Commit 80b18dfa authored by Andrea Arcangeli's avatar Andrea Arcangeli Committed by Linus Torvalds

ksm: optimize refile of stable_node_dup at the head of the chain

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.comSigned-off-by: default avatarAndrea Arcangeli <aarcange@redhat.com>
Cc: Evgheni Dereveanchin <ederevea@redhat.com>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Petr Holasek <pholasek@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Gavin Guo <gavin.guo@canonical.com>
Cc: Jay Vosburgh <jay.vosburgh@canonical.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 8dc5ffcd
...@@ -1367,13 +1367,14 @@ struct page *stable_node_dup(struct stable_node **_stable_node_dup, ...@@ -1367,13 +1367,14 @@ struct page *stable_node_dup(struct stable_node **_stable_node_dup,
put_page(_tree_page); put_page(_tree_page);
} }
/* if (found) {
* nr is relevant only if prune_stale_stable_nodes is true, /*
* otherwise we may break the loop at nr == 1 even if there * nr is counting all dups in the chain only if
* are multiple entries. * prune_stale_stable_nodes is true, otherwise we may
*/ * break the loop at nr == 1 even if there are
if (prune_stale_stable_nodes && found) { * multiple entries.
if (nr == 1) { */
if (prune_stale_stable_nodes && nr == 1) {
/* /*
* If there's not just one entry it would * If there's not just one entry it would
* corrupt memory, better BUG_ON. In KSM * corrupt memory, better BUG_ON. In KSM
...@@ -1404,12 +1405,22 @@ struct page *stable_node_dup(struct stable_node **_stable_node_dup, ...@@ -1404,12 +1405,22 @@ struct page *stable_node_dup(struct stable_node **_stable_node_dup,
* time. * time.
*/ */
stable_node = NULL; 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 * If the found stable_node dup can accept one
* after the prune if our candidate * more future merge (in addition to the one
* can accept one more future sharing * that is underway) and is not at the head of
* in addition to the one underway. * 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_del(&found->hlist_dup);
hlist_add_head(&found->hlist_dup, hlist_add_head(&found->hlist_dup,
......
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