linux-kernel - [PATCH -mm] do_migrate_pages() calls migrate_to_node() even if task is already on a correct node

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Message-ID: <4F6B6BFF.1020701@redhat.com>
Date:	Thu, 22 Mar 2012 14:14:23 -0400
From:	Larry Woodman <lwoodman@...hat.com>
To:	linux-mm@...ck.org
CC:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>,
	Motohiro Kosaki <mkosaki@...hat.com>
Subject: [PATCH -mm] do_migrate_pages() calls migrate_to_node() even if task
 is already on a correct node

While moving tasks between cpusets I noticed some strange behavior.  Specifically if the nodes of the destination
cpuset are a subset of the nodes of the source cpuset do_migrate_pages() will move pages that are already on a node
in the destination cpuset.  The reason for this is do_migrate_pages() does not check whether each node in the source
nodemask is in the destination nodemask before calling migrate_to_node().  If we simply do this check and skip them
when the source is in the destination moving we wont move nodes that dont need to be moved.

Adding a little debug printk to migrate_to_node():

Without this change migrating tasks from a cpuset containing nodes 0-7 to a cpuset containing nodes 3-4, we migrate
from ALL the nodes even if they are in the both the source and destination nodesets:

   Migrating 7 to 4
   Migrating 6 to 3
   Migrating 5 to 4
   Migrating 4 to 3
   Migrating 1 to 4
   Migrating 3 to 4
   Migrating 0 to 3
   Migrating 2 to 3

With this change we only migrate from nodes that are not in the destination nodesets:

   Migrating 7 to 4
   Migrating 6 to 3
   Migrating 5 to 4
   Migrating 2 to 3
   Migrating 1 to 4
   Migrating 0 to 3

Signed-off-by: Larry Woodman<lwoodman@...hat.com>

View attachment "upstream-do_migrate_pages.patch" of type "text/plain" (408 bytes)