[SRU][Yakkety][PATCH 5/5] ksm: optimize refile of stable_node_dup at the head of the chain

Gavin Guo gavin.guo at canonical.com
Tue Jul 18 12:05:56 UTC 2017


From: Andrea Arcangeli <aarcange at redhat.com>

BugLink: http://bugs.launchpad.net/bugs/1680513

If a candidate stable_node_dup has been found and it can accept further
merges it can be refiled to the head of the list to speedup next
searches without altering which dup is found and how the dups accumulate
in the chain.

We already refiled it back to the head in the prune_stale_stable_nodes
case, but we didn't refile it if not pruning (which is more common).
And we also refiled it when it was already at the head which is
unnecessary (in the prune_stale_stable_nodes case, nr > 1 means there's
more than one dup in the chain, it doesn't mean it's not already at the
head of the chain).

The stable_node_chain list is single threaded and there's no SMP locking
contention so it should be faster to refile it to the head of the list
also if prune_stale_stable_nodes is false.

Profiling shows the refile happens 1.9% of the time when a dup is found
with a max_page_sharing limit setting of 3 (with max_page_sharing of 2
the refile never happens of course as there's never space for one more
merge) which is reasonably low.  At higher max_page_sharing values it
should be much less frequent.

This is just an optimization.

Link: http://lkml.kernel.org/r/20170518173721.22316-4-aarcange@redhat.com
Signed-off-by: Andrea Arcangeli <aarcange at redhat.com>
Cc: Evgheni Dereveanchin <ederevea at redhat.com>
Cc: Andrey Ryabinin <aryabinin at virtuozzo.com>
Cc: Petr Holasek <pholasek at redhat.com>
Cc: Hugh Dickins <hughd at google.com>
Cc: Arjan van de Ven <arjan at linux.intel.com>
Cc: Davidlohr Bueso <dave at stgolabs.net>
Cc: Gavin Guo <gavin.guo at canonical.com>
Cc: Jay Vosburgh <jay.vosburgh at canonical.com>
Cc: Mel Gorman <mgorman at techsingularity.net>
Cc: Dan Carpenter <dan.carpenter at oracle.com>
Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
(cherry picked from commit 80b18dfa53bbb03085eba6401f5d29dad49205b7)
Signed-off-by: Gavin Guo <gavin.guo at canonical.com>
---
 mm/ksm.c | 35 +++++++++++++++++++++++------------
 1 file changed, 23 insertions(+), 12 deletions(-)

diff --git a/mm/ksm.c b/mm/ksm.c
index 470f28ba040f..4a72634846d3 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -1337,13 +1337,14 @@ struct page *stable_node_dup(struct stable_node **_stable_node_dup,
 		put_page(_tree_page);
 	}
 
-	/*
-	 * nr is relevant only if prune_stale_stable_nodes is true,
-	 * otherwise we may break the loop at nr == 1 even if there
-	 * are multiple entries.
-	 */
-	if (prune_stale_stable_nodes && found) {
-		if (nr == 1) {
+	if (found) {
+		/*
+		 * nr is counting all dups in the chain only if
+		 * prune_stale_stable_nodes is true, otherwise we may
+		 * break the loop at nr == 1 even if there are
+		 * multiple entries.
+		 */
+		if (prune_stale_stable_nodes && nr == 1) {
 			/*
 			 * If there's not just one entry it would
 			 * corrupt memory, better BUG_ON. In KSM
@@ -1374,12 +1375,22 @@ struct page *stable_node_dup(struct stable_node **_stable_node_dup,
 			 * time.
 			 */
 			stable_node = NULL;
-		} else if (__is_page_sharing_candidate(found, 1)) {
+		} else if (stable_node->hlist.first != &found->hlist_dup &&
+			   __is_page_sharing_candidate(found, 1)) {
 			/*
-			 * Refile our candidate at the head
-			 * after the prune if our candidate
-			 * can accept one more future sharing
-			 * in addition to the one underway.
+			 * If the found stable_node dup can accept one
+			 * more future merge (in addition to the one
+			 * that is underway) and is not at the head of
+			 * the chain, put it there so next search will
+			 * be quicker in the !prune_stale_stable_nodes
+			 * case.
+			 *
+			 * NOTE: it would be inaccurate to use nr > 1
+			 * instead of checking the hlist.first pointer
+			 * directly, because in the
+			 * prune_stale_stable_nodes case "nr" isn't
+			 * the position of the found dup in the chain,
+			 * but the total number of dups in the chain.
 			 */
 			hlist_del(&found->hlist_dup);
 			hlist_add_head(&found->hlist_dup,
-- 
2.7.4





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