14/mm-swap.c-clean-up-lru_cache_add-functions.patch
queue-3.14/vmalloc-use-rcu-list-iterator-to-reduce-vmap_area_lock-contention.patch
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