linux-kernel - [PATCH 31/31] mm: numa: Use a two-stage filter to restrict pages being migrated for unlikely task<->node relationships

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-Id: <1352805180-1607-32-git-send-email-mgorman@suse.de>
Date:	Tue, 13 Nov 2012 11:13:00 +0000
From:	Mel Gorman <mgorman@...e.de>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Ingo Molnar <mingo@...nel.org>
Cc:	Rik van Riel <riel@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Hugh Dickins <hughd@...gle.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>, Mel Gorman <mgorman@...e.de>
Subject: [PATCH 31/31] mm: numa: Use a two-stage filter to restrict pages being migrated for unlikely task<->node relationships

While it is desirable that all threads in a process run on its home
node, this is not always possible or necessary. There may be more
threads than exist within the node or the node might over-subscribed
with unrelated processes.

This can cause a situation whereby a page gets migrated off its home
node because the threads clearing pte_numa were running off-node. This
patch uses page->last_nid to build a two-stage filter before pages get
migrated to avoid problems with short or unlikely task<->node
relationships.

Signed-off-by: Mel Gorman <mgorman@...e.de>
---
 mm/mempolicy.c |   27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 076f8f8..89696d7 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -2425,6 +2425,8 @@ int mpol_misplaced(struct page *page, struct vm_area_struct *vma, unsigned long
 
 	/* Migrate pages towards their home node or the referencing node */
 	if (pol->flags & MPOL_F_HOME) {
+		int last_nid;
+
 		/*
 		 * Make a placement decision based on the home node.
 		 * NOTE: Potentially this can result in a remote->remote
@@ -2437,6 +2439,31 @@ int mpol_misplaced(struct page *page, struct vm_area_struct *vma, unsigned long
 			/* No home node, migrate to the referencing node */
 			polnid = numa_node_id();
 		}
+
+		/*
+		 * Multi-stage node selection is used in conjunction
+		 * with a periodic migration fault to build a temporal
+		 * task<->page relation. By using a two-stage filter we
+		 * remove short/unlikely relations.
+		 *
+		 * Using P(p) ~ n_p / n_t as per frequentist
+		 * probability, we can equate a task's usage of a
+		 * particular page (n_p) per total usage of this
+		 * page (n_t) (in a given time-span) to a probability.
+		 *
+		 * Our periodic faults will sample this probability and
+		 * getting the same result twice in a row, given these
+		 * samples are fully independent, is then given by
+		 * P(n)^2, provided our sample period is sufficiently
+		 * short compared to the usage pattern.
+		 *
+		 * This quadric squishes small probabilities, making
+		 * it less likely we act on an unlikely task<->page
+		 * relation.
+		 */
+		last_nid = page_xchg_last_nid(page, polnid);
+		if (last_nid != polnid)
+			goto out;
 	}
 
 	if (curnid != polnid)
-- 
1.7.9.2

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/