mm: make vmstat_update periodic run conditional

mm: make vmstat_update periodic run conditional

vmstat_update runs every second from the work queue to update statistics
and drain per cpu pages back into the global page allocator.

This is useful in most circumstances but is wasteful if the CPU doesn't
actually make any VM activity. This can happen in the situtation that
the CPU is idle or running a CPU bound long term task (e.g. CPU
isolation), in which case the periodic vmstate_update timer needlessly
itnerrupts the CPU.

This patch tries to make vmstat_update schedule itself for the next
round only if there was any work for it to do in the previous run.
The assumption is that if for a whole second we didn't see any VM
activity it is reasnoable to assume that the CPU is not using the
VM because it is idle or runs a long term single CPU bound task.

A new single unbound system work queue item is scheduled periodically
to monitor CPUs that have their vmstat_update work stopped and
re-schedule them if VM activity is detected.

Signed-off-by: Gilad Ben-Yossef <gilad@benyossef.com>
CC: Thomas Gleixner <tglx@linutronix.de>
CC: Tejun Heo <tj@kernel.org>
CC: John Stultz <johnstul@us.ibm.com>
CC: Andrew Morton <akpm@linux-foundation.org>
CC: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
CC: Mel Gorman <mel@csn.ul.ie>
CC: Mike Frysinger <vapier@gentoo.org>
CC: David Rientjes <rientjes@google.com>
CC: Hugh Dickins <hughd@google.com>
CC: Minchan Kim <minchan.kim@gmail.com>
CC: Konstantin Khlebnikov <khlebnikov@openvz.org>
CC: Christoph Lameter <cl@linux.com>
CC: Chris Metcalf <cmetcalf@tilera.com>
CC: Hakan Akkan <hakanakkan@gmail.com>
CC: Max Krasnyansky <maxk@qualcomm.com>
CC: Frederic Weisbecker <fweisbec@gmail.com>
CC: linux-kernel@vger.kernel.org
CC: linux-mm@kvack.org